MT

The earth has a great diversity of species and habitats, or places where these species can live. Some species live in terrestrial, or land, habitats such as grasslands, forests (see 

chapter-opening photo

), and deserts. These three major types of terrestrial ecosystems are called biomes—large terrestrial areas characterized by their climates and the plants and animals that live there. They represent one of the four components of biodiversity (

Figure 4.5

), which is the basis for one of the three scientific principles of sustainability.

Why do grasslands grow on some areas of the earth’s land while forests and deserts form in other areas? The answer lies largely in differences in climate, the average weather conditions in a given region over at least three decades to thousands of years. Differences in climate result mostly from long-term differences in weather, based primarily on average annual precipitation and temperature. These differences lead to three major types of climate—tropical (areas near the equator, receiving the most intense sunlight), polar (areas near the earth’s poles, receiving the least intense sunlight), and temperate (areas between the tropical and polar regions).

Throughout these regions, we find different types of ecosystems, vegetation, and animals in land-based biomes adapted to the various climate conditions. For example, in tropical areas, we find a type of grassland called a savanna. This biome typically contains scattered trees and usually has warm temperatures year-round with alternating dry and wet seasons. Savannas in East Africa are home to grazing (primarily grass-eating) and browsing (twig- and leaf-nibbling) hoofed animals. They include wildebeests, gazelles, antelopes, zebras, giraffes, and elephants (

Figure 7.1

), as well as their predators such as lions, hyenas, and humans.

Figure 7.1

Elephants on a tropical savanna in Kenya, Africa.

FOTOGRIN/ Shutterstock.com

Archeological evidence indicates that our species emerged from African savannas and survived by gathering edible vegetation and hunting animals for food and clothing made from animal hides. Early humans lived largely in trees but eventually came down to the ground and learned to walk upright. This freed them to use their hands for using tools such as clubs and spears. Later, they developed bows and arrows and other weapons that enhanced their abilities to hunt animals for food and clothing made from animal hides.

After the last ice age, about 10,000 years ago, the earth’s climate warmed and humans began their transition from hunter–gatherers to farmers growing food on the savanna and other grasslands. Later, they cleared patches of forest to expand farmland and created villages and eventually towns and cities.

Today, vast areas of African savanna have been plowed up and converted to cropland or used for grazing livestock. Towns are also expanding there, and this trend will continue as the human population in Africa—the continent with the world’s fastest population growth—increases. As a result, populations of elephants, lions, and other animals that roamed the savannas for millions of years have dwindled. Many of these animals face extinction in the next few decades because of the loss of their habitats and because people kill them for food and their valuable parts such as the ivory tusks of elephants.

In this chapter, we distinguish between weather and climate and examine the role that climate plays in the location and formation of the major terrestrial ecosystems. We also begin the study of human impacts on these important ecosystems.

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7.1Weather

· LO 7.1ADefine weather in terms of at least five factors that occur over periods of hours to days.

· LO 7.1BDescribe what happens at a warm front and at a cold front.

· LO 7.1CExplain how a high-pressure air mass affects weather.

· LO 7.1DExplain how a low-pressure air mass affects weather.

· LO 7.1EExplain how an ENSO, or El Niño, affects weather patterns.

· LO 7.1FDescribe the formation of a tornado and the formation of a tropical cyclone.

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7.1aWeather Is Affected by Moving Masses of Warm or Cold Air

Weather

 is the set of physical conditions of the lower atmosphere that includes temperature, precipitation, humidity, wind speed, cloud cover, and other factors that occur in a given area over a period of hours to days. The most important factors in an area’s weather are atmospheric temperature and precipitation.

Meteorologists use equipment mounted on weather balloons, aircraft, ships, and satellites, as well as radar and stationary sensors, to obtain data on weather variables. They feed these data into computer models to draw weather maps for various parts of the world. Other computer models project upcoming weather conditions based on probabilities that air masses, winds, and other factors will change in certain ways.

Much of the weather we experience results from interactions between the leading edges of moving masses of warm air and cold air. Weather changes when one air mass replaces or meets another. The most dramatic changes in weather occur along a 

front

, the boundary between two air masses with different temperatures and densities.

A 

warm front

 is the boundary between an advancing warm air mass and the cooler one it is replacing (

Figure 7.2

, left). Because warm air is less dense (weighs less per unit of volume) than cool air, an advancing warm air mass rises up over a mass of cool air. As the warm air rises, its moisture begins condensing into droplets, forming layers of clouds at different altitudes. Gradually, the clouds thicken, descend to a lower altitude, and often release their moisture as rainfall.

Figure 7.2

Weather fronts: A warm front (left) occurs when a moving mass of warm air meets and rises up over a mass of denser cool air. A cold front (right) forms when a moving mass of cold air wedges beneath a mass of less dense warm air.

A 

cold front

 (Figure 7.2, right) is the leading edge of an advancing mass of cold air. Because cold air is denser than warm air, an advancing cold front stays close to the ground and wedges beneath less dense warmer air. It pushes this warm, moist air up, which produces rapidly moving, towering clouds called thunderheads. As it passes through, it can cause high surface winds and thunderstorms, followed by cooler temperatures and a clear sky.

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7.1bWeather Is Affected by Changes in Atmospheric Pressure and Wind Patterns

Atmospheric pressure

 results from molecules of gases in the atmosphere (mostly nitrogen and oxygen) moving at high speeds and bouncing off everything they encounter. Atmospheric pressure is greater near the earth’s surface because the molecules in the atmosphere are squeezed together under the weight of the air above them.

An air mass with high pressure, called a 

high

, contains cool, dense air that descends slowly toward the earth’s surface and becomes warmer. Because of this warming, water molecules in the air do not form droplets—a process called condensation. Thus clouds, which are made of droplets, usually do not form in the presence of a high. Fair weather with clear skies follows as long as this high remains over the area.

A low-pressure air mass, called a 

low

, contains low-density, warm air at its center. This air rises, expands, and cools. When its temperature drops below a certain level, called the dew point, moisture in the air condenses and forms clouds. The condensation process usually requires that the air contain suspended tiny particles of dust, smoke, sea salts, or volcanic ash, called condensation nuclei, around which water droplets can form. If the droplets in the clouds coalesce into larger drops or snowflakes heavy enough to fall from the sky, precipitation occurs. Thus, a low tends to produce cloudy and sometimes stormy weather.

Movement of these air masses is influenced strongly by jet streams—powerful winds that circle the globe near the top of the troposphere. They are like fast-flowing rivers of air moving west to east, one in each hemisphere somewhere above and below the equator. They form because of the temperature difference between the equator and the poles, which causes air to move. As the air moves away from the equator, north and south, it is deflected by the earth’s rotation and flows generally west to east. Jet streams can influence weather by moving moist air masses from one area to another. We examine these patterns of airflow in more depth later in this chapter.

Every few years, normal wind patterns in the Pacific Ocean (Figure 7.3, left) are disrupted and this affects weather around much of the globe. This change in wind patterns is called the El Niño–Southern Oscillation, or ENSO (

Figure 7.3

, right).

Figure 7.3

El Niño: Normal trade winds blowing east to west cause shore upwellings of cold, nutrient-rich bottom water in the tropical Pacific Ocean near the coast of Peru (left). A zone of gradual temperature change, called the thermocline, separates the warm and cold water. Every few years, a shift in trade winds known as the El Niño–Southern Oscillation (ENSO) disrupts this pattern (right) for 1 to 2 years. This disrupts normal rainfall patterns.

In an ENSO, often called simply El Niño, winds that usually blow more-or-less constantly from east to west weaken or reverse direction. This allows the warmer waters of the western Pacific to move toward the coast of South America. A horizontal zone of gradual temperature change separating warm and cold waters, called the thermocline, sinks in the eastern Pacific. These changes result in drier weather in some areas and wetter weather in other areas. A strong ENSO can alter weather conditions over at least two-thirds of the globe (

Figure 7.4

)—especially on the coasts of the Pacific and Indian Oceans.

Figure 7.4

Typical global weather effects of an El Niño–Southern Oscillation.

Question:

1. How might an ENSO affect the weather where you live or go to school?

(Compiled by the authors using data from United Nations Food and Agriculture Organization and U.S. Weather Service.)

An ENSO is a 1- to 2-year natural weather event. Although it is not a climate event, it can raise the earth’s average temperature by as much as ‍ for a year or two. As a result, it can affect the climate by temporarily increasing the earth’s average temperature. ENSOs can be extreme in their effects. One such super ENSO occurred in 1997 and 1998. This 2-year period of extreme weather, including severe storms, flooding, and temperature extremes, caused $4.5 billion in damages and 23,000 deaths.

La Niña, the reverse of El Niño, cools some coastal surface waters. This natural weather event also occurs every few years and it typically leads to more Atlantic Ocean hurricanes, colder winters in Canada and the northeastern United States, and warmer and drier winters in the southeastern and southwestern United States. It also usually leads to wetter winters in the Pacific Northwest, torrential rains in Southeast Asia, and sometimes more wildfires in Florida. Scientists do not know the exact causes of these weather events or when they are likely to occur, but they do know how to detect and monitor them.

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7.1cWeather Extremes: Tornadoes and Tropical Cyclones

Sometimes we experience weather extremes. Two examples are violent storms called tornadoes (which form over land) and tropical cyclones (which form over warm ocean waters and sometimes pass over coastal land areas).

Tornadoes, or twisters, are swirling, funnel-shaped clouds that form over land. They can destroy houses and cause other serious damage in areas where they touch down. The United States is the world’s most tornado-prone country, followed by Australia.

Tornadoes in the plains of the Midwestern United States often occur when a large, dry, cold front moving southward from Canada runs into a large mass of warm humid air moving northward from the Gulf of Mexico. As the large warm front moves rapidly over the denser cold-air mass, it rises swiftly and forms strong vertical convection currents that suck air upward (

Figure 7.5

). Scientists hypothesize that the interaction of the cooler air nearer the ground and the rapidly rising warmer air above causes a spinning, vertically rising air mass, or vortex. Most tornadoes in the American Midwest occur in the spring and summer when cold fronts from the north penetrate deeply into the Great Plains and the Midwest.

Figure 7.5

Formation of a tornado, or twister. The most active tornado season in the United States is usually March through August.

Tropical cyclones (

Figure 7.6

) are spawned by the formation of low-pressure cells of air over warm tropical seas. Hurricanes are tropical cyclones that form in the Atlantic Ocean. Those forming in the Pacific Ocean usually are called typhoons. Hurricanes and typhoons kill and injure people, damage property, and hinder food production. Unlike tornadoes, however, tropical cyclones take a long time to form and gain strength. This allows meteorologists to track their paths and wind speeds, and to warn people in areas likely to be hit by these violent storms.

Figure 7.6

Formation of a tropical cyclone. Those forming in the Atlantic Ocean are called hurricanes. Those forming in the Pacific Ocean are called typhoons.

For a tropical cyclone to form, the temperature of ocean water has to be at least ‍ to a depth of 46 meters (150 feet). Areas of low pressure over these warm ocean waters draw in air from surrounding higher-pressure areas. The earth’s rotation makes these winds spiral counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Moist air, warmed by the heat of the ocean, rises in a vortex through the center of the storm until it becomes a tropical cyclone (

Figure 7.6).

The intensities of tropical cyclones are rated in different categories, based on their sustained wind speeds. The longer a tropical cyclone stays over warm waters, the stronger it gets. Significant hurricane-force winds can extend 64–161 kilometers (40–100 miles) from the center, or eye, of a tropical cyclone.

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7.2Climate

· LO 7.2AExplain the difference between weather and climate.

· LO 7.2BIdentify two global forces that distribute heat and precipitation between the tropics and other parts of the world.

· LO 7.2CExplain how convection works in the atmosphere to help determine regional climates.

· LO 7.2DExplain how uneven heating of the earth helps determine regional climates.

·
LO 7.2EDescribe the greenhouse effect by identifying four greenhouse gases and explaining how they warm the atmosphere.

· LO 7.2FExplain how the rain shadow effect helps create deserts.
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7.2aThe Earth Has Many Different Climates

It is important to understand the difference between weather and climate. Weather is the set of short-term atmospheric conditions over hours to days, whereas 

climate

 is the pattern of atmospheric conditions in a given area over periods ranging from at least three decades to thousands of years. Weather often fluctuates daily, from one season to another, and from one year to the next. However, climate tends to change slowly because it is the average of long-term atmospheric conditions over at least 30 years.

Climate varies among the earth’s different regions primarily because of global air circulation and 

ocean currents

, or mass movements of ocean water. Global winds and ocean currents distribute heat and precipitation unevenly between the tropics and other parts of the world. Scientists have described the various regions of the earth according to their climates (

Figure 7.7

).

Figure 7.7

Natural capital: This generalized map of the earth’s current climate zones also shows the major ocean currents and upwelling areas (where currents bring nutrients from the ocean bottom to the surface).

Question:

1. Based on this map, what is the general type of climate where you live?

Several major factors help determine regional climates. The first is the cyclical movement of air driven by solar energy. It is a form of convection, the movement of fluid matter (such as gas or water) caused when the warmer and less dense part of a body of such matter rises while the cooler, denser part of the fluid sinks due to gravity. In the atmosphere, convection occurs when the sun warms the air and causes some of it to rise, while cooler air sinks in a cyclical pattern called a 

convection cell

.

For example, the air over an ocean is heated when the sun evaporates water. This transfers moisture and heat from the ocean to the atmosphere, especially near the hot equator. This warm, moist air rises, then cools and releases heat and moisture as precipitation (

Figure 7.8

, right side and top, center). Then the cooler, denser, and drier air sinks, warms up, and absorbs moisture as it flows across the earth’s surface (Figure 7.8, left side and bottom) to begin the cycle again.

Figure 7.8

Convection cells play a key role in transferring energy (heat) and moisture through the atmosphere from place to place on the planet.

The second major climatic factor is the uneven heating of the earth’s surface by the sun. Air is heated much more at the equator, where the sun’s rays strike directly, than at the poles, where sunlight strikes at an angle and spreads out over a much greater area (

Figure 7.9

, left). Thus, solar heating varies with 

latitude

—the location between the equator and one of the poles. Latitudes are designated by degrees  north or south. The equator is at , the poles are at  north and  south, and areas between range from  to .

Figure 7.9

Global air circulation: Air rises and falls in giant convection cells (right). Air flowing away from the equator is deflected to the east and air flowing toward the equator is deflected west, due to the Coriolis Effect. This creates global patterns of prevailing winds (left) that help to distribute heat and moisture in the atmosphere, which leads to the earth’s variety of forests, grasslands, and deserts (right).

The input of solar energy in a given area, called 

insolation

, varies with latitude. This partly explains why tropical regions are hot, polar regions are cold, and temperate regions generally alternate between warm and cool temperatures (Figure 7.9, right).

A third major factor is the tilt of the earth’s axis and resulting seasonal changes. The earth’s axis—an imaginary line connecting the north and south poles—is tilted with respect to the sun’s rays. As a result, regions north and south of the equator are tipped toward or away from the sun at different times, as the earth makes its annual revolution around the sun (

Figure 7.10

). This means that most areas of the world experience widely varying amounts of solar energy, and thus very different seasons, throughout the year. This leads to seasonal changes in temperature and precipitation in most areas of the globe, and over three or more decades these changes help determine regional climates.

Figure 7.10

The earth’s axis is tilted about  with respect to the plane of the earth’s path around the sun. The resulting variations in solar energy reaching the northern and southern hemispheres throughout a year result in seasons.

:

1. How might your life be different if the earth’s axis was not tilted?

The fourth major climatic factor is the rotation of the earth on its axis. As the earth rotates to the east (to the right, looking at Figure 7.9), the equator spins faster than the regions to its north and south. This means that air masses moving to the north or south from the equator are deflected to the east, because they are also moving east faster than the land below them. This deflection is known as the Coriolis Effect.

Some of this high, moving mass of warm air cools as it flows northeast or southeast from the equator. It becomes more dense and heavier and sinks toward the earth’s surface at about  north and  south (Figure 7.9). Because it is part of a convection cell (Figure 7.8), it starts flowing back toward the equator in what is known as a Hadley cell. Because of the Coriolis Effect, this air moving toward the equator curls in a westerly direction. In the northern hemisphere, it flows southwest from northeast. In the southern hemisphere, it flows northwest from southeast.

These air flows or winds are known as the northeast trade winds (north of the equator) and the southeast trade winds (south of the equator). They were named long ago when sailing ships used them to move goods in trade between the continents. They are examples of prevailing winds—major surface winds that blow almost continuously.

The warm air that does not descend in the Hadley cells at  north and  south continues moving toward the poles and curving to the east due to the Coriolis Effect. These prevailing winds that blow generally from the west in temperate regions of the globe are known as westerlies (Figure 7.9, left).

This complex movement of air results in six huge regions between the equator and the poles in which warm air rises and cools, then falls and heats up again in great rolling patterns (Figure 7.9, right). The two nearest the equator are the Hadley cells. These convection cells and the resulting prevailing winds distribute heat and moisture over the earth’s surface, thus helping to determine regional climates.

A fifth major factor determining regional climates is ocean currents (

Figure 7.7). They help to redistribute heat from the sun, thereby influencing climate and vegetation, especially near coastal areas. This solar heat, along with differences in water density (mass per unit volume), creates warm and cold ocean currents. They are driven by prevailing winds and the earth’s rotation (the Coriolis Effect), and continental coastlines that change their directions. As a result, between the continents, ocean currents flow in roughly circular patterns, called 

gyres

, which move clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

Water also moves vertically in the oceans as denser water sinks while less dense water rises. This creates a connected loop of deep and shallow ocean currents (which are separate from those shown in Figure 7.7). This loop acts somewhat like a giant conveyer belt that moves heat from the surface to the deep sea and transfers warm and cold water between the tropics and the poles (

Figure 7.11

).

Figure 7.11

A connected loop of deep and shallow ocean currents transports warm and cool water to various parts of the earth.

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7.2bGreenhouse Gases and Climate

Gases in the lower atmosphere affect its temperature and thus the earth’s climates. As energy flows from the sun to the earth, some of it is reflected by the earth’s surface back into the atmosphere. Molecules of certain gases in the atmosphere, including water vapor , carbon dioxide , methane , and nitrous oxide , absorb some of this solar energy and release a portion of it as infrared radiation (heat) that warms the lower atmosphere and the earth’s surface. These gases, called 

greenhouse gases

, play a role in determining the lower atmosphere’s average temperatures and thus the earth’s climates.

The earth’s surface also absorbs much of the solar energy that strikes it and transforms it into longer-wavelength infrared radiation, which then rises into the lower atmosphere. Some of this heat escapes into space, but some is absorbed by molecules of greenhouse gases and emitted into the lower atmosphere as even longer-wavelength infrared radiation (see 

Figure 2.12

). Some of this released energy radiates into space, and some adds to the warming of the lower atmosphere and the earth’s surface. This natural warming of the troposphere, called the greenhouse effect (see 

Figure 3.3

). Without this natural warming effect, the earth would be a very cold and mostly lifeless planet with an average temperature of near  instead of a much warmer .

Human activities such as the production and burning of fossil fuels, clearing of forests, and growing of crops release large quantities of the greenhouse gases carbon dioxide, methane, and nitrous oxide into the atmosphere. An enormous body of scientific evidence, combined with climate model projections, indicates that human activities are emitting greenhouse gases into the atmosphere faster than natural processes such as the earth’s carbon and nitrogen cycles (see 

Figures 3.20

, and 

3.21

) can remove them.

Climate research and climate models indicate that these emissions have played a key role in warming the earth over the last 50 years and thus helping change its climate. In other words, human activities are enhancing the earth’s natural greenhouse effect, warming the lower atmosphere and the earth’s surface by altering the amount of the sun’s heat that flows back into space. If the earth’s average atmospheric temperature continues to rise as projected, this will alter temperature and precipitation patterns, raise average sea levels, and shift areas where we can grow crops and where many types of plants and animals (including humans) can live. We discuss this issue more fully in 

Chapter 19

.

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7.2cThe Earth’s Surface Features Affect Local Climates

Various topographic features of the earth’s surface can create local climatic conditions that differ from the general climate in some regions. For example, mountains interrupt the flow of prevailing surface winds and the movement of storms. When moist air from an ocean blows inland and reaches a mountain range, it is forced upward. As the air rises, it cools, expands, and loses most of its moisture as rain and snow that fall on the windward slope of the mountain.

As shown in 

Figure 7.12

, when the drier air mass passes over the mountaintops, it flows down the leeward slopes (facing away from the wind) and warms up. This warmer air can hold more moisture, but it typically does not release much of it. Instead, the air tends to dry out plants and soil below. This process is called the 

rain shadow effect

. Over many decades, it results in semiarid or arid conditions on the leeward side of a high mountain range. Sometimes this effect leads to the formation of deserts such as Death Valley, a part of the Mojave Desert, which lies on the leeward side of mountains in the southwest United States.

Figure 7.12

The rain shadow effect is a reduction of rainfall and loss of moisture from the landscape on the leeward side of a mountain. Warm, moist air in onshore winds loses most of its moisture as rain and snow that fall on the windward slopes of a mountain range. This leads to semiarid and arid conditions on the leeward side of the mountain range and on the land beyond.

Cities also create distinct microclimates based on their weather averaged over three decades or more. Bricks, concrete, asphalt, and other building materials absorb and hold heat, and buildings block wind. Motor vehicles and the heating and cooling systems of buildings release large quantities of heat and pollutants. As a result, cities on average tend to have more haze and smog, higher temperatures, and lower wind speeds than the surrounding countryside. These factors make cities heat islands.

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7.3Climate and Biomes

· LO 7.3ADefine biome.

· LO 7.3BExplain why biomes are not uniform with sharp boundaries such as those shown in 

Figure

7.14

.

· LO 7.3CDescribe three different types of desert biomes, three different types of grassland biomes, and three different types of forest biomes in terms of variations in annual temperature and precipitation.

· LO 7.3DExplain why the elephant is a keystone species on the African savanna.

·

LO 7.3EDescribe the important ecological roles played by mountains.

· LO 7.3FFor each major type of biome—deserts, grasslands, forests, and mountains—describe three harmful human impacts.

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7.3aClimate Affects Where Terrestrial Organisms Can Live

Differences in climate (Figure 7.7) help to explain why one area of the earth’s land surface is a desert, another a grassland, and another a forest. Different climates based on long-term average annual precipitation and temperatures, global air circulation patterns, and ocean currents, lead to the formation of tropical (hot), temperate (moderate), and polar (cold) deserts, grasslands, and forests, as summarized in 

Figure 7.13

.

Figure 7.13

Natural capital: Average precipitation and average temperature, acting together as limiting factors over a long time, help to determine the type of desert, grassland, or forest in any particular area, and thus the types of plants, animals, and decomposers found in that area (assuming it has not been disturbed by human activities).

Figure 7.14

 shows how scientists have divided the world into biomes—large terrestrial regions, each characterized by a certain type of climate and dominant forms of plant life. The variety of biomes and aquatic systems is one of the four components of the earth’s biodiversity (see Figure 4.5)—a vital part of the earth’s natural capital.

Figure 7.14

Natural capital: The earth’s major biomes result primarily from differences in climate.

By comparing Figure 7.14 with Figure 7.7, you can see how the world’s major biomes vary with climate. 

Figure 4.8

 shows how major biomes along the midsection of the United States are related to different climates.

On maps such as the one in Figure 7.14, biomes are shown with sharp boundaries, and each biome is covered with one general type of vegetation. In reality, biomes are not uniform. They consist of a variety of areas, each with somewhat different biological communities but with similarities typical of the biome. These areas occur because of the irregular distribution of the resources needed by plants and animals and because human activities have removed or altered the natural vegetation in many areas. There are also differences in vegetation along the transition zone or ecotone (see 

Biodiversity

) between any two different ecosystems or biomes.

Critical Thinking

1. Use Figure 7.7 to determine the general type of climate where you live and Figure 7.14 to determine the general type of biome that should exist where you live. Does that biome exist there? If not, how is it different?

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7.3bDesert Biomes

In a desert, annual precipitation is low and often scattered unevenly throughout the year. During the day, the baking sun warms the ground and evaporates water from plant leaves and from the soil. At night, most of the heat stored in the ground radiates quickly into the atmosphere. This explains why in a desert, you might roast during the day but shiver at night.

A combination of low rainfall and varying average temperatures over many decades creates a variety of desert types—tropical, temperate, and cold (

Figures 7.13

 and 7.14). Tropical deserts (

Figure 7.15

, top photo) such as the Sahara and the Namib of Africa are hot and dry most of the year (Figure 7.15, top graph). They have few plants and a hard, windblown surface strewn with rocks and sand.

Figure 7.15

These climate graphs track the typical variations in annual temperature (red) and precipitation (blue) in tropical, temperate, and cold deserts. Top photo: a tropical desert in Morocco. Center photo: a temperate desert in southeastern California, with saguaro cactus, a prominent species in this ecosystem. Bottom photo: a cold desert, Mongolia’s Gobi Desert.

Data Analysis:

1. Which month of the year has the highest temperature and which month has the lowest rainfall for each of the three types of deserts?

apstockphoto/ Shutterstock.com; Mike Norton/ Shutterstock.com; murosvur/ Shutterstock.com

In temperate deserts (

Figure 7.15, center photo), daytime temperatures are high in summer and low in winter and there is more precipitation than in tropical deserts (Figure 7.15, center graph). Their sparse vegetation consists mostly of widely dispersed, drought-resistant shrubs and cacti or other succulents adapted to the dry conditions and temperature variations.

In cold deserts such as the Gobi Desert in Mongolia, vegetation is sparse (Figure 7.15, bottom photo). Winters are cold, summers are warm or hot, and precipitation is low (Figure 7.15, bottom graph). In all types of deserts, plants and animals have evolved adaptations that help them to stay cool and to get enough water to survive (

Science Focus 7.1

).

Science Focus 7.1

Staying Alive in the Desert

Adaptations for survival in the desert have two themes: beat the heat and every drop of water counts.

Desert plants have evolved a number of strategies based on such adaptations. During long hot and dry spells, plants such as mesquite and creosote drop their leaves to survive in a dormant state. Succulent (fleshy) plants such as the saguaro (“sah-WAH-ro”) cactus (

Figure 7.A

 and Figure 7.15, middle photo) have no leaves that can lose water to the atmosphere through transpiration. They reduce water loss by opening their pores only at night to take up carbon dioxide . Succulents also store water and synthesize food in their expandable, fleshy tissue. The spines of these and many other desert plants guard them from being eaten by herbivores seeking the precious water they hold.

Figure 7.A

After a brief rain, these wildflowers bloomed in this temperate desert in Picacho Peak State Park in the U.S. state of Arizona.

Anton Foltin/ Shutterstock.com

Some desert plants use deep roots to tap into groundwater. Others such as prickly pear and saguaro cacti use widely spread shallow roots to collect water after brief showers and store it in their spongy tissues.

Other desert plants have wax-coated leaves that reduce water loss. Annual wildflowers and grasses store much of their biomass in seeds that remain inactive, sometimes for years, until they receive enough water to germinate. Shortly after a rain, these seeds germinate, grow, and carpet some deserts with dazzling arrays of colorful flowers (

Figure 7.A) that last several weeks.

Most desert animals are small. Some beat the heat by hiding in cool burrows or rocky crevices by day and coming out at night or in the early morning when it is cooler. Others become dormant during periods of extreme heat or drought. Some larger animals such as camels can drink massive quantities of water when it is available and store it in their fat for use as needed. In addition, the camel’s thick fur helps it keep cool because the air spaces in the fur insulate the camel’s skin against the outside heat. In addition, camels do not sweat, which reduces their water loss through evaporation. Kangaroo rats never drink water. They get the water they need by breaking down fats in seeds that they consume.

Insects and reptiles such as rattlesnakes have thick outer coverings to minimize water loss through evaporation, and their wastes are dry feces and a dried concentrate of urine. Many spiders and insects get their water from dew or from the food they eat.

Critical Thinking

1. What are three steps you would take to survive in the open desert if you had to?

Desert ecosystems are vulnerable to disruption because they have slow plant growth, low species diversity, slow nutrient cycling due to low bacterial activity in the soils, and little water. It can take decades to centuries for desert soils to recover from disturbances such as off-road vehicle traffic, which can also destroy the habitats for a variety of animal species that live underground. The lack of vegetation, especially in tropical and polar deserts, also makes them vulnerable to heavy wind erosion from sandstorms.

Learning from Nature

The shell of the African Namib Desert beetle collects drinking water on water-repellent ridges on its shell, which channel water droplets condensed from fog to the beetle’s mouth. Researchers have mimicked this design in devices that harvest water from fog for use in cooling devices or as drinking water in dry areas.

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7.3cGrassland Biomes

Grasslands occur primarily in the interiors of continents in areas that are too moist for deserts to form and too dry for forests to grow (Figures 7.13 and 7.14). Grasslands persist because of a combination of seasonal drought, grazing by large herbivores, and occasional fires—all of which keep shrubs and trees from growing in large numbers. Fires and droughts are common in grasslands.

The three main types of grassland—tropical, temperate, and cold (arctic tundra)—result from long-term combinations of low average precipitation and varying average temperatures (

Figure 7.16

). One type of tropical grassland is savanna (Core Case Study and 

Science Focus 7.2

). It contains widely scattered clumps of trees and usually has warm temperatures year-round with alternating dry and wet seasons (Figure 7.16, top graph).

Figure 7.16

These climate graphs track the typical variations in annual temperature (red) and precipitation (blue) in tropical, temperate, and cold (arctic tundra) grasslands. Top photo: savanna (tropical grassland) in Kenya, Africa, with zebras grazing (Core Case Study). Center photo: prairie (temperate grassland) in the U.S. state of Illinois. Bottom photo: arctic tundra (cold grassland) in Iceland in fall.

Data Analysis:

1. Which month of the year has the highest temperature and which month has the lowest rainfall for each of the three types of grassland?

erichon/ Shutterstock.com; biletskiy/ Shutterstock.com; Fexel/ Shutterstock.com

Science Focus 7.2

Revisiting the Savanna: Elephants as a Keystone Species

As in all biomes, the African savanna (

Core Case Study) has food webs, (

Figure 7.B

). Its food webs often include one or more keystone species that play a major role in maintaining the structure and functioning of the ecosystem.

Figure 7.B

A savanna food web.

Ecologists view elephants as a keystone species in the African savanna. They eat woody shrubs and young trees. This helps keep the savanna from being overgrown by these woody plants and prevents the grasses, which form the foundation of the food web, from dying out. If this were to happen, antelopes, zebras, and other grass-eaters would leave the savanna in search of food and with them would go the carnivores such as lions and hyenas that feast on these grass-eaters. Elephants also dig for water during drought periods, creating or enlarging waterholes that are used by other animals. Without African elephants, savanna food webs would collapse and the savanna would become shrubland.

Conservation scientists classify the African elephant as vulnerable to extinction. In 1979, there were an estimated 1.3 million wild African elephants. Today, an estimated 415,000 remain in the wild, according to the World Wildlife Fund. This sharp decline is due mostly to the illegal killing of elephants for their valuable ivory tusks (Figure 7.B, left). Since 1990, there has been an international ban on the sale of ivory, and in some areas, elephants are protected as threatened or endangered species but the illegal killing of elephants for their valuable ivory continues (

Figure 7.C

, right).

Figure 7.C

One reason African elephants are being threatened with extinction is their financially valuable ivory tusks.

Pearl Media/ Shutterstock.com; Martin Harvey/Alamy Stock Photo

Another major threat to elephants is the loss and fragmentation of their habitats as human populations have expanded and taken over more land. In 2018, less than 20% of African elephant habitat was formally protected. Elephants are eating or trampling the crops of settlers who have moved into elephant habitat areas, and this has led to the killing of some elephants by farmers. If these multiple threats are not curtailed, elephants may disappear from the African savanna within your lifetime.

Critical Thinking

1. Do you think African governments would be justified in setting aside large areas of elephant habitat and prohibiting development there? Why or why not?

Tropical savannas in East Africa (

Core Case Study) are home to grazing (primarily grass-eating) and browsing (twig- and leaf-nibbling) hoofed animals, including wildebeests, gazelles, zebras, giraffes, and antelopes, as well as their predators such as lions, hyenas, and humans. Elephants eat a variety of foods including grass, tree leaves, tree bark, twigs, and shrubs and serve as keystone species (Science Focus 7.2). Herds of grazing and browsing animals migrate across the tropical savannas of East Africa to find water and food in response to seasonal and year-to-year variations in rainfall (Figure 7.16, blue areas in top graph) and food availability. Savanna plants, like those in deserts, are adapted to survive drought and extreme heat. Many have deep roots that tap into groundwater.

Connections

Savanna Grassland Niches and Feeding Habits

As an example of differing niches, some large herbivores have evolved specialized eating habits that minimize competition among species for the vegetation found on the savanna (Core Case Study). For example, giraffes eat leaves and shoots from the tops of trees, elephants eat leaves and branches farther down, wildebeests prefer short grasses, and zebras graze on longer grasses and stems.

In a temperate grassland, winters can be bitterly cold, summers are hot and dry, and annual precipitation is sparse and falls unevenly throughout the year (Figure 7.16, center graph). Because the aboveground parts of most of the grasses die and decompose each year, organic matter accumulates to produce deep, fertile topsoil (

Figure 3.10

). This topsoil is held in place by a thick network of the grasses’ intertwined roots. However, if the topsoil is plowed, it can be blown away by high winds. This biome’s grasses are adapted to droughts and to fires that burn the plant parts above the ground but do not harm the roots, from which new grass can grow.

In the midwestern and western areas of the United States, we find two types of temperate grasslands depending primarily on average rainfall: short-grass prairies (Figure 7.16, center photo) and the tallgrass prairies (which get more rain). In all prairies, winds blow almost continuously and evaporation is rapid, often leading to fires in the summer and fall. This combination of winds and fires helps to maintain such grasslands by hindering tree growth. Many of the world’s natural temperate grasslands have been converted to farmland, because their fertile soils are useful for growing crops (

Figure 7.17

) and grazing cattle.

Figure 7.17

Natural capital degradation: This intensively cultivated cropland is an example of the replacement of biologically diverse temperate grasslands (such as in the center photo of Figure 7.16) with a monoculture crop.

EPA Documerica/National Archives and Records Administration

Learning from Nature

Scientists at The Land Institute have been using the prairie as a model for more sustainable agriculture based on the use of deep-rooted perennial plants that need little or no fertilizer, pesticides, or irrigation. They can provide economical yields of grains while preserving water and soil quality.

Cold grasslands, or arctic tundra, lie south of the arctic polar ice cap (Figure 7.14). During most of the year, these treeless plains are bitterly cold (Figure 7.16, bottom graph), swept by frigid winds, and covered with ice and snow. Winters are long with few hours of daylight, and the scant precipitation falls primarily as snow.

A thick, spongy mat of low-growing plants lies under the snow. Trees and tall plants cannot survive in the cold and windy tundra because they would lose too much of their heat. Most of the annual growth of the tundra’s plants occurs during the short 7- to 8-week summer, when there is daylight almost around the clock.

One outcome of the extreme cold is the formation of 

permafrost

, underground soil in which captured water stays frozen for more than 2 consecutive years. During the brief summer, the permafrost layer keeps melted snow and ice from draining into the ground. Thus, shallow lakes, marshes, bogs, ponds, and other seasonal wetlands form when snow and frozen surface soil melt. Hordes of mosquitoes, black flies, and other insects thrive in these shallow surface pools. They serve as food for large colonies of migratory birds (especially waterfowl) that migrate from the south to nest and breed in the tundra’s summer bogs and ponds.

Animals in this biome survive the intense winter cold through adaptations such as thick coats of fur (arctic wolf, arctic fox, and musk oxen) or feathers (snowy owl) and living underground (arctic lemming). In the summer, caribou (often called reindeer) and other types of deer migrate to the tundra to graze on its vegetation.

Tundra is vulnerable to disruption. Because of the short growing season, tundra soil and vegetation recover very slowly from damage or disturbance. Human activities in the arctic tundra—primarily on and around oil and natural gas drilling sites, pipelines, mines, and military bases—leave scars that persist for centuries.

Another type of tundra, called alpine tundra, occurs above the limit of tree growth but below the permanent snow line on high mountains. The vegetation is similar to that found in arctic tundra, but it receives more sunlight than arctic vegetation gets. During the brief summer, alpine tundra can be covered with an array of beautiful wildflowers.

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7.3dChaparral—a Dry Temperate Biome

In many coastal regions that border on deserts, we find a biome known as temperate shrubland or chaparral. Because it is close to the sea, it has a slightly longer winter rainy season than the bordering desert has and experiences fogs during the spring and fall seasons. Chaparral is found along coastal areas of southern California, the Mediterranean Sea, central Chile, southern Australia, and southwestern South Africa.

This biome consists mostly of dense growths of low-growing evergreen shrubs and occasional small trees with leathery leaves (

Figure 7.18

). Its animal species include mule deer, chipmunks, jackrabbits, lizards, and a variety of birds. The soil is thin and not very fertile. During the long, hot, and dry summers, chaparral vegetation dries out. In the late summer and fall, fires started by lightning or human activities spread swiftly.

Figure 7.18

Lowland chaparral in the Rio Grande River Valley, New Mexico (USA).

Gerry Ellis/Minden Pictures/Superstock

Research reveals that chaparral is adapted to and maintained by occasional fires. Many of the shrubs store food reserves in their fire-resistant roots and have seeds that sprout only after a hot fire. With the first rain, annual grasses and wildflowers spring up and use nutrients released by the fire. New shrubs grow quickly and crowd out the grasses.

People like living in this biome because of its moderate, sunny climate. As a result, humans have moved in and modified this biome so much that little natural chaparral exists. The downside is that people living in chaparral assume the high risk of frequent fires, which are often followed by flooding during winter rainy seasons. When heavy rains come, torrents of water pour off the unprotected burned hillsides to flood lowland areas, often causing mudslides.

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7.3eForest Biomes

Forests are lands that are dominated by trees. The three main types of forest—tropical, temperate, and cold (northern coniferous, or boreal)—result from combinations of varying precipitation levels and temperatures averaged over three decades or longer (Figures 7.13 and 7.14).

Tropical rain forests (

Figure 7.19

, top photo) are found near the equator (Figure 7.14), where hot, moisture-laden air rises and dumps its moisture (Figure 7.9). These lush forests have year-round, warm temperatures, high humidity, and almost daily heavy rainfall (Figure 7.19, top graph). This almost constant warm and wet climate is ideal for a wide variety of plants and animals.

Figure 7.19

These climate graphs track the typical variations in annual temperature (red) and precipitation (blue) in tropical, temperate, and cold (northern coniferous, or boreal) forests. Top photo: the closed canopy of a tropical rain forest in Costa Rica. Middle photo: a temperate deciduous forest in autumn. Bottom photo: a northern coniferous forest in Canada.

Data Analysis:

1. Which month of the year has the highest temperature and which month has the lowest rainfall for each of the three types of forest?

Antonio Jorge Nunes/ Shutterstock.com; Marc von Hacht/ Shutterstock.com; Polina Katritch/ Shutterstock.com

Broadleaf evergreen plants that keep most of their leaves year-round dominate tropical rain forests. The tops of the trees form a dense canopy (

Figure 7.19, top photo) that blocks most light from reaching the forest floor. Many of the relatively few plants that live at the ground level have enormous leaves to capture what little sunlight filters down to them.

Some trees are draped with vines (called lianas) that reach for the treetops to gain access to sunlight. In the canopy, the vines grow from one tree to another, providing walkways for many species living there. When a large tree is cut down, its network of lianas can pull down other trees.

Tropical rain forests have a high net primary productivity (see 

Figure 3.18

). They are teeming with life and possess incredible biological diversity. Although tropical rain forests cover only about 2% of the earth’s land surface, ecologists estimate that they contain at least 50% of the known terrestrial plant and animal species. A single tree in these forests may support several thousand different insect species. Plants from tropical rain forests are a source of a variety of chemicals, many of which have been used as blueprints for making most of the world’s prescription drugs.

Rain forest species occupy a variety of specialized niches in distinct layers, which contribute to their high species diversity. Vegetation layers are structured, for the most part, according to the plants’ needs for sunlight, as shown in 

Figure 7.20

. Much of the animal life, particularly insects, bats, and birds, lives in the sunny canopy layer, with its abundant shelter and supplies of leaves, flowers, and fruits.

Figure 7.20

Specialized plant and animal niches are stratified, or arranged roughly in layers, in a tropical rain forest. Filling such specialized niches enables many species to avoid or minimize competition for resources and results in the coexistence of a great variety of species.

Dropped leaves, fallen trees, and dead animals decompose quickly in tropical rain forests because of the warm, moist conditions and the hordes of decomposers. About 90% of the nutrients released by this rapid decomposition are quickly taken up and stored by trees, vines, and other plants. Nutrients that are not taken up are soon leached from the thin topsoil by the frequent rainfall and little plant litter builds up on the ground. The resulting lack of fertile soil (see 

Figure 3.11

) helps explain why rain forests are not good places to clear and grow crops or graze cattle on a sustainable basis.

At least half of the world’s tropical rain forests have been destroyed or disturbed by human activities such as farming, and the pace of this destruction and degradation is increasing (see 

Chapter 3

 Core Case Study). Ecologists warn that without strong protective measures, most of these forests, along with their rich species biodiversity and highly valuable ecosystem services, could be gone by the end of this century.

The second major type of forest is the temperate forest, the most common of which is the temperate deciduous forest (Figure 7.19, middle photo). Such forests typically have warm summers, cold winters, and abundant precipitation—rain in summer and snow in winter months (Figure 7.19, middle graph). They are dominated by a few species of broadleaf deciduous trees such as oak, hickory, maple, aspen, and birch. Animal species living in these forests include predators such as wolves, foxes, and wildcats. They feed on herbivores such as white-tailed deer, squirrels, rabbits, and mice. Warblers, robins, and other bird species live in these forests during the spring and summer, mating and raising their young.

In these forests, most of the trees’ leaves, after developing their vibrant colors in the fall (Figure 7.19, center photo), drop off the trees. This allows the trees to survive the cold winters by becoming dormant. Each spring, the trees sprout new leaves and spend their summers growing and producing until the cold weather returns.

Because they have cooler temperatures and fewer decomposers than tropical forests have, temperate forests also have a slower rate of decomposition. As a result, they accumulate a thick layer of slowly decaying leaf litter, which becomes a storehouse of soil nutrients (Figure 3.10).

On a global basis, temperate forests have been degraded by various human activities, especially logging and urban expansion, more than any other terrestrial biome. However, within 100 to 200 years, forests of this type that have been cleared can return through secondary ecological succession (see 

Figure 5.13

).

Another type of temperate forest, the coastal coniferous forests or temperate rain forests (

Figure 7.21

), is found in scattered coastal temperate areas with ample rainfall and moisture from dense ocean fogs. These forests contain thick stands of large cone bearing, or coniferous, trees that keep most of their leaves (or needles) year-round. Most of these species have small, needle-shaped, wax-coated leaves that can withstand the intense cold and drought of winter. Examples are Sitka spruce, Douglas fir, giant sequoia, and redwoods that once dominated undisturbed areas of these biomes along the coast of North America, from Canada to Northern California in the United States.

Figure 7.21

Temperate rain forest in Olympic National Park in the U.S. state of Washington.

CrackerClips Stock Media/ Shutterstock.com

In this biome, the ocean moderates the temperature so winters are mild and summers are cool. The trees in these moist forests depend on frequent rains and moisture from summer fogs. Most of the trees are evergreen because the abundance of water means that they have no need to shed their leaves. Tree trunks and the ground are frequently covered with mosses and ferns in this cool and moist environment. As in tropical rain forests, little light reaches the forest floor.

Many of the redwood, Douglas fir, and western cedar forests have been cleared for their valuable timber and there is constant pressure to cut what remains. This threatens species such as the spotted owl and marbled murrelet that depend on these ecosystems. Clear-cutting also loads streams in these ecosystems with eroded sediment and threatens species such as salmon that depend on clear streams for laying their eggs.

Cold, or northern coniferous forests (

Figure 7.19, bottom photo), also called boreal forests or taigas are found south of arctic tundra (Figure 7.14). In this subarctic, moist climate, winters are long and extremely cold, with winter sunlight available only 6 to 8 hours per day. Summers are short, with cool to warm temperatures (Figure 7.19, bottom graph), and the sun shines as long as 19 hours a day during mid-summer.

Most boreal forests are dominated by a few species of coniferous evergreen trees or conifers such as spruce, fir, cedar, hemlock, and pine. They produce their seeds in cones. Plant diversity is low because few species can survive the winters when soil moisture is frozen.

Beneath the stands of trees in these forests is a deep layer of partially decomposed conifer needles. Decomposition is slow because of low temperatures, the waxy coating on the needles, and high soil acidity. The decomposing conifer needles make the thin, nutrient-poor topsoil acidic (Figure 3.11), which prevents most other plants (except certain shrubs) from growing on the forest floor.

Year-round wildlife in this biome includes bears, wolves, moose, lynx, and many burrowing rodent species. Caribou spend winter in the taiga and summer in the arctic tundra (Figure 7.16, bottom). During the brief summer, warblers and other insect-eating birds feed on flies, mosquitoes, and caterpillars.

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7.3fMountains Play Important Ecological Roles

Some of the world’s most spectacular environments are high on mountains (

Figure 7.22

), steep or high-elevation lands that cover about one-fourth of the earth’s land surface.

Figure 7.22

Mountains, such as these surrounding a lake, provide vital ecosystem services.

Dmitry Pichugin/ Shutterstock.com

Mountains are places where dramatic changes take place over very short distances. In fact, climate and vegetation vary according to elevation, or height above sea level, just as they do with latitude (

Figure 7.23

). If you climb a tall mountain, from its base to its summit, you can observe changes in plant life similar to those you would encounter in traveling from a temperate region to the earth’s northern polar region.

Figure 7.23

Climate and vegetation represented by different biomes change with elevation as well as with latitude.

About 1.2 billion people (16% of the world’s population) live in mountain ranges or in their foothills, and 4 billion people (53% of the world’s population) depend on mountain systems for all or some of their water. The soil on the steep slopes of mountains erodes easily when the vegetation holding them in place is removed by natural disturbances such as landslides and avalanches or by human activities such as timber cutting and agriculture. Many mountains are islands of biodiversity surrounded by a sea of lower-elevation landscapes transformed by human activities.

Mountains play an important ecological role. They contain a large portion of the world’s forests, which are habitats for much of the planet’s terrestrial biodiversity. They often are habitats for endemic species—those that are found nowhere else on earth. They also serve as sanctuaries for animals that are capable of migrating to higher altitudes and surviving in such environments. Every year, more of these animals are driven from lowland areas to mountain habitats by human activities and by the warming climate.

Connections

Mountains and Climate

Mountains help regulate the earth’s climate. Many mountaintops are covered with glacial ice and snow that reflect some solar radiation back into space, which helps to cool the earth. However, many mountain glaciers are melting, primarily because the atmosphere has warmed over recent decades. Whereas glaciers reflect solar energy, the darker rocks exposed by melting glaciers absorb that energy. This helps to warm the atmosphere above them, which melts more ice and warms the atmosphere more—in an escalating positive feedback loop.

Mountains also play a critical role in the hydrologic cycle (see 

Figure 3.19

) by serving as major storehouses of water. During winter, precipitation is stored as ice and snow. In the warmer weather of spring and summer, much of this snow and ice melts, releasing water to streams for use by wildlife and by humans for drinking and for irrigating crops. Because the atmosphere has warmed over the past 40 years, some mountaintop snow packs and glaciers have been melting earlier in the spring each year. This has led to lower food production in certain areas, because much of the water needed throughout the summer to irrigate crops is released too quickly and too early in the season.

Scientific measurements and climate models indicate that a large number of the world’s mountaintop glaciers may disappear during this century if the atmosphere keeps getting warmer as projected. This could force many people to move from their homelands in search of new water supplies and places to grow their crops.

Despite the ecological, economic, and cultural importance of mountain ecosystems, governments and many environmental organizations have not focused on protecting these areas. However, conservationist, mountain explorer, and National Geographic Explorer Gregg Treinish is trying to change this. He founded the nonprofit Adventure Scientists. It connects outdoor adventurers who are able to collect data during their travels with researchers who are focused on environmental issues, such as identifying the effects of climate change on mountain ecosystems. Treinish, who was National Geographic Adventurer of the Year in 2008–2009, has also led his own expeditions to many of the world’s rugged mountain regions.

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7.3gHumans Have Disturbed Much of the Earth’s Land

About 60% of the world’s major terrestrial ecosystems are being degraded or used unsustainably, as the human ecological footprint gets bigger and spreads across the globe (see 

Figure 1.9

), according to the 2005 Millennium Ecosystem Assessment and later updates of such research. 

Figure 7.24

 summarizes the most harmful human impacts on the world’s deserts, grasslands, forests, and mountains.

Figure 7.24

Human activities have had major impacts on the world’s deserts, grasslands, forests, and mountains, as summarized here.

Critical Thinking:

1. For each of these biomes, which two of the impacts listed do you think are the most harmful? Explain.

Left: somchaij/ Shutterstock.com. Left center: Orientaly/ Shutterstock.com. Right center: Timothy Epp/ Dreamstime.com. Right: Vasik Olga/ Shutterstock.com.

How long can we keep eating away at these terrestrial forms of natural capital without threatening our economies and the long-term survival of our own and many other species? No one knows, but there are increasing signs that we need to come to grips with this vital issue.

Many environmental scientists call for a global effort to better understand the nature and state of the world’s major terrestrial ecosystems and biomes and to use such scientific data to protect the world’s remaining wild areas from harmful forms of development. In addition, they call for restoration of many of the land areas that have been degraded, especially in areas that are rich in biodiversity. However, such efforts are highly controversial because of the timber, mineral, fossil fuel, and other resources found on or under many of the earth’s remaining wild land areas. These issues are discussed in 

Chapter 10

.

Big Ideas

· Differences in climate, based mostly on long-term differences in average temperature and precipitation, largely determine the types and locations of the earth’s deserts, grasslands, and forests.

· The earth’s terrestrial ecosystems provide important economic and ecosystem services.

· Human activities are degrading and disrupting many of the ecosystem and economic services provided by the earth’s terrestrial ecosystems.

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Tying It All TogetherTropical African Savanna and Sustainability

Paul Banton/ Shutterstock.com

This chapter’s 

Core Case Study

 began with questions about the earth’s diversity of terrestrial ecosystems and how they form. We examined the difference between weather and climate and how climate is a major factor in the formation and distribution of these biomes—the world’s deserts, grasslands, and forests—as well as the life forms that live in those systems. In particular, we focused on the savanna, a grassland biome that is threatened by expansion of the human population.

The relationships among weather, climate, and biomes and their living inhabitants are in keeping with the three scientific principles of sustainability. The earth’s dynamic climate system helps to distribute heat from solar energy and to recycle the earth’s nutrients. This in turn helps to generate and support the biodiversity found in the earth’s various biomes.

Scientists have made progress in understanding the ecology of the world’s terrestrial systems, as well as how the vital ecosystem and economic services they provide are being degraded and disrupted. One of the major lessons from their research is: in nature, everything is connected. According to these scientists, we urgently need more research on the components and workings of the world’s biomes, on how they are interconnected, and on which connections are in the greatest danger of being disrupted by human activities. With such information, we will have a clearer picture of what we can do to help sustain the natural capital on which we and all other species depend.

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Chapter Review

Critical Thinking

1. Why is the African savanna (

Core Case Study) a good example of the three scientific principles of sustainability in action? For each of these principles, give an example of how it applies to the African savanna and explain how it is being violated by human activities that now affect the savanna.

2. For each of the following, decide whether it represents a likely trend in weather or in climate:

1. an increase in the number of thunderstorms in your area from one summer to the next;

2. a decrease of 20% in the depth of a mountain snowpack between 1975 and 2018;

3. a rise in the average winter temperatures in a particular area over a decade; and

4. an increase in the earth’s average global temperature since 1980.

3. Review the five major climatic factors explained in 

Section 7.2

 and explain how each of them has helped to define the climate in the area where you live or go to school.

4. Why do deserts and arctic tundra support a much smaller number and variety of animals than do tropical forests? Why do most animals in a tropical rain forest live in its trees?

5. How might the distribution of the world’s forests, grasslands, and deserts shown in Figure 7.14 differ if the prevailing winds shown in Figure 7.9 did not exist?

6. Which biomes are best suited for

1. raising crops and

2. grazing livestock? Use the three scientific principles of sustainability to come up with three guidelines for growing crops and grazing livestock more sustainably in these biomes.

7. What type of biome do you live in? (If you live in a developed area, what type of biome was the area before it was developed?) List three ways in which your lifestyle could be contributing to the degradation of this biome. What are three lifestyle changes that you could make in order to reduce your contribution?

8. You are a defense attorney arguing in court for sparing a large area of tropical rain forest from being cut down and used to produce food. Give your three best arguments for the defense of this ecosystem.

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Chapter Review

Doing Environmental Science

1. Find a natural terrestrial ecosystem near where you live or go to school. Study and write a description of the system, including its dominant vegetation and any animal life that you are aware of. Also, note how any human disturbances have changed the system. Return to the system after a month or two and note any changes, based on your earlier notes. Compare your notes with those of your classmates.

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Chapter Review

Data Analysis

In this chapter, you learned how long-term variations in average temperatures and average precipitation play a major role in determining the types of deserts, forests, and grasslands found in different parts of the world. Below are typical annual climate graphs for a tropical grassland (savanna) in Africa (Core Case Study) and a temperate grassland in the Midwestern United States.

1. In what month (or months) does the most precipitation fall in each of these areas?

2. What are the driest months in each of these areas?

3. What is the coldest month in the tropical grassland?

4. What is the warmest month in the temperate grassland?

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Chapter Introduction

A clownfish gains protection by living among sea anemones and helps protect the anemones from some of their predators.

cbpix/

Shutterstock.com

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Core

The Southern Sea Otter: A Species in Recovery

Learning Objective

· LO 5.1Explain what could happen to the Pacific coast kelp forest ecosystem if the southern sea otters were eliminated.

Southern sea otters (

Figure 5.1

, left) live in giant kelp forests (Figure 5.1, right) in shallow waters along parts of the Pacific coast of North America. Most of the members of this endangered species are found off the California coast between the cities of Santa Cruz and Santa Barbara.

Figure 5.1

An endangered southern sea otter in Monterey Bay, California (USA) uses a stone to crack the shells of the clams that it feeds on (left). It lives in a bed of seaweed called giant kelp (right).

Left: Kirsten Wahlquist/Dreamstime.com.; Right: Paul Whitted/ Shutterstock.com.

Southern sea otters are fast and agile swimmers that dive to the ocean bottom looking for shellfish and other prey. They swim on their backs on the ocean surface and use their bellies as a table to eat their prey (

Figure 5.1, left). Each day, a sea otter consumes 20–35% of its weight in clams, mussels, crabs, sea urchins, abalone, and other species of bottom-dwelling organisms. Their thick, dense fur traps air bubbles and keeps them warm.

An estimated 16,000 southern sea otters once lived in California’s coastal waters. By the early 1900s, they had been hunted almost to extinction in this region by fur traders who killed them for their luxurious fur. Commercial fishers also killed the sea otters because they competed with them for valuable abalone and other shellfish.

The southern sea otter population grew from a low of 50 in 1938 to 1,850 in 1977 when the U.S. Fish and Wildlife listed the species as endangered. In 2018, there were 3,128 otters.

Why should we care about the southern sea otters of California? One reason is ethical:

Many

people believe it is wrong to allow human activities to cause the extinction of a species. Another reason is that people love to look at these appealing and highly intelligent animals as they play in the water. As a result, the otters help to generate millions of dollars a year in tourism revenues. A third reason—and a key reason in our study of environmental science—is that biologists classify the southern sea otter as a keystone species (see 

Chapter 4

). Scientists hypothesize that in the absence of southern sea otters, sea urchins and other kelp-eating species would probably destroy the Pacific coast kelp forests and much of the rich biodiversity they support.

Biodiversity is an important part of the earth’s natural capital and is the focus of one of the three scientific principles of sustainability. In this chapter, we look at how species interact and help control one another’s population sizes. We also explore how communities, ecosystems, and populations of species respond to changes in environmental conditions.

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5.1aCompetition for Resources

Ecologists have identified five basic types of interactions among species as they share limited resources such as food, shelter, and space. These types of interactions are called interspecific competition, predation, parasitism, mutualism, and commensalism. They each have a role in limiting the population size and resource use of the interacting species in an ecosystem.

Competition is the most common interaction among species. It occurs when members of one or more species try to use the same limited resources such as food, water, light, and space. Competition between different species is called 

interspecific competition

. It plays a larger role in most ecosystems than intraspecific competition—competition among members of the same species.

When two species compete with one another for the same resources, their ecological niches (

Figure 4.9

) overlap. The greater this overlap, the more they compete for key resources. If one species can take over the largest share of one or more key resources, each of the other competing species must move to another area (if possible), suffer a population decline, or become extinct in that area. The niches of two different species can overlap but they cannot simultaneously fully occupy the same niche, a concept called the 

competitive exclusion principle

.

Humans compete with many other species for space, food, and other resources. As our ecological footprints grow and spread, we take over or degrade the habitats of many of those species and deprive them of resources they need to survive.

Species evolve to reduce competition for resources by reducing their niche overlap. An example is 

resource partitioning

, which occurs when different species competing for similar scarce resources evolve specialized traits that allow them to “share” the same resources. Sharing resources can mean using parts of the resources, or using the resources at different times or in different ways. 

Figure 5.2

 shows resource partitioning by insect-eating bird species. Adaptations allow the birds to reduce competition by feeding in different portions of certain spruce trees and by feeding on different insect species.

Figure 5.2

Sharing the wealth: Resource partitioning among five species of insect-eating warblers in the spruce forests of the U.S. state of Maine. Each species spends at least half its feeding time in its associated yellow-highlighted areas of these spruce trees.

After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,” Ecology 36:533–536, 1958.

Another example of resource partitioning through natural selection involves birds called honeycreepers that live in the U.S. state of Hawaii (

Figure 5.3

). 

Figure 4.10

 shows how the evolution of specialized feeding niches has reduced competition for resources among bird species in a coastal wetland.

Figure 5.3

Specialist species of honeycreepers: Through natural selection, different species of honeycreepers have shared resources by evolving specialized beaks to take advantage of certain types of food such as insects, seeds, fruits, and nectar from certain flowers.

Question:

1. Look at each bird’s beak and guess what sort of food that bird might eat.

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5.1bPredation

In 

predation

, a member of one species that feeds on all or part of a member of another species is called a 

predator

, while the species that is fed upon is called the 

prey

. Together, they are engaged in a 

predator–prey relationship

 (

Figure 5.4

). Predation has a strong effect on the population sizes of the competing species.

Figure 5.4

Predator–prey relationship: This brown bear (the predator) in the U.S. state of Alaska has captured and will feed on this salmon (the prey).

Steve Hilebrand/U.S. Fish and Wildlife Service

Connections

Grizzly Bears and Moths

During the summer months, the grizzly bears of the Greater Yellowstone ecosystem in the western United States eat huge amounts of army cutworm moths, which huddle in masses high on remote mountain slopes. In this predator–prey interaction, one grizzly bear can dig out and lap up as many as 40,000 cutworm moths in a day. Consisting of 50–70% fat, the moths offer a nutrient that the bear can store in its fatty tissues and draw on during its winter hibernation.

In a giant kelp forest ecosystem, sea urchins prey on kelp, a type of seaweed (

Science Focus 5.1

). As a keystone species, southern sea otters (

Core Case Study

) prey on the sea urchins and prevent them from destroying the kelp forests. An adult southern sea otter can eat as many as 1,500 sea urchins a day.

Science Focus 5.1

Threats to Kelp Forests

A kelp forest contains large concentrations of seaweed called giant kelp. Anchored to the ocean floor, its long blades grow toward the sunlit surface waters (Figure 5.1, right). Under good conditions, the blades can grow 0.6 meter (2 feet) in a day and the plant can grow as tall as a 10-story building. The kelp blades are flexible and can survive all but the most violent storms and waves.

Kelp forests support many marine plants and animals and are one of the most biologically diverse marine ecosystems. These forests also reduce shore erosion by blunting the force of incoming waves and trapping some of the outgoing sand.

Sea urchins, such as the purple urchin (

Figure 5.A

), prey on kelp plants. Large populations of these predators can rapidly devastate a kelp forest because they eat the bases of young kelp plants. Scientific studies by biologists, including James Estes of the University of California at Santa Cruz, indicate that the southern sea otter (Core Case Study) is a keystone species that helps sustain kelp forests by controlling populations of purple and other sea urchin species.

Figure 5.A

The purple sea urchin inhabits the coastal waters of the U.S. state of California and feeds on kelp.

Kokhanchikov/ Shutterstock.com

Another threat to kelp forests is polluted water running off the land. The pollutants in runoff can include pesticides and herbicides that can kill kelp plants and other species and upset the food webs in these aquatic forests. Another runoff pollutant is fertilizer. Its plant nutrients (mostly nitrates) can cause excessive growth of algae and other aquatic plants. This growth blocks some of the sunlight needed to support the growth of giant kelp.

Some scientists warn that the warming of the world’s oceans is a growing threat to kelp forests, which require cool water. If coastal waters get warmer during this century, as projected by climate models, many or most of California’s coastal kelp forests could disappear.

Critical Thinking

1. List three ways in which we could reduce the degradation of giant kelp forest ecosystems.

Predators use a variety of ways to capture prey. Herbivores can walk, swim, or fly to the plants they feed on. Many carnivores, such as cheetahs, use their speed to chase down and kill prey, such as zebras. Eagles and hawks have keen enough eyesight to spot their prey from the air as they fly. Some predators such as female African lions work in groups to capture large or fast-running prey.

Other predators use camouflage to hide in plain sight and ambush their prey. For example, praying mantises (see 

Figure 4.4

, right sit on flowers or plants of a color similar to their own and ambush visiting insects. White ermines (a type of weasel), snowy owls, and arctic foxes (

Figure 5.5

) hunt their prey in snow-covered areas. Some predators use chemical warfare to attack their prey. For example, some spiders and poisonous snakes use venom to paralyze their prey and to defend against their predators.

Figure 5.5

A white arctic fox hunts its prey by blending into its snowy background to avoid being detected.

Paul Nicklen/National Geographic Image Collection

Prey species have evolved many ways to avoid predators. Some can run, swim, or fly fast and some have highly developed senses of sight, sound, or smell that alert them to the presence of predators. Other adaptations include protective shells (abalone and turtles), thick bark (giant sequoia trees), spines (porcupines and sea urchins), and thorns (cacti and rose bushes).

Other prey species use camouflage to blend into their surroundings. Some insect species resemble twigs (

Figure 5.6a

), or bird droppings on leaves. A leaf insect can be almost invisible against its background (

Figure 5.6b

), as can an arctic hare in its white winter fur.

Figure 5.6

These prey species have developed specialized ways to avoid their predators: (a, b) camouflage, (c, d, e) chemical warfare, (d, e, f) warning coloration, (f) mimicry, (g) deceptive looks, and (h) deceptive behavior.

Prey species also use chemical warfare. Some discourage predators by containing or emitting chemicals that are poisonous (oleander plants), irritating (stinging nettles and bombardier beetles, 

Figure 5.6c

), foul smelling (skunks and stinkbugs), or bad tasting (buttercups and monarch butterflies, 

Figure 5.6d

). When attacked, some species of squid and octopus emit clouds of black ink, allowing them to escape by confusing their predators.

Learning from Nature

Researchers have studied the bombardier beetle’s high-pressure combustion chamber in its abdomen, used to expel a poison that forces predators to vomit the beetle after eating it. Engineers hope to apply this research to industrial or medical spray technology.

Many bad-tasting, bad-smelling, toxic, or stinging prey species flash a warning coloration that eating them is risky. Examples are the brilliantly colored, foul–tasting monarch butterflies (Figure 5.6d) and poisonous frogs (

Figure 5.6e

). When a bird eats a monarch butterfly, it usually vomits and learns to avoid monarchs.

Some butterfly species gain protection by looking and acting like other, more dangerous species, a protective device known as mimicry. For example, the nonpoisonous viceroy butterfly (

Figure 5.6f

) mimics the monarch butterfly. Other prey species use behavioral strategies to avoid predation. Some attempt to scare off predators by puffing up (blowfish), spreading their wings (peacocks), or mimicking a predator (

Figure 5.6h

). Some moths have wings that look like the eyes of much larger animals (

Figure 5.6g

). Other prey species gain some protection by living in large groups such as schools of fish and herds of antelope.

Biologist Edward O. Wilson (

Individuals Matter 4.1

) proposed two criteria for evaluating the dangers posed by various brightly colored animal species. First, if they are small and strikingly beautiful, they are probably poisonous. Second, if they are strikingly beautiful and easy to catch, they are probably deadly.

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5.1cCoevolution

Over time, a prey species develops traits that make it more difficult to catch. Its predators then face selection pressures that favor traits that increase their ability to catch their prey. Then the prey species must get better at eluding the more effective predators.

This back-and-forth adaptation is called 

coevolution

, a natural selection process in which changes in the gene pool of one species lead to changes in the gene pool of another species. It can play an important role in controlling population growth of predator and prey species. When populations of two species interact as predator and prey over a long time, coevolution can help the predator succeed and it can help the prey avoid being eaten.

For example, bats prey on certain species of moths (

Figure 5.7

) that they hunt at night using echolocation. They emit pulses of high-frequency sound that bounce off their prey and capture the returning echoes that tell them where their prey is located. Over time, certain moth species have evolved ears that are sensitive to the sound frequencies that bats use to find them. When they hear these frequencies, they drop to the ground or fly evasively. Some bat species evolved ways to counter this defense by changing the frequency of their sound pulses. In turn, some moths evolved their own high-frequency clicks to jam the bats’ echolocation systems. Some bat species then adapted by turning off their echolocation systems and using the moths’ clicks to locate their prey. This is a classic example of coevolution.

Figure 5.7

Coevolution: This bat is using ultrasound to hunt a moth. As the bats evolve traits to increase their chances of getting a meal, the moths evolve traits to help them avoid being eaten.

Michael Duham/Minden Pictures/Superstock

Learning from Nature

Bats and dolphins use echolocation to navigate and locate prey in the darkness of night and in the ocean’s murky water. Scientists are studying how they do this to improve our sonar systems, sonic imaging tools for detecting underground mineral deposits, and medical ultrasound imaging systems.

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5.1dParasitism, Mutualism, and Commensalism

Parasitism

 occurs when one species (the parasite) lives in or on another organism (the host). The parasite benefits by extracting nutrients from its host. The parasite weakens its host but rarely kills it, since doing so eliminates the source of its benefits. Parasites can be plants, animals, or microorganisms.

Tapeworms are parasites that live part of their life cycle inside their hosts. Others such as mistletoe plants and blood-sucking sea lampreys (

Figure 5.8

) attach themselves to the outsides of their hosts and suck nutrients from them. Some parasites move from one host to another (fleas and ticks) while others (such as certain protozoa) spend their adult lives within a single host. Parasites help keep their host populations in check.

Figure 5.8

Parasitism: This blood-sucking, parasitic sea lamprey has attached itself to an adult lake trout from one of the Great Lakes of the United States and Canada.

Great Lakes Fishery Commission

In 

mutualism

, two species interact in ways that benefit both species by providing each with food, shelter, or some other resource. An example is pollination of flowering plants by species such as honeybees, hummingbirds, and butterflies that feed on the nectar of flowers. These pollinators get food in the form of nectar and spread the pollen from flower to flower, which helps the flower species produce seeds and reproduce.

Figure 5.9

 shows an example of a mutualistic relationship that combines nutrition and protection. It involves birds that ride on the backs or heads of large animals such as elephants, rhinoceroses, and impalas. The birds remove and eat parasites and pests (such as ticks and flies) from the animals’ bodies and often make noises warning the animals when predators are approaching.

Figure 5.9

Mutualism: This red-billed oxpecker feeds on parasitic ticks that infest animals such as this impala and warns of approaching predators.

Uwe Bergwitz/ Shutterstock.com

Another example of mutualism involves clownfish, which usually live within sea anemones (see chapter-opening photo), whose tentacles sting and paralyze most fish that touch them. The clownfish, which are not harmed by the tentacles, gain protection from predators and feed on the waste matter left from the anemones’ meals. The sea anemones benefit because the clownfish protect them from some of their predators and parasites.

Mutualism might appear to be a form of cooperation between species. However, each species is acting for its own survival.

Commensalism

 is an interaction between two species in which one species benefits and the other species is unaffected. For example, plants called epiphytes (air plants), attach themselves to the trunks or branches of trees (

Figure 5.10

) in tropical and subtropical forests. The epiphytes gain better access to sunlight, water from the humid air and rain, and nutrients falling from the tree’s upper leaves and limbs. Their presence apparently does not harm the tree. Similarly, birds benefit by nesting in trees, generally without harming them.

Figure 5.10

Commensalism: This pitcher plant is attached to a branch of a tree without penetrating or harming the tree. This carnivorous plant feeds on insects that become trapped inside it.

all_about_people/ Shutterstock.com

Succession

Watch this animation to see the difference between primary and secondary succession.

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5.2aEcological Succession

The types and numbers of species in biological communities and ecosystems change in response to changing environmental conditions. The gradual change in species composition in a given community or ecosystem is called 

ecological succession

.

There are two major types of ecological succession, depending on the conditions present at the beginning of the process. 

Primary ecological succession

 involves the gradual establishment of communities of different species in lifeless areas—in terrestrial systems with no soil or in aquatic systems with no bottom sediments. Examples include bare rock exposed by a retreating glacier (

Figure 5.11

), an abandoned highway or parking lot, and a newly created shallow pond or lake (

Figure 5.12

). Primary succession can take hundreds to thousands of years because of the need to build up fertile soil or aquatic sediments to provide the nutrients needed to establish a community of producers.

Figure 5.11

Primary ecological succession: Over almost a thousand years, these plant communities developed, starting on bare rock exposed by a retreating glacier on Isle Royal, Michigan (USA), in western Lake Superior. The details of this process vary from one site to another.

Figure 5.12

Primary ecological succession in a lake basin in which sediments and plants have been gouged out by a glacier. When the glacier melts, the lake basin begins accumulating sediments and plant and animal life. Over hundreds to thousands of years, the lake can fill with sediments and become a terrestrial habitat.

Pioneer species

 are the first species to occupy the barren environment and are often carried there by wind or water. Common pioneer species are mosses and lichens (Figure 5.12) because they can grow on rock. They spread and break the rock into pieces that start the long soil formation process. When they die and decompose, they provide nutrients for the thin soil layer.

The other, more common type of ecological succession is called 

secondary ecological succession

, in which a community or ecosystem develops on the site of an existing community or ecosystem and replaces or adds to the existing set of resident species. This type of succession begins in an area where an ecosystem has been disturbed, removed, or destroyed, but where some soil or bottom sediment remains. Candidates for secondary succession include abandoned farmland (

Figure

5.13

), burned or cut forests, heavily polluted streams, and flooded land. Because some soil or sediment is present, new vegetation can begin to grow, usually within a few weeks. On land, growth begins with the germination of seeds already in the soil and seeds imported by wind or in the droppings of birds and other animals.

Figure 5.13

Secondary ecological succession: Natural restoration of disturbed land on an abandoned farm field in the U.S. state of North Carolina. It took 150 to 200 years after the farmland was abandoned for the area to become covered with a mature oak and hickory forest. Primary and secondary ecological succession are examples of natural ecological restoration.

Succession

Watch this animation to see the difference between primary and secondary succession.
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Ecological succession is an important ecosystem service that tends to enrich the biodiversity of communities and ecosystems by increasing species diversity and interactions among species. Such interactions enhance an ecosystem’s sustainability by promoting population control and by increasing the complexity of food webs, which enhances energy flow and nutrient cycling.

Ecologists have identified three factors that affect how and at what rate ecological succession occurs. One is facilitation, in which one set of species makes an area suitable for species with different niche requirements, and often less suitable for itself. For example, as lichens and mosses gradually build up soil on a rock in primary succession, herbs and grasses can move in and crowd out the lichens and mosses (

Figure 5.11).

A second factor is inhibition, in which some species hinder the establishment and growth of other species. For example, needles dropping off some pine trees make the soil beneath the trees too acidic for most other plants to grow there. A third factor is tolerance, in which plants in the late stages of succession succeed because they are not in direct competition with other plants for key resources. Shade-tolerant plants, for example, can live in shady forests because they do not need as much sunlight as the trees above them do (

Figures 5.11

 and 5.13).

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5.2bIs There a Balance of Nature?

According to the traditional view, ecological succession proceeds in an orderly sequence along an expected path until a certain stable type of climax community (Figures 5.11 and 5.13), which is assumed to be in balance with its environment, occupies an area. This equilibrium model of succession is what ecologists once meant when they talked about the balance of nature.

Over the last several decades, many ecologists have changed their views about balance and equilibrium in nature based on ecological research. There is a general tendency for succession to lead to more complex, diverse, and presumably more resilient ecosystems that can withstand changes in environmental conditions if the changes are not too large or too sudden. However, the current scientific view is that we cannot predict a given course of succession or view it as inevitable progress toward an ideally adapted climax plant community or ecosystem. Rather, ecological succession reflects the ongoing struggle by different species for enough light, water, nutrients, food, space, and other key resources. In other words, research shows that there is no balance of nature consisting of a permanent and stable state.

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5.2cLiving Systems Are Sustained through Constant Change

All living systems, from a cell to the biosphere, are constantly changing in response to changing environmental conditions. Living systems have processes that interact to provide some degree of stability, or sustainability. However, this stability, or the capacity to withstand external stress and disturbance, is maintained by constant change in response to changing environmental conditions. Nature is dynamic, not static, and is not fragile as revealed by how the earth’s life has changed and evolved for 3.8 billion years in response to drastic changes in environmental conditions.

Ecologists distinguish between two aspects of sustainability in ecosystems. 

Ecological inertia

, or 

persistence

 is the ability of an ecosystem to survive moderate disturbances. A second factor, 

resilience

, is the ability of an ecosystem to be restored through secondary ecological succession after a severe disturbance.

Evidence suggests that some ecosystems have one of these properties but not the other. However, once a large tract of tropical rain forest is cleared or severely damaged, the resilience of the degraded forest ecosystem may be so low that the degradation reaches an ecological tipping point. Once it exceeds that point, the forest might not be restored by secondary ecological succession. One reason is that most of the nutrients in a tropical rain forest are stored in its vegetation, not in the topsoil. Once the nutrient-rich vegetation is gone, frequent rains can remove most of the remaining soil nutrients and thus prevent the return of a tropical rain forest to a large cleared area.

By contrast, grasslands are much less diverse than most forests. Thus, they have low inertia and can burn easily. Because most of their plant matter is stored in underground roots, these ecosystems have high resilience and can recover quickly after a fire because their root systems produce new grasses. Grassland can be destroyed only if its roots are plowed up and something else is planted in its place, or if it is severely overgrazed by livestock or other herbivores.

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5.3aPopulations Can Grow, Shrink, or Remain Stable

A population is a group of interbreeding individuals of the same species (

Figure 5.14

). Most populations live together in clumps or groups such as packs of wolves, schools of fish (Figure 5.14), and flocks of birds. Living in groups allows them to cluster where resources are available, provides some protection from predators, and helps some predator species to find and capture prey.

Figure 5.14

A population, or school, of Big Eye Trevally Jack in Baja California Sur, Mexico.

Leonardo Gonzalez/ Shutterstock.com

 is the number of individual organisms in a population at a given time. Four variables—births, deaths, immigration, and emigration—govern changes in population size. A population increases through birth and immigration (the arrival of individuals from outside the population). Populations decrease through death and emigration (the departure of individuals from the population):

Scientists use sampling techniques to estimate the sizes of large populations of species such as oak trees that are spread over a large area and squirrels that move around and are hard to count. Typically, they count the number of individuals in one or more small sample areas and use this information to estimate the number of individuals in a larger area.

A population’s 

age structure

—its distribution of individuals among various age groups—can have a strong effect on how rapidly its numbers grow or decline. Age groups are usually described in terms of organisms not mature enough to reproduce (the pre-reproductive stage), those capable of reproduction (the reproductive stage), and those too old to reproduce (the post-reproductive stage).

The size of a population will likely increase if it is made up mostly of individuals in their reproductive stage, or soon to enter this stage. In contrast, the size of a population dominated by individuals in their post-reproductive stage will tend to decrease over time.

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5.3bSeveral Factors Can Limit Population Size

Each population in an ecosystem has a 

range of tolerance

—a range of variations in its physical and chemical environment within which it is most likely to survive. For example, a trout population (

Figure 5.15

) will thrive within a narrow band of temperatures (optimum level or range), although a few individuals can survive above and below that band (Figure 5.15). If the water becomes too hot or too cold, none of the trout can survive.

Figure 5.15

Range of tolerance for a population of trout to changes in water temperature.

Various physical or chemical factors can determine the number of organisms in a population and how fast a population grows or declines. Sometimes one or more factors, known as 

limiting factors

, are more important than other factors in regulating population growth.

Learning from Nature

Biomimicry researchers are hoping to learn how plants that have a high tolerance for salty seawater can teach us how to design better ways of providing fresh drinking water in drought-prone areas.

On land, precipitation often is the limiting factor.

Low

precipitation levels in desert ecosystems limit desert plant growth. Lack of key soil nutrients limits the growth of plants, which in turn limits populations of animals that eat plants, and animals that feed on such plant-eating animals.

Limiting physical factors for populations in aquatic systems include water temperature (Figure 5.15), depth, and clarity (allowing for more or less sunlight). Other important factors are nutrient availability, acidity, salinity, and the level of oxygen gas in the water (dissolved oxygen content).

Too much of a physical or chemical factor can also be limiting. For example, too much water or fertilizer can kill land plants. If acidity levels are too high in an aquatic environment, some of its organisms can be harmed.

An additional factor that can limit the sizes of some populations is 

population density

, the number of individuals in a population found within a defined area or volume. Density-dependent factors are variables that become more important as a population’s density increases. In a dense population, parasites and diseases can spread more easily, resulting in higher death rates, and competition for resources such as food and water can intensify. On the other hand, a higher population density can help sexually reproducing individuals to find mates more easily to produce offspring. Other factors such as drought, and climate change are considered density-independent factors, because they can affect population sizes regardless of density.

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5.3cNo Population Can Grow Indefinitely: J-Curves and S-Curves

The populations of some species, such as bacteria and many insect species, have an ability to increase their numbers exponentially. For example, with no controls on its population growth, a species of bacteria that can reproduce every 20 minutes would generate enough offspring to form a 0.3-meter-deep (1-foot-deep) layer over the surface of the entire earth in only 36 hours. Plotting such numbers against time yields a J-shaped curve of exponential growth (

Figure 5.16

, left). Members of such populations typically reproduce at an early age, have many offspring each time they reproduce, and reproduce many times with short intervals between generations.

Figure 5.16

According to this idealized mathematical model, populations of species can undergo exponential growth represented by a J-shaped curve (left) when resource supplies are plentiful. As resource supplies become limited, a population undergoes logistic growth, represented by an S-shaped curve (right), when the size of the population approaches the carrying capacity of its habitat.

However, there are always limits to population growth in nature. Research reveals that a rapidly growing population of any species eventually reaches some size limit imposed by limiting factors. These factors include sunlight, water, temperature, space, nutrients, or exposure to predators or infectious diseases. Environmental resistance is the sum of all such factors in a habitat.

Limiting factors largely determine an area’s carrying capacity, the maximum population of a given species that a particular habitat can sustain indefinitely. As a population approaches the carrying capacity of its habitat, the J-shaped curve of its exponential growth (Figure 5.16, left) is converted to an S-shaped curve of logistic growth, or growth that often fluctuates around the carrying capacity of its habitat (Figure 5.16, right).

However, the rate of population growth and the carrying capacity for a population are not fixed and can rise or fall as environmental conditions change the factors that promote and limit the population’s growth. Nature is constantly changing and is never in balance. In other words, the curve in Figure 5.16 is a simplified and idealized mathematical model of the growth rate and carrying capacity of populations in nature.

Some populations do not make a smooth transition from exponential growth to logistic growth. Instead, they use up their resource supplies and temporarily overshoot, or exceed, the carrying capacity of their environment. In such cases, the population suffers a sharp decline, called a dieback, or population crash, unless part of the population can switch to new resources or move to an area that has more resources. Such a crash occurred when reindeer were introduced onto a small island in the Bering Sea in the early 1900s (

Figure 5.17

).

Figure 5.17

Exponential growth, overshoot, and population crash of a population of reindeer introduced onto the small Bering Sea island of St. Paul in 1910.

Data Analysis:

1. By what percentage did the population of reindeer grow between 1923 and 1940?

Patterns of Population growth

Watch this animation to see what factors affect population size.

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5.3dReproductive Patterns

Species vary in their reproductive patterns. Species with a capacity for a high rate of population growth (r) (Figure 5.16, left) are called 

r-selected species

. These species tend to have short life spans and produce many, usually small offspring and give them little or no parental care. As a result, many of the offspring die at an early age. To overcome such losses, r-selected species produce large numbers of offspring so a few will likely survive and have many offspring to sustain the species. Examples of r-selected species include algae, bacteria, frogs, and most insects.

Such species tend to be opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance such as a fire or clear-cutting of a forest opens up a new habitat or niches for invasion. Once established, their populations may crash because of unfavorable changes in environmental conditions or invasion by more competitive species. This explains why most opportunist species go through irregular and unstable boom-and-bust cycles in their population sizes.

At the other extreme are 

K-selected species

. They tend to reproduce later in life, have few offspring, and have long life spans. Typically, the offspring of K-selected mammal species develop inside their mothers (where they are safe). After birth, they mature slowly and one or both parents care for and protect them. In some cases, they live in herds or groups until they reach reproductive age.

The population size of K-selected species tends to be near the carrying capacity (K) of its environment (Figure 5.16, right). Examples of K-selected species include most large mammals such as elephants, whales, and humans, birds of prey, large and long-lived plants such as the saguaro cactus, and most tropical rain forest trees. Many of these species—especially those with low reproductive rates, such as elephants, sharks, giant redwood trees, and California’s southern sea otters (Core Case Study and 

Science Focus 5.2

)—are vulnerable to extinction. Most organisms have reproductive patterns between the extremes of r-selected and K-selected species. 

Table 5.1

 compares typical traits of r-selected and K-selected species.

Table 5.1

Typical Traits of r-Selected and K-Selected Species

Many

Few

Low

High

Short

Long

Science Focus 5.2

The Future of California’s Southern Sea Otters

The population of southern sea otters (Core Case Study) has fluctuated in response to changes in environmental conditions (

Figure 5.B

). One change was a rise in populations of the orcas (killer whales) that feed on them. Scientists hypothesize that orcas started feeding more on southern sea otters when populations of their normal prey, sea lions and seals, began declining. In addition, between 2010 and 2015, the number of sea otters killed or injured by sharks increased, possibly because warmer ocean water brought some sharks closer to the shore.

Figure 5.B

Changes in the population size of southern sea otters off the coast of the U.S. state of California, 1983–2018.

(Compiled by the authors using data from U.S. Geological Survey.)

Another factor affecting sea otters may be parasites that breed in the intestines of cats. Scientists hypothesize that some southern sea otters are dying because coastal area cat owners flush feces-laden cat litter down their toilets or dump it in storm drains that empty into coastal waters. The feces contain parasites that can infect otters.

Toxic algae blooms also threaten otters. The algae thrive on urea, a nitrogen-containing ingredient in fertilizer that washes into coastal waters. Other pollutants released by human activities include PCBs and other fat-soluble toxic chemicals. These chemicals can kill otters by accumulating to high levels in the tissues of the shellfish that otters eat. Because southern sea otters feed at high trophic levels and live close to the shore, they are vulnerable to these and other pollutants in coastal waters.

Other threats to otters include oil spills from ships. The entire California southern sea otter population could be wiped out by a large oil spill from a single tanker off the central west coast or by the rupture of an offshore oil well, should drilling for oil be allowed off this coast. Some sea otters die when they are trapped in underwater nets and traps for shellfish. Others are killed by boat strikes and gunshots.

Figure 5.B shows the change in the population size of the southern sea otter since 1967, ten years before it was protected as an endangered species. In 2016, the sea otter population was 3,272 the highest it has been since 1985. In 2017, the population was 3,186 and in 2018, it was 3,128. Thus, for three years the sea otter population has averaged above 3,090, which means it can be considered for removal from the federal endangered species list. Such a delisting would be a success story for the U.S. Endangered Species Act and the otters would still be protected under a California state law.

Critical Thinking

1. How would you design a controlled experiment to test the hypothesis that cat litter flushed down toilets might be killing southern sea otters?

The reproductive pattern of a species may give it a temporary advantage. However, the key factor in determining the ultimate population size of a species is the availability of suitable habitat with adequate resources. Changes in habitat or other environmental conditions can reduce the populations of some species while increasing the populations of other species, such as white-tailed deer in the United States (see Case Study that follows).

Case Study

Exploding White-Tailed Deer Populations in the United States

By 1900, habitat destruction and uncontrolled hunting had reduced the white-tailed deer (

Figure 5.18

) population in the United States to about 500,000 animals. In the 1920s and 1930s, laws were passed to protect the remaining deer. Hunting was restricted and predators, including wolves and mountain lions that preyed on the deer, were nearly eliminated.

Figure 5.18

White-tailed deer populations in the United States have been growing.

Roy Toft/National Geographic Image Collection

These protections worked, perhaps too well for some suburbanites and farmers. Today there are over 30 million white-tailed deer in the United States. During the last 50 years, suburbs have expanded and many Americans have moved into the wooded habitat of deer. The gardens and landscaping around their homes provide deer with flowers, shrubs, garden crops, and other plants they like to eat.

Deer prefer to live in the edge areas of forests and woodlots for security and go to nearby fields, orchards, lawns, and gardens for food, so white-tailed deer populations have soared in suburban areas.

In woodlands, larger populations of the deer are consuming native ground-cover vegetation, which has allowed nonnative weed species to take over and upset ecosystem food webs. The deer also help to spread Lyme disease (carried by deer ticks) to humans. In addition, each year about 1.5 million deer–vehicle collisions injure thousands and kill more than 160 people per year, on average—the highest human death toll from encounters with any wild animal in the United States.

There are no easy solutions to the deer population problem in the suburbs. Changes in hunting regulations that allow for the killing of more female deer have cut down the overall deer population. However, this has had a limited effect on deer populations in suburban areas because it is too dangerous to allow widespread hunting with guns in such populated communities. Some areas have hired experienced and licensed archers who use bows and arrows to help reduce deer numbers, being careful not to endanger nearby residents.

Some communities spray the scent of deer predators or of rotting deer meat in edge areas to scare off deer. Others scare off deer by using electronic equipment that emits high-frequency sounds that humans cannot hear. Some homeowners surround their gardens and yards with high, black plastic mesh fencing.

Deer can be trapped and moved from one area to another, but this is expensive and must be repeated whenever they move back into an area. In addition, there are questions concerning where to move the deer and how to pay for such programs.

Darts loaded with contraceptives can be shot into female deer to hold down their birth rates, but this is expensive and must be repeated every year. Another approach is to trap dominant males and use chemical injections to sterilize them. However, this is costly and will require years of testing. In addition, ethical questions about this approach would have to be considered.

Meanwhile, suburbanites can expect deer to chow down on their shrubs, flowers, and garden plants unless they can protect their properties with fences, repellents, or other methods. Suburban dwellers could also stop planting trees, shrubs, and flowers that attract deer around their homes.

Critical Thinking

1. If the earth experiences significant warming during this century as projected, is this likely to favor r-selected or K-selected species? Explain.

Critical Thinking

1. Some people blame the white-tailed deer for invading farms and suburban yards and gardens to eat food that humans have made easily available to them. Others say humans are mostly to blame because they have invaded deer territory, eliminated most of the predators that kept deer populations under control, and provided the deer with plenty to eat in their lawns, gardens, and crop fields. Which view do you hold? Why? Do you see a solution to this problem?

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5.3eSurvivorship Curves

Individuals of species with different reproductive strategies tend to have different life expectancies. This can be illustrated by a 

survivorship curve

, which shows the percentages of the members of a population surviving at different ages. There are three generalized types of survivorship curves: late loss, early loss, and constant loss (

Figure 5.19

). A late loss population (K-selected species such as elephants and rhinoceroses) typically has high survivorship to a certain age, and then high mortality. A constant loss population (such as many songbirds) typically has a constant death rate at all ages. For an early loss population (many r-selected species and annual plants), survivorship is low early in life. These generalized survivorship curves only approximate the realities of nature.

Figure 5.19

Survivorship curves for populations of different species, obtained by showing the percentages of the members of a population surviving at different ages.

Critical Thinking:

1. Which type of survivorship curve applies to the human species?

Top: Gualtiero boffi/ Shutterstock.com. Center: IrinaK/ Shutterstock.com. Bottom: ultimathule/ Shutterstock.com.

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5.3fHumans Are Not Exempt from Nature’s Population Controls

Humans are not exempt from population crashes. In 1845, Ireland experienced such a crash after a fungus destroyed its potato crop. About 1 million people died from hunger or diseases related to malnutrition and millions more migrated to other countries, sharply reducing the Irish population.

During the 14th century, bubonic plague spread through densely populated European cities and killed at least 25 million people—one-third of the European population. The bacterium that causes this disease normally lives in rodents. It was transferred to humans by fleas that fed on infected rodents and then bit humans. The disease spread like wildfire through crowded cities, where sanitary conditions were poor and rats were abundant. Today several antibiotics can be used to treat bubonic plague.

So far, technological, social, and other cultural changes have expanded the earth’s carrying capacity for the human species. We have used large amounts of energy and matter resources to occupy formerly uninhabitable areas. We have expanded agriculture and controlled the populations of other species that compete with us for resources. Some say we can keep expanding our ecological footprint in this way indefinitely because of our technological ingenuity. Others say that at some point, we will reach the limits that nature eventually imposes on any population that exceeds or degrades its resource base. We discuss these issues in 

Chapter 6

.

Big Ideas

· Certain interactions among species affect their use of resources and their population sizes.

· The species composition and population sizes of a community or ecosystem can change in response to changing environmental conditions through a process called ecological succession.

· No population can escape natural limiting factors and grow indefinitely.

Patterns of Population growth
Watch this animation to see what factors affect population size.
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Tying It All TogetherSouthern Sea Otters and Sustainability

fred goldstein/ Shutterstock.com

The southern sea otters of California are part of a complex ecosystem made up of large underwater kelp forests, bottom-dwelling creatures, and other species that depend on one another for survival. The sea otters act as a keystone species, mostly by feeding on sea urchins and keeping them from destroying the kelp.

In this chapter, we focused on how biodiversity promotes sustainability, provides a variety of species to restore damaged ecosystems through ecological succession, and limits the sizes of populations. Populations of most plants and animals depend, directly or indirectly, on solar energy, and all populations play roles in the cycling of nutrients in the ecosystems where they live. In addition, the biodiversity in different terrestrial and aquatic ecosystems provides alternative paths for energy flow and nutrient cycling, better opportunities for natural selection as environmental conditions change, and natural population control mechanisms. When we disrupt these paths, we violate the three scientific principles of sustainability.

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Chapter Review

Critical Thinking

1. What difference would it make if the southern sea otter (

Core Case Study) became extinct primarily because of human activities? What are three things we could do to help prevent the extinction of this species?

2. Use the second law of thermodynamics (

Chapter 2

) and the concept of food chains and food webs to explain why predators are generally less abundant than their prey.

3. How would you reply to someone who argues that we should not worry about the effects that human activities have on natural systems because ecological succession will repair whatever damage we do?

4. How would you reply to someone who contends that efforts to preserve species and ecosystems are not worthwhile because nature is largely unpredictable?

5. What is the reproductive strategy of most species of insect pests and harmful bacteria? Why does this make it difficult for us to control their populations?

6. List three examples of how your life might be affected if changing environmental conditions favor r-selected species during the latter half of this century.

7. List two factors that may limit human population growth in the future. Do you think that we are close to reaching those limits? Explain.

8. If the human species were to suffer a population crash, name three species that might move in to occupy part of our ecological niche. What are three species that would likely decline as a result? Explain why these other species would decline.

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Chapter Review

Doing Environmental Science

1. Visit a nearby land area, such as a partially cleared or burned forest, grassland, or an abandoned crop field, and record signs of secondary ecological succession. Take notes on your observations and formulate a hypothesis about what sort of disturbance led to this succession. Include your thoughts about whether this disturbance was natural or caused by humans. Study the area carefully to see whether you can find patches that are at different stages of succession and record your thoughts about what sorts of disturbances have caused these differences. You might want to research the topic of ecological succession in such an area.

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Chapter Review

Data Analysis

The graph below shows changes in the size of an Emperor penguin population in terms of numbers of breeding pairs on the island of Terre Adelie in the Antarctic. Scientists used this data along with data on the penguins’ shrinking ice habitat to project a general decline in the island’s Emperor penguin population, to the point where they will be endangered in 2100. Use the graph to answer the following questions.

1. If the penguin population fluctuates around the carrying capacity, what was the approximate carrying capacity of the island for the penguin population from 1960 to 1975? What was the approximate carrying capacity of the island for the penguin population from 1980 to 2010?

2. What was the overall percentage decline in the penguin population from 1975 to 2010?

3. What is the projected overall percentage decline in the penguin population between 2010 and 2100?

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Core

Why Are Amphibians Vanishing?

Learning Objective

· LO 4.1List three reasons why we need to care about the growing rate of amphibian extinctions.

Amphibians are a class of animals that includes frogs (chapter 

opening photo

, toads, and salamanders. Amphibians were among the first vertebrates (animals with backbones) to leave the earth’s waters and live on land. They have adjusted to and survived environmental changes more effectively than many other species, but their environment is changing rapidly.

An amphibian lives part of its life in water and part on land. Human activities such as the use of pesticides and other chemicals can pollute the land and water habitats of amphibians. Many of the more than 8,000 known amphibian species (90% of them frogs) have problems adapting to these changes.

Since 1980, populations of hundreds of amphibian species have declined or vanished (

Figure 4.1

). According to the International Union for Conservation of Nature (IUCN), about 33% of known amphibian species face extinction. A 2015 study by biodiversity expert Peter Crane found that 200 frog species have gone extinct since the 1970s, and frogs are going extinct 10,000 times faster than their historical rates.

Figure 4.1

Specimens of some of the nearly 200 amphibian species that have gone extinct since the 1970s.

Joel Sartore/National Geographic Image Collection

No single cause can account for the decline of many amphibian species, but scientists have identified a number of factors that affect amphibians at various points in their life cycles. For example, frog eggs lack shells to protect the embryos they contain from water pollutants and adult frogs ingest the insecticides contained in many of the insects they eat. We explore these and other factors later in this chapter.

Why should we care if some amphibian species become extinct? Scientists give three reasons. First, amphibians are sensitive biological indicators of changes in environmental conditions. These changes include habitat loss, air and water pollution, ultraviolet (UV) radiation from the sun, and a warming climate. The growing threats to the survival of an increasing number of amphibian species indicate that environmental conditions for amphibians and many other species are deteriorating in many parts of the world.

Second, adult amphibians play important roles in biological communities. They eat more insects (including mosquitoes) than do many species of birds. In some habitats, the extinction of certain amphibian species could lead to population declines or extinction of animals that eat amphibians or their larvae, such as reptiles, birds, fish, mammals, and other amphibians.

Third, amphibians play a role in human health. A number of pharmaceutical products come from compounds found in secretions from the skin of certain amphibians. Many of these compounds have been isolated and used as painkillers and antibiotics and in treatments for burns and heart disease. If amphibians vanish, these potential medical benefits and others that scientists have not yet discovered would vanish with them.

The threat to amphibians is part of a greater threat to the earth’s biodiversity. In this chapter, we discuss biodiversity, how it arose on the earth, why it is important, and how it is threatened. We will also consider possible solutions to these threats.

4.1aEarth’s Organisms Are Many and Varied

Every organism is composed of one or more cells. Based on their cell structure, organisms can be classified as eukaryotic or prokaryotic. All organisms except bacteria are 

eukaryotic

. Their cells are encased in a membrane and have a distinct nucleus (a membrane-bounded structure containing genetic material in the form of DNA) and several other internal parts enclosed by membranes (

Figure 4.2

, right). Bacterial cells are 

prokaryotic

, enclosed by a membrane but containing no distinct nucleus or other internal parts enclosed by membranes (Figure 4.2, left).

Figure 4.2

Comparison of key components of a eukaryotic cell (left) and prokaryotic cell (right).

Scientists group organisms into categories based on their varying characteristics, a process called taxonomic classification. The largest category is the kingdom, which includes all organisms that have one or several common features. Biologists recognize six kingdoms. Two are different types of bacteria (eubacteria and archaebacteria) with single cells that are prokaryotic (Figure 4.2, left). The other four kingdoms are protists, plants, fungi, and animals (Figure 4.2 right).

Protists are mostly many-celled eukaryotic organisms such as golden brown and yellow-green algae, and protozoans. Most fungi are many-celled organisms such as mushroom, molds, mildews, and yeasts.

Plants include certain types of algae (including red, brown, and green algae), mosses, ferns, trees, and flowering plants whose flowers produce seeds. Flowering plant species make up about 90% of the plant kingdom. Some flowering plants such as corn and marigolds are 

annuals

 that live for one growing season, die, and have to be replanted. Others are 

perennials

, such as roses and grapes, which can live for two or more seasons before they die and have to be replanted.

Animals are many-celled eukaryotic organisms. Most are 

invertebrates

, with no backbones. They include jellyfish, worms, insects, shrimp, snails, clams, and octopuses. Other animals, called 

vertebrates

, have backbones. Examples include amphibians (

Core Case Study

), fishes, reptiles (alligators and snakes), birds (robins and eagles), and mammals (humans, whales, elephants, bats, and tigers).

Kingdoms are divided into phyla, which are divided into subgroups called classes. Classes are subdivided into orders, which are further divided into families. Families consist of genera (singular, genus), and each genus contains one or more species. 

Figure 4.3

 shows the detailed taxonomic classification for the current human species: Homo sapiens sapiens.

Figure 4.3

How the current human species got its name: Homo sapiens sapiens.

A species is a group of living organisms with characteristics that distinguish it from other groups of organisms. In sexually reproducing organisms, individuals must be able to mate with similar individuals and produce fertile offspring in order to be classified as a species.

Estimates of the number of species range from 7 million to 100 million, with a best guess of 7 million to 10 million species. Biologists have identified about 2 million species.

2 Million

Number of species that scientists have identified out of the world’s estimated 7 million to 100 million species

Well over half of the world’s identified species are insects that play important ecological roles in sustaining the earth’s life. For example, pollination is a vital ecosystem service that allows flowering plants to reproduce. When pollen grains are transferred from the flower of one plant to a receptive part of the flower of another plant of the same species, reproduction occurs. Many flowering species depend on bees and other insects to pollinate their flowers (

Figure 4.4

, left). In addition, insects that eat other insects—such as the praying mantis (Figure 4.4, right)—help to control the populations of at least half the species of insects that we call pests. This free pest control service is another vital ecosystem service. In addition, insects make up an increasing part of the human food supply in some parts of the world.

Figure 4.4

Importance of insects: Bees (left) and numerous other insects pollinate flowering plants that serve as food for many plant eaters, including humans. This praying mantis, which is eating a moth (right), and many other insect species help to control the populations of most of the insect species we classify as pests.

Klagyivik Viktor/ Shutterstock.com; Dr. Morley Read/ Shutterstock.com

Some insect species reproduce at an astounding rate and can rapidly develop new genetic traits such as resistance to pesticides. They also have an exceptional ability to evolve into new species when faced with changing environmental conditions.

Research indicates that some human activities are threatening insect populations such as honeybees. We discuss this environmental problem more fully in 

Chapter 9

.

Learning from Nature

Even the lowly mosquito provides benefits to humans by serving as a model for a new type of hypodermic needle, based on the mosquito’s use of a multi-part mouth to work its way through a layer of skin without creating pain. (The pain of a mosquito bite comes from a chemical injected into the skin after it has been penetrated.)

4.2aBiodiversity

Biodiversity, or 

biological diversity

, is the variety of life on the earth. It has four components, as shown in 

Figure 4.5

.

Figure 4.5

Natural capital: The major components of the earth’s biodiversity—one of the planet’s most important renewable resources and a key component of its natural capital (

Figure 1.3

).

Right side, top left: Laborant/ Shutterstock.com; right side, top right: leungchopan/ Shutterstock.com; right side, top center: Elenamiv/ Shutterstock.com; bottom right: Juriah Mosin/ Shutterstock.com.

One is 

species diversity

, the number and abundance of the different kinds of species living in an ecosystem. Species diversity has two components, one being 

species richness

, the number of different species in an ecosystem. The other is 

species evenness

, a measure of the comparative abundance of all species in an ecosystem.

A species-rich ecosystem has a large number of different species. However, this tells us nothing about how many members of each species are present. If it has many of one or more species and just a few of others, its species evenness is low. If it has roughly equal numbers of each species, its species evenness is high. For example, if an ecosystem has only three species, its species richness is low. However, if there are roughly equal numbers of each of the three species, the species evenness is high. Species-rich ecosystems such as rain forests tend to have high species evenness. Ecosystems with low species richness, such as tree farms, tend to have low species evenness.

Species diversity can enhance the stability of ecosystems. For example, a forest with many different tree species is more stable than a forest with just one tree species, which is the case with a tree farm.

The species diversity of ecosystems varies with their geographical location. For most terrestrial plants and animals, species diversity (primarily species richness) is highest in the tropics and declines as we move from the equator toward the poles. The most species-rich environments are tropical rain forests, large tropical lakes, coral reefs, and the ocean-bottom zone.

The second component of biodiversity is 

genetic diversity

, which is the variety of genes found in a population or in a species (

Figure 4.6

). Genes contain genetic information that give rise to specific traits, or characteristics, that are passed on to offspring through reproduction. Species whose populations have greater genetic diversity have a better chance of surviving and adapting to environmental changes.

Figure 4.6

Genetic diversity in this population of a Caribbean snail species is reflected in the variations of shell color and banding patterns. Genetic diversity can also include other variations such as slight differences in chemical makeup, sensitivity to various chemicals, and behavior.

The third component of biodiversity, 

ecosystem diversity

, refers to the earth’s diversity of biological communities such as deserts, grasslands, forests, mountains, oceans, lakes, rivers, and wetlands. Biologists classify terrestrial (land) ecosystems into 

biomes

—large regions such as forests, deserts, and grasslands characterized by distinct climates and certain prominent species (especially vegetation). Biomes differ in their community structure based on the types, relative sizes, and stratification of their plant species (

Figure 4.7

). 

Figure 4.8

 shows the major biomes found across the midsection of the United States. We discuss biomes in detail in 

Chapter 7

.

Figure 4.7

Community structure: Generalized types, relative sizes, and stratification of plant species in communities or ecosystems in major terrestrial biomes.

Figure 4.8

The variety of biomes found across the midsection of the United States.

First: Zack Frank/ Shutterstock.com; second: Robert Crum/ Shutterstock.com; third: Joe Belanger/ Shutterstock.com; fourth: Protasov AN/ Shutterstock.com; fifth: Maya Kruchankova/ Shutterstock.com; sixth: Marc von Hacht/ Shutterstock.com

Large areas of forest and other biomes tend to have a core habitat and edge habitats with different environmental conditions and species, called 

edge effects

. For example, a forest edge is usually more open, bright, and windy and has greater variations in temperature and humidity than a forest interior. Humans have fragmented many forests, grasslands, and other biomes into isolated patches with less core habitat and more edge habitat that supports fewer species.

Natural ecosystems within biomes rarely have distinct boundaries. Instead, one ecosystem tends to merge with the next in a transitional zone called an 

ecotone

. It is a region containing a mixture of species from adjacent ecosystems along with some migrant species not found in either of the bordering ecosystems.

The fourth component of biodiversity is 

functional diversity

—the variety of processes such as energy flow and matter cycling that occur within ecosystems (

Figure 3.9

) as species interact with one another in food chains and food webs. This component of biodiversity includes the variety of ecological roles organisms play in their biological communities and the impacts these roles have on their overall ecosystems.

A more biologically diverse ecosystem with a greater variety of producers can produce more plant biomass, which in turn can support a greater variety of consumer species. Biologically diverse ecosystems also tend to be more stable because they are more likely to include species with traits that enable them to adapt to changes in the environment, such as disease or drought.

We should care about and avoid degrading the earth’s biodiversity because it is vital to maintaining the natural capital (

Figure 1.3) that keeps us alive and supports our economies. We use biodiversity as a source of food, medicine, building materials, and fuel. Biodiversity also provides natural ecosystem services such as air and water purification, renewal of topsoil, decomposition of wastes, and pollination. In addition, the earth’s variety of genetic information, species, and ecosystems provide raw materials for the evolution of new species and ecosystem services, as they respond to changing environmental conditions. Biodiversity is an ecological life insurance policy. When we celebrate, protect, and enhance the earth’s biodiversity, we are helping to preserve our own species and economic systems, which depend on the natural capital that biodiversity provides. We owe much of what we know about biodiversity to researchers such Edward O. Wilson (

Individuals Matter 4.1

).

Individuals Matter 4.1

Edward O. Wilson: A Champion of Biodiversity

Jim Harrison

As a boy growing up in the southeastern United States, Edward O. Wilson became interested in insects at age 9. He has said, “Every kid has a bug period. I never grew out of mine.”

Before entering college, Wilson had decided he would specialize in the study of ants. He became one of the world’s experts on ants and then widened his focus to include the entire biosphere. One of Wilson’s landmark works is The Diversity of Life, published in 1992. In that book, he presented the principles and practical issues of biodiversity more completely than anyone had to that point. Today, he is recognized as one of the world’s leading experts on biodiversity—often referred to as “the father of biodiversity.”

Wilson continues actively writing and lecturing about the importance of species and the need for global biodiversity discovery and inventory in order to better understand our planet and identify conservation priorities. In 2016, he published Half-Earth: Our Planet’s Fight for Life, a call to conserve half the Earth’s lands and seas in order to ensure species have the space they need to thrive in perpetuity. The E. O. Wilson Biodiversity Foundation is now bringing this vision to life through the Half-Earth Project.

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4.3aEach Species Plays a Role

Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’ 

ecological niche

. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its 

habitat

, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.

Ecologists use the niches of species to classify them as generalists or specialists. A 

generalist species

 such as a raccoon has a broad niche (

Figure 4.9

, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.

Figure 4.9

Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).

In contrast, a 

specialist species

, such as the giant panda, occupies a narrow niche (
Figure 4.9
, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (

Figure 4.10

).

Figure 4.10

Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.

Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (
Figure 4.9
, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.

Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.

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4.3aEach Species Plays a Role
Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’ 
ecological niche
. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its 
habitat
, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.
Ecologists use the niches of species to classify them as generalists or specialists. A 
generalist species
 such as a raccoon has a broad niche (
Figure 4.9
, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.
Figure 4.9
Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).

In contrast, a 
specialist species
, such as the giant panda, occupies a narrow niche (
Figure 4.9
, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (
Figure 4.10
).
Figure 4.10
Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.

Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (
Figure 4.9
, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.
Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.
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4.3cIndicator Species

Species that provide early warnings of changes in environmental conditions in an ecosystem are called 

indicator species

. They are like biological smoke alarms. In this chapter’s Core Case Study, you learned that some amphibians are classified as indicator species. Scientists have been working hard to identify some of the possible causes of the declines in amphibian populations (

Science Focus 4.1

).

Science Focus 4.1

Causes of Amphibian Declines

Scientists who study amphibians have identified natural and human-related factors that can cause the decline and disappearance of these indicator species.

One natural threat is parasites such as flatworms that feed on certain amphibian eggs. Research indicates that this has caused birth defects such as missing limbs or extra limbs in some amphibians.

Another natural threat comes from viral and fungal diseases. An example is the chytrid fungus that infects a frog’s skin and causes it to thicken. This reduces the frog’s ability to take in water through its skin and leads to death from dehydration. An even deadlier fungal disease is Batrachochytrium dendrobatidis (or Bd), which invades skin cells and multiplies, causing the frog’s skin to peel away. Such diseases can spread easily, because adults of many amphibian species congregate in large numbers to breed.

Habitat loss and fragmentation is another major threat to amphibians. It is mostly a human-caused problem resulting from the clearing of forests and the draining and filling of freshwater wetlands for farming and urban development.

Another human-related problem is higher levels of UV radiation from the sun. Ozone  that forms in the stratosphere protects the earth’s life from harmful UV radiation emitted by the sun. During the past few decades, ozone-depleting chemicals released into the troposphere by human activities have drifted into the stratosphere and have destroyed some of the stratosphere’s protective ozone. The resulting increase in UV radiation can kill embryos of amphibians in shallow ponds as well as adult amphibians basking in the sun for warmth. International action has been taken to reduce the threat of stratospheric ozone depletion, but it will take about 50 years for ozone levels to recover to levels that existed before this threat arose.

Pollution from human activities also threatens amphibians. Frogs and other species are exposed to pesticides in ponds and in the bodies of insects that they eat. This can make them more vulnerable to bacterial, viral, and fungal diseases and to some parasites. Amphibian expert and National Geographic Explorer Tyrone Hayes, a professor of biology at University of California-Berkeley, conducts research on how some pesticides can harm frogs and other animals by disrupting their endocrine systems.

Climate change is also a concern. Amphibians are sensitive to even slight changes in temperature and moisture. Warmer temperatures may lead amphibians to breed too early. Extended dry periods also lead to a decline in amphibian populations by drying up breeding pools that frogs and other amphibians depend on for reproduction and survival through their early stages of life (

Figure 4.A

).

Figure 4.A

This golden toad lived in Costa Rica’s high-altitude Monteverde Cloud Forest Reserve. The species became extinct in 1989, apparently because its habitat dried up.

Charles H. Smith/U.S. Fish and Wildlife Service

Overhunting is another human-related threat, especially in areas of Asia and Europe, where frogs are hunted for their leg meat. Another threat is the invasion of amphibian habitats by nonnative predators and competitors, such as certain fish species. Some of this immigration into habitats is natural, but humans accidentally or deliberately transport many species to amphibian habitats.

According to most amphibian experts, a combination of these factors, which vary from place to place, is responsible for most of the decline and extinctions of amphibian species. This amounts to a biological “fire alarm.”

Critical Thinking

1. Of the factors listed above, which three do you think could be most effectively controlled by human efforts?

Birds are excellent biological indicators. They are found almost everywhere and are affected quickly by environmental changes such as the loss or fragmentation of their habitats and the introduction of chemical pesticides.

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4.3dKeystone Species

A keystone is the wedge-shaped stone placed at the top of a stone archway. Remove this stone and the arch collapses. In some communities and ecosystems, ecologists hypothesize that certain species play a similar role. A 

keystone species

 has such a large effect on the types and abundance of other species in an ecosystem that without it, the ecosystem would be dramatically different or might cease to exist.

Keystone species play several critical roles in helping to sustain ecosystems. One is the pollination of flowering plant species by butterflies, honeybees (Figure 4.4, left), hummingbirds, bats, and other species. In addition, top predator keystone species feed on and help to regulate the populations of other species. Examples are wolves, leopards, lions, some shark species, and the American alligator (see the following Case Study).

Case Study

The American Alligator—A Keystone Species That Almost Went Extinct

The American alligator (

Figure 4.12

) is a keystone species in wetland ecosystems where it is found in the southeastern United States. These alligators play several important ecological roles. They dig deep depressions, or gator holes. These depressions hold freshwater during dry spells and serve as refuges for aquatic life. They supply freshwater and food for fishes, insects, snakes, turtles, birds, and other animals.

Figure 4.12

Keystone species: The American alligator plays an important ecological role in its marsh and swamp habitats in the southeastern United States by helping support many other species.

Arto Hakola/ Shutterstock.com

The large nesting mounds that alligators build provide nesting and feeding sites for some herons and egrets, and red-bellied turtles lay their eggs in old gator nests. In addition, by eating large numbers of gar, a predatory fish, alligators help maintain populations of game fish that gar eat, such as bass and bream. When alligators excavate holes and build nesting mounds, they help keep vegetation from invading shorelines and open-water areas. Without this ecosystem service, freshwater ponds and coastal wetlands where alligators live would fill in with shrubs and trees, and dozens of species could disappear from these ecosystems.

In the 1930s, hunters began killing American alligators for their exotic meat and their soft belly skin, used to make expensive shoes, belts, and pocketbooks. Other people hunted alligators for sport or out of dislike for the large reptile. By the 1960s, hunters and poachers had wiped out 90% of the alligators in the state of Louisiana, and the Florida Everglades population waswas near extinction.

In 1967, the U.S. government placed the American alligator on the endangered species list. By 1987, because it was protected, its populations had made a strong comeback and the alligator was removed from the endangered species list. Today, there are well over a million alligators in Florida. The state now allows property owners to kill alligators that stray onto their land.

To conservation biologists, the comeback of the American alligator is an important success story in wildlife conservation. Recently, however, large and rapidly reproducing Burmese and African pythons released deliberately or accidently by humans have invaded the Florida Everglades. These nonnative invaders feed on young alligators, and could threaten the long-term survival of this keystone species in the Everglades.

Critical Thinking

The American Alligator and Biodiversity

1. What are two ways in which the American alligator supports one or more of the four components of biodiversity (

Figure 4.5) within its environment?

The loss of a keystone species in an ecosystem can lead to population declines and, in some cases, to extinctions of other species that depend on them for certain ecosystem services. This is why it important for scientists to identify keystone species and work to protect them.

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4.4aEvolution Explains How Life Changes Over Time

How did the earth end up with such an amazing diversity of species? The scientific answer is 

biological evolution

 or simply 

evolution

—the process by which the genes of populations of species change genetically over time. According to this scientific theory, species have evolved from earlier, ancestral species through 

natural selection

—the process in which individuals with certain genetic traits are more likely to survive and reproduce under a specific set of environmental conditions. These individuals then pass these traits on to their offspring.

A huge body of scientific evidence supports this idea. As a result, biological evolution through natural selection is the most widely accepted scientific theory that explains how the earth’s life has changed over the past 3.8 billion years and why we have today’s diversity of species.

Most of what we know about the history of life on the earth comes from 

fossils

—the remains or traces of past organisms. Fossils include mineralized or petrified replicas of skeletons, bones, teeth, shells, leaves, and seeds, or impressions of such items found in rocks (

Figure 4.13

). Scientists have discovered fossil evidence in successive layers of sedimentary rock such as limestone and sandstone. They have also studied evidence of ancient life contained in ice core samples drilled from glacial ice at the earth’s poles and on mountaintops.

Figure 4.13

This fossil shows the mineralized remains of an early ancestor of the present-day horse. It roamed the earth more than 35 million years ago. Notice that you can also see fish skeletons on this fossil.

Ira Block/National Geographic Image Collection

This body of evidence is called the fossil record. It is uneven and incomplete because many past forms of life left no fossils and some fossils have decomposed. Scientists estimate that the fossils found so far represent probably only 1% of all species that have ever lived. There are still many unanswered scientific questions about the details of evolution by natural selection, and research continues in this area.

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4.4bEvolution Depends on Genetic Variability and Natural Selection

The idea that organisms change over time and are descended from a single common ancestor has been discussed since the early Greek philosophers. There was no convincing explanation of how this could happen until 1858 when naturalists Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently proposed the concept of natural selection as a mechanism for biological evolution. Darwin gathered evidence for this idea and published it in his 1859 book, On the Origin of Species by Means of Natural Selection.

Biological evolution by natural selection involves changes in a population’s genetic makeup through successive generations. Populations—not individuals—evolve by becoming genetically different.

The first step in this process is the development of 

genetic variability

: a variety in the genetic makeup of individuals in a population. This occurs primarily through 

mutations

, or changes in the coded genetic instructions in the DNA in a gene. During an organism’s lifetime, the DNA in its cells is copied each time one of its cells divides and whenever it reproduces. In a lifetime, this happens millions of times and results in various mutations.

Most mutations result from random changes in the DNA’s coded genetic instructions that occur in only a tiny fraction of these millions of divisions. Some mutations also occur from exposure to external agents such as radioactivity, ultraviolet (UV) radiation from the sun, and certain natural and human-made chemicals called mutagens.

Mutations can occur in any cell, but only those that take place in the genes of reproductive cells are passed on to offspring. Sometimes a mutation can result in a new genetic trait, called a heritable trait, which can be passed from one generation to the next. In this way, populations develop genetic differences among their individuals. Biologists refer to the genes of a population as a 

gene pool

.

The next step in biological evolution is natural selection, which explains how populations evolve in response to changes in environmental conditions by changing their genetic makeup. Through natural selection, environmental conditions favor increased survival and reproduction of certain individuals in a population. These favored individuals possess heritable traits that give them an advantage over other individuals in the population. Such a trait is called an 

adaptation

, or 

adaptive trait

. An adaptive trait improves the ability of an individual organism to survive and to reproduce at a higher rate than other individuals in a population can under prevailing environmental conditions.

An example of natural selection at work is genetic resistance. It occurs when one or more organisms in a population have genes that can tolerate a chemical (such as a pesticide or antibiotic) that normally would be fatal. The resistant individuals survive and reproduce more rapidly than the members of the population that do not have such genetic traits. Genetic resistance can develop quickly in populations of organisms such as bacteria and insects that produce large numbers of offspring. For example, some disease-causing bacteria have developed genetic resistance to widely used antibacterial drugs, or antibiotics (

Figure 4.14

).

Figure 4.14

Evolution by natural selection: (a) A population of bacteria is exposed to an antibiotic, which (b) kills all individuals except those possessing a trait that makes them resistant to the drug; (c) the resistant bacteria multiply and eventually (d) replace all or most of the nonresistant bacteria.

Through natural selection, humans have evolved traits that have enabled them to survive in many different environments and to reproduce successfully. If we think of the earth’s 4.6 billion years of geological and biological history as one 24-hour day, the human species arrived about a tenth of one second before midnight. In that short time, we have dominated most of the earth’s land (

Figure 1.9

) and aquatic systems. Evolutionary biologists attribute our ability to dominate the earth to three major adaptations (

Figure 4.15

):

· Strong opposable thumbs that allowed humans to grip and use tools better than the few other animals that have thumbs

· The ability to walk upright, which gave humans agility and freed up their hands for many uses

· A complex brain, which allowed humans to develop many skills, including the ability to communicate complex ideas.

Figure 4.15

Homo sapiens sapiens: Three advantages over other mammals have helped us to become the earth’s dominant species within an eye blink of time in the 3.8-billion-year history of life on the earth.

To summarize the process of biological evolution by natural selection: Genes mutate, individuals are selected, and populations that are better adapted to survive and reproduce under existing environmental conditions evolve.

Evolutionary biologists study patterns of evolution by examining the similarities and differences among species based on their physical and genetic characteristics. They use this information to develop branching 

evolutionary tree

, or 

phylogenetic tree

, diagrams that depict the hypothetical evolution of various species from common ancestors (

Figure 4.16

). They use fossil, DNA, and other evidence to hypothesize the evolutionary pathways and connections among species.

Figure 4.16

Simplified phylogenetic tree (or tree of life) diagram showing the hypothesized evolution of life on the earth into six major kingdoms of species over 3.8 billion years.

On an evolutionary timescale, as new species arise, they have new genetic traits that can enhance their survival as long as environmental conditions do not change dramatically. The older species from which they originated and the new species evolve and branch out along different lines or lineages of species that can be recorded in phylogenetic tree diagrams (
Figure 4.16
).

4.4cLimits to Adaptation through Natural Selection

In the not-too-distant future, will adaptations to new environmental conditions through natural selection protect us from harm? For example, will adaptations allow the skin of our descendants to become more resistant to the harmful effects of the sun’s UV radiation, enable their lungs to cope with air pollutants, and improve the ability of our livers to detoxify pollutants in our bodies?

Scientists in this field say not likely because of two limitations on adaptation through natural selection. First, a change in environmental conditions leads to adaptation only for genetic traits already present in a population’s gene pool, or if they arise from random mutations.

Second, even if a beneficial heritable trait is present in a population, the population’s ability to adapt may be limited by its reproductive capacity. Populations of genetically diverse species that reproduce quickly often can adapt to a change in environmental conditions in a short time (days to years). Examples are dandelions, mosquitoes, rats, bacteria, and cockroaches. By contrast, species that cannot produce large numbers of offspring rapidly—such as elephants, tigers, sharks, and humans—take thousands or even millions of years to adapt through natural selection.

4.4dMyths about Evolution through Natural Selection

There are a number of misconceptions about biological evolution through natural selection. Here are five common myths:

· Survival of the fittest means survival of the strongest. To biologists, fitness is a measure of reproductive success, not strength. Thus, the fittest individuals are those that leave the most descendants, not those that are physically the strongest.

· Evolution explains the origin of life. It does not. However, it does explain how species have evolved after life came into being around 3.8 billion years ago.

· Humans evolved from apes or monkeys. Fossil and other evidence shows that humans, apes, and monkeys evolved along different paths from a common ancestor that lived 5 million to 8 million years ago.

· Evolution by natural selection is part of a grand plan in nature in which species are to become more perfectly adapted. There is no evidence of such a plan. Instead, evidence indicates that the forces of natural selection and random mutations can push evolution along any number of paths.

· Evolution by natural selection is not important because it is just a theory. This reveals a misunderstanding of the concept of a scientific theory, which is based on extensive evidence and is accepted widely by the scientific experts in a particular field of study. Numerous polls show that evolution by natural selection is widely accepted by over 95% of biologists because it best explains the earth’s biodiversity and how populations of different species have adapted to changes in the earth’s environmental conditions over billions of years.

4.5aHow Do New Species Arise?

Under certain circumstances, natural selection can lead to an entirely new species. Through this process, called 

speciation

, one species evolves into two or more different species. For sexually reproducing organisms, a new species forms when a separated population of a species evolves to the point where its members can no longer interbreed and produce fertile offspring with members of another population of its species that did not change or that evolved differently.

Speciation, especially among sexually reproducing species, happens in two phases: first geographic isolation, and then reproductive isolation. 

Geographic isolation

 occurs when different groups of the same population of a species become physically isolated from one another for a long time. Part of a population may migrate in search of food and then begin living as a separate population in an area with different environmental conditions. Winds and flowing water may carry a few individuals far away where they establish a new population. A flooding stream, a new road, a hurricane, an earthquake, or a volcanic eruption, as well as long-term geological processes (

Science Focus 4.2

), can also separate populations. Human activities, such as the creation of large reservoirs behind dams and the clearing of forests, can also create physical barriers for certain species. The separated populations can develop different genetic characteristics because they are no longer exchanging genes.

Science Focus 4.2

Geological Processes Affect Biodiversity

The earth’s surface has changed dramatically over its long history. Scientists discovered that huge flows of molten rock within the earth’s interior have broken its surface into a number of gigantic solid plates, called tectonic plates. For hundreds of millions of years, these plates have drifted slowly on the planet’s mantle formed today’s continents (

Figure 4.B

).

Figure 4.B

Over millions of years, the earth’s landmasses have moved very slowly on several gigantic tectonic plates to different locations and formed today’s continents (right).

Critical Thinking:

1. How might an area of land splitting apart cause the extinction of a species?

Rock and fossil evidence indicates that 200–250 million years ago, all of the earth’s present-day continents were connected in a supercontinent called Pangaea (Figure 4.B, left). About 175 million years ago, Pangaea began splitting apart as the earth’s tectonic plates moved. Eventually tectonic movement resulted in the present-day locations of the continents (Figure 4.B, right).

The movement of tectonic plates has had two important effects on the evolution and distribution of life on the earth. First, the locations of continents and oceanic basins have greatly influenced the earth’s climate, which plays a key role in where plants and animals can live. Second, the breakup, movement, and joining of continents have allowed species to move and adapt to new environments. This led to the formation of a large number of new species through speciation.

Along boundaries where they meet, tectonic plates may pull away from, collide with, or slide alongside each other. Tremendous forces produced by these interactions along plate boundaries can lead to earthquakes and volcanic eruptions. These geological activities can also affect biological evolution by causing fissures in the earth’s crust, which can isolate populations of species on either side of the fissure. Over long periods, this can lead to the formation of new species as each isolated population changes genetically in response to new environmental conditions.

Volcanic eruptions that occur along the boundaries of tectonic plates can also affect extinction and speciation by destroying habitats and reducing, isolating, or wiping out populations of species. These geological processes are further discussed in 

Chapter 14

.

Critical Thinking

1. The earth’s tectonic plates, including the one you are riding on, typically move at about the rate at which your fingernails grow. If they stopped moving, how might this affect the future biodiversity of the planet?

In 

reproductive isolation

, mutation and change by natural selection operate independently in the gene pools of geographically isolated populations. If this process continues long enough, members of isolated populations of sexually reproducing species can become different in genetic makeup. Then they cannot produce live, fertile offspring if they rejoin their original populations and attempt to interbreed. When that is the case, speciation has occurred and one species has become two (

Figure 4.17

).

Figure 4.17

Geographic isolation can lead to reproductive isolation, divergence of gene pools, and speciation.

4.5bArtificial Selection, Genetic Engineering, Gene Editing, and Synthetic Biology

For thousands of years, humans have used 

artificial selection

 to change the genetic characteristics of populations with similar genes. First, they select one or more desirable genetic traits that already exist in the population of a plant or animal. Then they use selective breeding, or crossbreeding, to control which members of a population have the opportunity to reproduce to increase the numbers of individuals in a population with the desired traits (

Figure 4.18

).

Figure 4.18

Artificial selection involves the crossbreeding of species that are close to one another genetically. In this example, similar fruits are being crossbred to yield a pear with a certain color.

Learning from Nature

Artificial selection is a classic case of learning from nature. It involves learning how natural processes produce a particular trait in a fruit or vegetable and then using crossbreeding to enhance that trait.

Artificial selection is not a form of speciation. It is limited to crossbreeding between genetic varieties of the same species or between species that are genetically similar to one another. Most of the grains, fruits, and vegetables we eat are produced by artificial selection. Artificial selection has also given us food crops with higher yields, cows that give more milk, trees that grow faster, and many different varieties of dogs and cats. However, traditional crossbreeding is a slow process.

Scientists have learned how to speed this process of manipulating genes in order to get desirable genetic traits or eliminate undesirable ones. They do this by transferring segments of DNA with the desired trait from one species to another through a process called 

genetic engineering

. In this process, also known as gene splicing, scientists alter an organism’s genetic material by adding, deleting, or changing segments of its DNA to produce desirable traits or to eliminate undesirable ones. Scientists have used genetic engineering to develop modified crop plants, new drugs, pest-resistant plants, and animals that grow rapidly.

There are five steps in this process:

1. Identify a gene with the desired trait in the DNA found in the nucleus of a cell from the donor organism.

2. Extract a small circular DNA molecule, called a plasmid, from a bacterial cell.

3. Insert the desired gene from 

step 1

 into the plasmid to form a recombinant DNA plasmid.

4. Insert the recombinant DNA plasmid into a cell of another bacterium, which rapidly divides and reproduces large numbers of bacterial cells with the desired DNA trait.

5. Transfer the genetically modified bacterial cells to a plant or animal that is to be genetically modified.

The result is a 

genetically modified organism (GMO)

 with its genetic information modified in a way not found in natural organisms. Genetic engineering enables scientists to transfer genes between different species that would not interbreed in nature. For example, scientists can put genes from a cold-water fish species into a tomato plant to give it properties that help it resist cold weather. Recently, scientists have learned how to treat certain genetic diseases by altering or replacing the genes that cause them. Genetic engineering has revolutionized agriculture and medicine. However, it is a controversial technology, as we discuss in 

Chapter 12

.

In 2012, scientists developed a new gene editing technique called RISPRCRISPR. This easy and cheap technique allows researchers to snip, insert, delete, or modify genetic material at targeted spots in DNA molecules with increased precision.

Gene editing has great promise for correcting disease-causing mutations in DNA molecules. It could be used treat cancers and other human diseases and to modify the DNA in human embryos to remove disease-causing mutations. It has been used to cure mice with HIV and hemophilia and to engineer pigs to make them suitable organ donors for humans. Gene editing can be done to cells outside the human body. Then the modified cells can be implanted in the human body. These genetic changes can then be passed on to future generations.

One worry is that gene editing could become so easy and cheap (a gene editing kit can be ordered online for $150) that anybody could do it and not be subject to regulations. Individuals could possibly alter cells in human embryos, eggs, and sperm to come up with “designer babies” with genes that favor certain traits. This could lead to discrimination against certain groups and a host of other major ethical challenges.

A new and rapidly growing form of genetic engineering is synthetic biology. It enables scientists to make new sequences of DNA and use such genetic information to design and create artificial cells, tissues, body parts, and organisms not found in nature. Synthetic biology can bypass the long process of evolution by natural selection and create new forms of life in a short time.

This process starts with a computer code of an organism’s entire genetic sequence (genome). Then engineers insert new sequences of the four nucleotide bases, adenine (A), cytosine (C), guanine (G), and thymine (T) (

Figure 2.9

), to create a new and different genetic sequence, or genome. Next, they transplant the new genome into the cell of a bacterium to transform it into a different, human-created species of bacteria. This technology uses science and engineering to alter the planet’s life by reducing the cell to a machine that can assemble forms of life like products in a factory.

Proponents of synthetic biology want to use it to create bacteria that can use sunlight to produce clean-burning hydrogen gas, which can be used to fuel motor vehicles. This could help us reduce our dependence on fossil fuels. Synthetic biology might also be used to create bacteria and algae that break down oil, industrial wastes, toxic heavy metals, pesticides, and radioactive waste in contaminated soil and water. It could be used to create vaccines to prevent diseases and drugs to combat parasitic diseases such as malaria. It might be used to develop instructions for three-dimensional printers to print human body parts, car parts, and clothing.

Scientists are a long way from achieving such goals, but the potential is there. If used properly and ethically, this new technology could help us live more sustainably. The problem is that, like any technology, synthetic biology can be used for good or bad. For example, it could be used to create biological weapons such as deadly bacteria that spread new diseases, to destroy existing oil deposits, or to interfere with the chemical cycles that keep us alive. It might also end up hindering the ability of decomposers to breakdown and recycle wastes, or it might add new pollutants to soil and water. This is why many scientists call for increased monitoring and regulation of this new technology to help control its use.

Learning from Nature

Scientists are applying synthetic biology by studying how organisms in nature operate. For example, some bacteria can consume substances that are harmful to humans, and scientists hope to create a bacterium that can be used to cleanse the human body of such substances.

4.5cExtinction Eliminates Species

Another factor affecting the number and types of species on the earth is 

biological extinction

, or simply extinction, which occurs when an entire species ceases to exist. When environmental conditions change dramatically, a population of a species faces three possible futures: adapt to the new conditions through natural selection, migrate (if possible) to another area with more favorable conditions, or become extinct in the area where they are found.

Species found in only one area, called 

endemic species

, are especially vulnerable to extinction. They exist on islands and in other isolated areas. For example, many species in tropical rain forests have highly specialized roles and are vulnerable to extinction. These organisms are unlikely to be able to migrate or adapt to rapidly changing environmental conditions. Many of these endangered species are amphibians (Core Case Study).

Extinction is a natural and ongoing process. Fossils and other scientific evidence indicate that 99.9% of all species that have existed on the earth are now extinct. Throughout most of the earth’s long history, species have disappeared at a low rate, called the 

background extinction rate

. Based on the fossil record and analysis of ice cores, biologists estimate that the average annual background extinction rate has been about 0.0001% of all species per year, which amounts to 1 species lost for every million species on the earth per year. At this rate, if there were 10 million species on the earth, about 10 of them, on average, would go extinct every year.

Evidence indicates that life on the earth has been sharply reduced by several periods of 

mass extinction

 during which extinction rates rise well above the background rate. In such a catastrophic, widespread, and often global event, 50-95% of all species are wiped out because of major, widespread environmental changes such as long-term climate change, massive flooding because of rising sea levels, huge meteorites striking the earth’s surface, and gigantic volcanic eruptions. Such events can trigger drastic environmental changes on a global scale, including massive releases of debris into the atmosphere that block sunlight for an extended period. This can kill off most plant species and the consumers that depend on them for food. Fossil and geological evidence indicates that there have been five mass extinctions (at intervals of 25–60 million years) during the past 500 million years (

Figure 4.19

).

Figure 4.19

Scientific evidence indicates that the earth has experienced five mass extinctions over the past 500 million years and that human activities have initiated a new sixth mass extinction.

A mass extinction provides an opportunity for the evolution of new species that can fill unoccupied ecological niches or newly created ones. Scientific evidence indicates that each past mass extinction has been followed by an increase in species diversity as shown by the wedges in Figure 4.19). However, this recovery process takes millions of years.

As environmental conditions change, the balance between speciation and extinction determines the earth’s biodiversity. The existence of millions of species today means that speciation, on average, has kept ahead of extinction. However, evidence indicates that the global extinction rate is rising sharply. Many scientists see this is as evidence that we are experiencing the beginning of a new sixth mass extinction caused mostly by human activities (Figure 4.19). We examine this issue and ways to deal with it in Chapter 9. The 

Case Study 

that follows discusses the threat of extinction for the monarch butterfly because of human activities.

Case Study

The Threatened Monarch Butterfly

The beautiful North American monarch butterfly (

Figure 4.20

 and the front cover of this book) is in trouble. This species is known for its annual 3,200- to 4,800-kilometer (2,000- to 3,000-mile) migration from the northern United States and Canada to a small number of tropical forest areas in central Mexico. They arrive on a predictable schedule and later return to their North American home. Another monarch population in the Midwestern United States makes a shorter annual journey to coastal northern California and then returns home.

Figure 4.20

Monarch butterflies in Mexico.

Melinda Fawer/ Shutterstock.com

During their annual round-trip journeys, these two populations of monarchs face serious threats from bad weather and numerous predators. In 2002, a single winter storm killed an estimated 75% of the monarch population migrating to Mexico.

During their migration, the monarchs need access to milkweed plants to lay their eggs. Once the butterfly larvae hatch, the caterpillar survives to become a butterfly by feeding on the milkweed plant. Without milkweed, the monarch butterfly cannot reproduce and faces extinction.

Once the monarchs reach their winter forest destinations in Mexico and California, they cling to trees (

Figure 4.20) by the millions as they rest. Each year, biologists estimate the monarch’s population size by measuring the total areas of forest they occupy at these destinations.

The overall estimated monarch population varies from year to year, mostly because of changes in weather and other natural conditions. However, the U.S. Fish and Wildlife Service estimates that since 1975, this overall population has dropped by nearly a billion. The size of Monarch butterfly populations wintering in Mexican forests varies from year to year, often because of weather and other environmental factors. However, since 1996, there has been an overall decline in their annual populations.

The monarchs face three serious threats from human activities in addition to the historic natural threats. One threat is the steady loss of their winter forest habitat in Mexico, due to logging (most of it illegal), and loss of their northern California habitat due to coastal development. A second threat is reduced access to milkweed plants essential for their survival during their migration. Almost all of the natural prairies in the United States, which were abundant with milkweed plants, have been replaced by croplands where milkweed plants grow much more sparsely only as weeds between rows of crops and on roadsides.

A third threat over the past decade is the explosive growth of cropland in the American Midwest planted with corn and soybean varieties genetically engineered to resist herbicides that are used to kill weeds, including milkweed. Some of these herbicides are thought to be killing monarchs as well as their food source.

So why should we care if the monarch butterfly becomes extinct, largely because of human activities? One reason is that monarchs provide an important ecological service by pollinating a variety of flowering plants (including corn) along their migration routes as they feed on the nectar from the blossoms of such plants. Another reason for many people is the belief that it is ethically wrong for us to cause the premature extinction of the monarch butterfly or other species.

What can we do to reduce the threat to this amazing species? Researchers call for protecting their migratory pathways and for the government to protect the monarch by classifying it as a threatened species. They propose that we sharply reduce the use of herbicides to kill milkweed. In addition, many people are trying to help by planting milkweed and other plants that attract pollinators such as butterflies and honeybees (whose populations are also decreasing, as we discuss in Chapter 9).

Learning from Nature

Scientists are studying the Monarch butterfly to find out how they are able to navigate their age-old annual migration routes and arrive at the same places in Mexico and California on the same day of each year. This knowledge could have benefits for human aviation.

Big Ideas

· Each species plays a specific ecological role, called its niche, in the ecosystems where it is found.

· As environmental conditions change, the genes in some individuals mutate and give those individuals genetic traits that enhance their abilities to survive and to produce offspring with these traits.

· The degree of balance between speciation and extinction in response to changing environmental conditions determines the earth’s biodiversity, which helps to sustain the earth’s life and our economies.

Tying It All TogetherAmphibians and Sustainability

Robert King/ Shutterstock.com

This chapter’s 

Core Case Study describes the increasing losses of amphibian species and explains why these species are important ecologically. In this chapter, we studied the importance of biodiversity—the numbers and varieties of species found in different parts of the world, along with genetic, ecosystem, and functional diversity.

We examined the variety of niches, or roles played by species in ecosystems. For example, we saw that some species, including many amphibians, are indicator species that warn us about threats to biodiversity, ecosystems, and the biosphere. Others such as the American alligator are keystone species that play vital roles in sustaining the ecosystems where they live.

We also studied the scientific theory of biological evolution through natural selection, which explains how life on the earth changes over time due to changes in the genes of populations and how new species can arise. We learned that the earth’s species biodiversity is the result of a balance between the formation of new species (speciation) and extinction of species due to changing environmental conditions.

The ecosystems where amphibians and other species live are functioning examples of the three scientific principles of sustainability in action. These species depend on solar energy, the cycling of nutrients, and biodiversity. Disruptions in any of these forms of natural capital can result in degradation of these species’ populations and their ecosystems.

Chapter Review

Critical Thinking

1. What might happen to humans and a number of other species if most or all amphibian species (Core Case Study) were to go extinct?

2. How might a reduction in species diversity affect the other three components of biodiversity?

3. Is the human species a keystone species? Explain. If humans become extinct, what are three species that might also become extinct and what are three species whose populations might grow?

4. Why should we care about saving the monarch butterfly from extinction caused by human activities? Do you care? Why or why not?

5. How would you respond to someone who tells you:

1. We should not believe in biological evolution because it is “just a theory.”

2. We should not worry about air pollution because natural selection will enable humans to develop lungs that can detoxify pollutants.

6. How would you respond to someone who says that because extinction is a natural process, we should not worry about the loss of biodiversity when species become extinct largely because of our activities?

7.

List three aspects of your lifestyle that could be contributing to some of the losses of the earth’s biodiversity. For each of these, what are some ways to avoid making this contribution?

8. Congratulations! You are in charge of the future evolution of life on the earth. What are the three most important things that you would do? Explain.

Chapter Review
Doing Environmental Science

1. Study an ecosystem of your choice, such as a meadow, a patch of forest, a garden, or an area of wetland. (If you cannot do this physically, do so virtually by reading about an ecosystem online or in a library.) Determine and list five major plant species and five major animal species in your ecosystem. Which, if any, of these species are indicator species and which of them, if any, are keystone species? Explain how you arrived at these hypotheses. Then design an experiment to test each of your hypotheses, assuming you would have unlimited means to carry them out.

Chapter Review
Data Analysis

The following table is a sample of a very large body of data reported by J. P. Collins, M. L. Crump, and T. E. Lovejoy III in their book Extinction in Our Times—Global Amphibian Decline. It compares various areas of the world in terms of the number of amphibian species found and the number of amphibian species that were endemic, or unique to each area. Scientists like to know these percentages because endemic species tend to be more vulnerable to extinction than do non-endemic species. Study the table and then answer the following questions.

Area

Number of Species

Number of Endemic Species

Percentage Endemic

Pacific/Cascades/Sierra Nevada Mountains of North America

52

43

Southern Appalachian Mountains of the United States

101

37

Southern Coastal Plain of the United States

68

27

Southern Sierra Madre of Mexico

118

74

Highlands of Western Central America

126

70

Highlands of Costa Rica and Western Panama

133

68

Tropical Southern Andes Mountains of Bolivia and Peru

132

101

Upper Amazon Basin of Southern Peru

102

22

1. Fill in the fourth column by calculating the percentage of amphibian species that are endemic to each area. 

2. Which two areas have the highest numbers of endemic species? Name the two areas with the highest percentages of endemic species.

3. Which two areas have the lowest numbers of endemic species? Which two areas have the lowest percentages of endemic species?

4. Which two areas have the highest percentages of non-endemic species?

10.4bWilderness

A

reas

One way to protect existing wildlands from human exploitation is to designate them as 

wilderness

—areas essentially undisturbed by humans that are protected by federal law from harmful human activities (

Figure 10.23

). For example, forestry, road and trail development, mining, and building construction are not allowed. Theodore Roosevelt (see 

Figure 1.14

), the first U.S. president to set aside protected areas, summarized his thoughts on what to do with wilderness: “Leave it as it is. You cannot improve it.”

Figure 10.23

D

iablo Lake lies in a wilderness area of North

C

ascades National Park in the U.S. state of Washington.

tusharkoley/ Shutterstock.com

Most developers and resource extractors oppose establishing protected wilderness areas because they contain resources that could provide short-term economic benefits.

E

cologists and conservation biologists take a longer view. To them, wilderness areas are protected islands of biodiversity and ecosystem services needed to support life and human economies both now and in the future and to serve as centers for future evolution in response to changes in environmental conditions.

In 1964, the U.S. Congress passed the Wilderness Act, which allowed the government to protect undeveloped tracts of U.S. public land from development as part of the National Wilderness Preservation System (

Figure 10.23). The country’s area of protected wilderness grew nearly 12-fold between 1964 and 2015. Even so, less than 5% of all U.S. land is protected as wilderness—more than 54% of it in Alaska. Only about 2% of the land of the lower 48 states is protected as wilderness, most of it in the West.

2%

Percentage of land protected as wilderness in the lower 48 U.S. states

As the human population and its ecological footprint expand, it will be increasingly difficult and expensive to protect existing wilderness areas and to establish new ones. In addition, climate change is projected to threaten the biodiversity and composition of many existing wilderness areas.

10.4cParks and Other Nature Reserves

According to the IUCN, there are more than 6,500 major national parks located in more than 120 countries (see chapter-opening

photo

). However, most of these parks are too small to sustain many large animal species. In addition, many of them are “paper parks” that receive little protection, especially in less-developed countries.

Many parks also suffer from invasions by harmful nonnative species that can outcompete and reduce the populations of native species. Some national parks are so popular that large numbers of visitors are degrading the natural features that make them attractive (see the 

 that follows).

Case Study

Stresses on U.S. Public Parks

The U.S. National Park System, established in 1912, includes 59 major national parks, sometimes called the country’s crown jewels, that are owned jointly by all U.S. citizens (see chapter-opening 

photo). The U.S. National Park System also has 358 monuments, recreational areas, battlefields, historic sites, and other sites. States, counties, and cities also operate public parks.

In 1872, Congress set aside public land for Yellowstone National Park—the world’s first national park. Historian, conservationist, and writer Wallace Stegner called it “the best idea America ever had.”

Popularity threatens many parks.

B

etween 1960 and 2017, the number of recreational visitors to U.S. national parks more than tripled, reaching about 331 million. In 2017, the three most visited places in the National Park System are the Blue Ridge Parkway, Golden Gate National Recreation Area, and the Great Smoky Mountain National Park.

In some U.S. parks and other public lands, dirt bikes, dune buggies, jet skis, snowmobiles, and other off-road vehicles destroy or damage vegetation, disturb wildlife, and degrade the park experience for many visitors. Some visitors expect parks to have grocery stores, laundries, bars, and other such conveniences. Cell phone towers now degrade the pristine nature of some parks.

A number of parks also suffer damage from the migration or deliberate introduction of nonnative species. European wild boars (see 

Figure 9.11

), imported into the state of North Carolina in 1912 for hunting, threaten vegetation in parts of the popular Great Smoky Mountains National Park. Nonnative mountain goats in Washington State’s Olympic National Park trample and destroy the root systems of native vegetation and accelerate soil erosion.

Native species—some of them threatened or endangered—are killed in, or illegally removed from, almost half of all U.S. national parks. However, the endangered gray wolf was successfully reintroduced into Yellowstone National Park after a 50-year absence (

Science Focus 10.3

).

Many U.S. national parks have become threatened islands of biodiversity surrounded by commercial development. The parks’ wildlife and recreational value are threatened by nearby activities such as mining, logging, livestock grazing, coal-fired power plants, and urban development. The National Park Service reports that air pollution, mainly caused by coal-fired power plants and dense vehicle traffic, impairs scenic views more than 90% of the time in many U.S. national parks. In 2018, scientists at Colorado State University found high noise levels from road traffic, air traffic, and nearby mining, drilling, and logging in more than 60% of the 492 areas they studied. Such noise pollution can diminish the park experience for many visitors.

The National Park Service estimated that in 2018, the national parks had at least an $11.6 billion backlog for long overdue maintenance and repairs to trails, buildings, and other park facilities. Some analysts say that some of the funds needed for such purposes could come from private concessionaires who provide campgrounds, restaurants, hotels, and other services for park visitors. They pay the government franchise fees averaging only about 6–7% of their gross receipts, and many large concessionaires with long-term contracts pay as little as 0.75%. Analysts say these percentages could reasonably be increased to around 20%.

Since the 1930s, there have been efforts to sell U.S. National Parks and other public lands to private owners and developers. These pressures are increasing, as discussed in 

Chapter 25

.

Parks in less-developed countries have the greatest biodiversity of all the world’s parks, but only about 1% of these parklands are protected. Local people in many of these countries enter the parks illegally in search of wood, game animals, and other natural products that they need for their daily survival. Loggers and miners also operate illegally in many of these parks, as do wildlife poachers who kill animals to obtain and sell items such as rhino horns, elephant tusks, and furs. Park services in most of the less-developed countries have too little money and too few personnel to fight these invasions, either by force or through education.

Science Focus 10.3

Reintroducing the Gray Wolf to Yellowstone National Park

In the 1800s, at least 3

50,000

gray wolves (

Figure 10.B

) roamed over 75% of America’s lower 48 states—especially in the West. The wolves preyed on bison, elk, caribou, and deer. Between 1850 and 1900, most of them were shot, trapped, or poisoned by ranchers, hunters, and government employees. This drove the gray wolf to near extinction in the lower 48 states.

Figure 10.B

After becoming almost extinct in much of the western United States, the gray wolf was listed and protected as an endangered species in 1974.

Volodymyr Burdiak/ Shutterstock.com

Ecologists recognize the important role that this keystone predator species once played in the Yellowstone National Park region. The wolves culled herds of bison, elk, moose, and mule deer, and kept down coyote populations. By leaving some of their kills partially uneaten, they provided meat for scavengers such as ravens, bald eagles, ermines, grizzly bears, and foxes.

When the number of gray wolves declined, herds of plant-browsing elk, moose, and mule deer expanded and over browsed the willow and aspen trees growing near streams and rivers. This led to increased soil erosion and declining populations of other wildlife species such as beaver, which eat willow and aspen. This in turn affected species that depend on wetlands created by dam-building beavers.

In 1974, the gray wolf was listed as an endangered species in the lower 48 states. However, in 2019, the Interior Department proposed removing endangered species protection for the gray wolf in the lower 48 states because of an increase in its population to more than 5,000.

In 1987, the U.S. Fish and Wildlife Service (USFWS) proposed reintroducing gray wolves into Yellowstone National Park to try to help stabilize the ecosystem. The proposal brought angry protests from ranchers who feared the wolves would leave the park and attack large numbers of their cattle and sheep and from hunters who feared the wolves would kill too many big-game animals. Mining and logging companies objected, fearing that the government would halt their operations on wolf-populated federal lands.

In 1996, USFWS officials captured 41 gray wolves in Canada and northwest Montana and relocated them in Yellowstone National Park. Scientists estimate that the long-term carrying capacity of the park is 110 to 150 gray wolves. In December 2018, the park had 104 wolves in 11 packs.

The reintroduction of this keystone species has turned the park into a living ecological laboratory. Wildlife ecologist Robert Crabtree and other scientists have been using radio collars to track some of the wolves and study the ecological effects of reintroducing the wolves. Their research indicates that the return of this keystone predator has decreased populations of elk, the wolves’ primary food source. The leftovers of elk killed by wolves have also been an important food source for scavengers such as bald eagles and ravens.

The wolves’ presence, with a projected decline in elk numbers, was supposed to promote the regrowth of young aspen trees that elk feed on and had depleted. However, a study led by U.S. Geological Survey scientist Matthew Kauffman indicated that the aspen were not recovering despite a 60% decline in elk numbers. Declining populations of elk were also supposed to allow for the return of willow trees along streams. Research indicates that willows have only partly recovered.

The wolves have cut in half the Yellowstone population of coyotes—the top predators in the absence of wolves. This has reduced coyote attacks on cattle from area ranches and has led to larger populations of small animals such as ground squirrels, mice, and gophers, which are hunted by coyotes, eagles, and hawks.

Overall, this experiment has had some important ecological benefits for the Yellowstone ecosystem, but more research is needed. The focus has been on the gray wolf, but other factors such as drought and the rise of populations of bears and cougars may play a role in the observed ecological changes and need to be examined. Some scientists hypothesize that the long-term absence of wolves led to a number of changes in plant and animal numbers and diversity that are difficult to reverse

The wolf reintroduction has also produced economic benefits for the region. One of the main attractions of the park for many visitors is the hope of spotting wolves chasing their prey across its vast meadows.

Critical Thinking

1. If the gray wolf population in the park were to reach its estimated carrying capacity of 110 to 150 wolves, would you support a program to kill wolves to maintain this population level? Why or why not? Can you think of other alternatives?

2. 10.4dDesigning and Managing Nature Reserves

3. In establishing nature reserves, the size and design of the reserve is important. Research by Thomas E. Lovejoy (

Chapter 3

, Individuals Matter) and other scientists indicates that large nature reserves typically sustain more species and provide greater habitat diversity than do small reserves. Research also indicates that in some areas, several well-placed medium-size reserves may better protect a variety of habitats and sustain more biodiversity than a single large reserve can.

4. Establishing protected habitat corridors between isolated reserves can benefit more species and allow migration by vertebrates that need large ranges. Corridors also allow some species to move to areas that are more favorable if climate change alters their existing areas.

5. On the other hand, corridors can threaten isolated populations by allowing movement of fire, disease, and pest and invasive species between reserves. They can also increase exposure of migrating species to natural predators, human hunters, and pollution. Some research suggests that the benefits of corridors outweigh their potential harmful effects, especially as the climate changes.

6.

Conservation biologists call for using the buffer zone concept, whenever possible, to design and manage nature reserves. Establishing a buffer zone means strictly protecting an inner core of a reserve, usually by establishing one or more buffer zones in which local people can extract resources sustainably without harming the inner core (see the Case Study that follows). By 2018, the United Nations had used this concept to create a global network of 686 biosphere reserves in 122 countries. However, most biosphere reserves fall short of these design ideals and receive too little funding for their protection and management.

7.

Case Study

8. Identifying and Protecting Biodiversity in Costa Rica

9. For several decades, Costa Rica (

Core Case Study

) has been using government and private research agencies to identify the plants and animals that make it one of the world’s most biologically diverse countries (

Figure 10.24

). The government consolidated the country’s parks and reserves into several large conservation areas, or megareserves, with the goal of protecting and sustaining 80% of the country’s biodiversity (

Figure 10.25

).

10. Figure 10.24

11. This scarlet macaw parrot is one of the more than half a million species found in Costa Rica.

12.

13.

Vladimir Melnik/ Shutterstock.com

14. Figure 10.25

15. Solutions: Costa Rica has created several megareserves. Green areas are protected natural parklands and yellow areas are the surrounding buffer zones.

16.

17.

Each reserve contains a protected inner core surrounded by two buffer zones that local and indigenous people can use for sustainable logging, crop farming, cattle grazing, hunting, fishing, and ecotourism. Instead of shutting local people out of reserve areas, this approach enlists local people as partners in protecting a reserve from activities such as illegal logging and poaching. It is an application of the biodiversity and win-win principles of sustainability.

18. In addition to its ecological benefits, this strategy has paid off financially. Today, Costa Rica’s largest source of income is its $2.9-billion-a-year travel and tourism industry, almost two-thirds of which involves ecotourism.

There are potential threats to Costa Rica’s conservation efforts. One is the clearing of forests to grow pineapples in plantations for export to China. Ecotourism helps to fund parks and conservation efforts and reduces exploitation of conservation areas by providing income for local people in visited areas, but excessive numbers of ecotourists can degrade sensitive ar0.4eSustaining Terrestrial Biodiversity: An Ecosystem Approach

Most wildlife biologists and conservationists believe that the best way to keep from hastening the extinction of wild species through human activities is to protect threatened habitats and ecosystem services. This ecosystems approach would generally employ the following five-point plan:

1. Map the world’s terrestrial ecosystems and create an inventory of the species contained in each of them, along with the ecosystem services they provide.

2. Identify terrestrial ecosystems that are resilient and can recover if not overwhelmed by harmful human activities, along with ecosystems that are fragile and need protection.

3. Protect the most endangered terrestrial ecosystems and species, with emphasis on protecting plant biodiversity and ecosystem services.

4. Restore as many degraded ecosystems as possible.

5. Make development biodiversity-friendly by providing significant financial incentives (such as tax breaks and subsidies) and technical help to private landowners who agree to help protect endangered ecosystems.

0.4fProtect Biodiversity Hotspots and Ecosystem Services

The ecosystem approach calls for identifying and taking emergency action to protect the earth’s 

biodiversity hotspots

. They are areas rich in highly endangered species found nowhere else and threatened by human activities. These areas have suffered serious ecological disruption, mainly due to rapid population growth and the resulting pressure on natural resources and ecosystem services.

Figure 10.26

 shows 34 terrestrial biodiversity hotspots biologists have identified. According to the IUCN, these areas cover only about 2% of the earth’s land surface, but are home for the majority of the world’s endangered species, as well as for 1.2 billion people.

Figure 10.26

Endangered natural capital: Biologists have identified these 34 biodiversity hotspots. Compare this map with the global map of the human ecological footprint, shown in 

Figure 1.9

.

Critical Thinking:

1. Why do you think so many hotspots are located near coastal areas?

(Compiled by the authors using data from the Center for Applied Biodiversity Science at Conservation International.)

This approach can conserve nearly half of the world’s terrestrial plant and animal species by preserving only about 2.3% of the earth’s land surface. However, only 5% of the total area of these hotspots is truly protected with government funding and law enforcement, as described in the Case Study that follows.

Case Study

Madagascar: An Endangered Biodiversity Hotspot

Madagascar, the world’s fourth largest island, lies in the Indian Ocean off the east coast of Africa. Most of its numerous species have evolved in near isolation from mainland Africa and all other land areas for at least 40 million years. As a result, roughly 90% of the more than 200,000 plant and animal species (

Figure 10.27

) found in this Texas-size biodiversity hotspot are found nowhere else on the earth.

Figure 10.27

Madagascar is the only home for six of the world’s eight baobab tree species (left), old-growth trees that are disappearing. This tree survives the desert-like conditions on part of the island by storing water in its large bottle-shaped trunk. The island is also the only home of more than 70 species of lemurs, including the threatened Verreaux’s sifaka, or dancing lemur (right).

Left: David Thyberg/ Shutterstock.com. Right: Richlindie/ Dreamstime.com.

Many of Madagascar’s plant and animal species are among the world’s most endangered, primarily because of habitat loss. People have cut down or burned more than 90% of Madagascar’s original forests to get firewood and lumber and to make way for small farms, large rice plantations, and cattle grazing.

Only about 17% of the island’s original vegetation remains, which means most of its topsoil is exposed. Hence, Madagascar is one of the world’s most eroded countries. Huge quantities of its topsoil have run off its hills, flowing as sediment in its rivers and emptying into its coastal waters. This explains why Madagascar is one of the world’s most threatened biodiversity hotspots.

Since 1984, the government, conservation organizations, and scientists worldwide have mounted efforts to slow the country’s rapid loss of biodiversity. Such efforts are hampered by Madagascar’s rapid population growth. Between 1994 and 2018, its population grew from 12 million to 26 million and is projected to grow to 54 million by 2050. The country is also very poor, with 90% of its population struggling to survive on the equivalent of less than $2.25 per day. This puts pressure on its dwindling forest resources.

Despite the efforts to preserve Madagascar’s biodiversity, less than 3% of its land area is officially protected. To reduce the rapid losses of biodiversity, the country will need to slow its population growth drastically and teach many of its people how to make a living from reforestation, ecotourism, and more sustainable uses of its forests, wildlife, and soil resources.

6.
7. 10.4gProtecting Ecosystem Services

8. Another way to sustain the earth’s biodiversity is to identify and protect areas in which vital ecosystem services (see the orange boxed labels in 

Figure 1.3

) are being impaired. Scientists call for identifying highly stressed areas with high poverty levels where most people are dependent on ecosystem services for survival. This ecosystem services approach recognizes that most of the world’s ecosystems are already dominated or influenced by human activities and that such pressures are increasing as the human population, urbanization, resource use, and the human ecological footprint all expand.

9. Proponents of this approach recognize that setting aside and protecting reserves and wilderness areas, especially highly endangered biodiversity hotspots and ecosystems, is vital. They also call for protecting the human communities that exist in these areas. Without addressing such issues as poverty, population growth, urbanization, and resource use, ecosystem services will continue to decline. Environmental scientist Gretchen Daily has developed tools to guide investments in restoring ecosystem services, in poor countries as well as more-developed countries (

Individuals Matter 10.2

).

10. Individuals Matter 10.2

11. Gretchen Daily—Shining a Light on Ecosystem Services

12.

13. Courtesy of Gretchen Daily

14. Gretchen Daily, Professor of Environmental Science at Stanford University, is alarmed by the growing threats to natural capital. She is considered one of the world’s experts on natural capital and ecosystem services.

15. In 2006, Daily cofounded the Natural Capital Project with Stanford University, World Wildlife Fund, and The Nature Conservancy. It has three major goals. First, develop credible and practical methods for measuring the economic and other values of ecosystem services. Second, find ways to integrate these values into major decisions made by governments and businesses. Third, tailor and replicate models that work across world regions or industry sectors.

16. The group has developed a software tool called InVEST (for Integrated Valuation of Ecosystem Services and Trade-offs). It helps users compare how different choices, such as where and how to develop or conserve land, can affect the ecosystem service benefits provided by forests or wetlands. It assigns long-term economic values to services such as flood control, water purification, and climate stability and projects how these services are likely to grow or shrink as a result of choices such as clear-cutting, selective logging, restoration, and preservation. This tool is being used in more than 185 countries, helping landowners and investors evaluate the effects of these and other factors in deciding how to use a piece of forested land. After heavy deforestation through the 1990s, China has invested over $100 billion in reforestation, for example, and is now reaping financial and well-being benefits of hydropower production efficiency, a more secure water supply, and flood control.

10.4hRestoring Damaged Ecosystems

Almost every natural place on the earth has been impacted to some degree by human activities, often in harmful ways. Much of the damage can be partially reversed through 

ecological restoration

, the process of repairing damage to ecosystems caused by human activities. Examples include replanting forests (see the Case Study that follows), reintroducing keystone native species (Science Focus 10.3), removing harmful invasive species, freeing river flows by removing dams, and restoring grasslands, coral reefs, wetlands, and stream banks (

Figure 10.21

, right). This is an important way to expand our beneficial environmental impact.

Case Study

Ecological Restoration of a Tropical Dry Forest in Costa Rica

Costa Rica (Core Case Study) is the site of one of the world’s largest ecological restoration projects. In the lowlands of its Guanacaste National Park, a tropical dry forest was burned, degraded, and fragmented for conversion to cattle ranches and farms. Now it is being restored and reconnected to a rain forest on nearby mountain slopes.

Daniel Janzen, professor of conservation biology at the University of Pennsylvania and a leader in the field of restoration ecology, used his own MacArthur Foundation grant money to purchase the Guanacaste forestland for designation as a national park. He has also raised more than $10 million for restoring the park.

Janzen recognizes that ecological restoration and protection of the park will fail unless the people living in the surrounding area believe they will benefit from such efforts. His vision is to see that the nearly 40,000 people who live near the park play an essential role in the restoration of the forest.

In the park, local farmers are paid to remove nonnative species and to plant tree seeds and seedlings started in Janzen’s laboratory. Local grade school, high school, and university students and citizens’ groups study the park’s ecology during field trips. The park’s location near the Pan American Highway makes it an ideal area for ecotourism, which stimulates the local economy.

This project also serves as a training ground in tropical forest restoration for scientists from around the world. Research scientists working on the project give guest classroom lectures and lead field trips. Janzen believes that education, awareness, and involvement—not guards and fences—are the best ways to protect largely intact ecosystems from unsustainable use. This is an application of the biodiversity and win-win principles of sustainability.

By studying how natural ecosystems recover, scientists are learning how to employ and enhance ecological succession processes by using a variety of approaches, including the following four:

· Restoration: returning a degraded habitat or ecosystem to a condition as close as possible to its original one.

· Rehabilitation: turning a degraded ecosystem into a functional or useful ecosystem without trying to restore it to its original condition. Examples include removing pollutants from abandoned mining or industrial sites and replanting trees to reduce soil erosion in clear-cut forests.

· Replacement: replacing a degraded ecosystem with another type of ecosystem. For example, a degraded forest might be replaced by a productive pasture or tree plantation.

· Creating artificial ecosystems: for example, artificial wetlands have been created in some areas to help reduce flooding and to treat sewage.

Researchers have suggested the following four-step strategy for carrying out most forms of ecological restoration and rehabilitation.

1. Identify the causes of the degradation, such as pollution, farming, overgrazing, mining, or invasive species.

2. Stop the degradation by eliminating or sharply reducing these factors.

3.

Reintroduce keystone species to help restore natural ecological processes, as was done with gray wolves in the Yellowstone ecosystem (

Science Focus 10.3).

4. Protect the area from further degradation to allow natural recovery (

Figure 10.19

, right).

By following this general plan, conservationist and National Geographic Explorer Sean Gerrity is working with his 35-person team in the U.S. state of Montana to create American Prairie Reserve, the largest nature reserve in the continental United States, a refuge for people and wildlife preserved forever as a part of America’s heritage. Their goal is to restore the wildlife and ecosystem services that were common to this unique area of North America’s grasslands for more than 11,000 years.

10.4iSharing Ecosystems with Other Species

We dominate most of the world’s ecosystems, which is a cause of species extinction and loss of ecosystem services. Ecologist Michael L. Rosenzweig calls for us to share some of the spaces we dominate with other species—an approach he calls 

reconciliation ecology

. It focuses on establishing and maintaining new habitats to conserve species diversity in places where people live, work, or play. This is a way for us to increase our beneficial environmental impact.

Learning from Nature

Researchers from the carpet manufacturer Interface examined a forest floor to get ideas for designing a carpet pattern. They decided to mimic the forest floor by creating a random, non-repeating design that became one of their best-selling lines of carpeting. It also saved the company money by allowing installers to cut waste dramatically, as the installations were not dependent on a repeating pattern.

By encouraging sustainable forms of ecotourism, people can protect local wildlife and ecosystems and provide economic resources for their communities. In the Central American country of Belize, for instance, conservation biologist Robert Horwich helped establish a local sanctuary for the black howler monkey. He convinced local farmers to set aside strips of forest to serve as habitats and corridors through which these monkeys can travel. The reserve, run by a local women’s cooperative, has attracted ecotourists and biologists. Local residents receive income for housing and guiding these visitors.

Without proper controls, ecotourism can lead to degradation of popular sites if they are overrun by visitors or are degraded by the construction of nearby hotels and other tourist facilities. However, when managed properly, ecotourism can be a useful form of reconciliation ecology.

Reconciliation ecology is also a way to protect vital ecosystem services. For example, some people are learning how to protect insect pollinators, such as butterflies and honeybees (see 

Chapter 9

 Core Case Study), which are vulnerable to pesticides and habitat loss. Neighborhoods and municipal governments are doing so by reducing or eliminating the use of pesticides on their lawns, fields, golf courses, and parks. People can also plant gardens of flowering plants as a source of food for bees, butterflies, and other pollinators. According to honeybee experts, people trying to help bees in this way should avoid using glyphosate herbicides and plants that contain neonicotinoid insecticides.

People have also worked together to protect bluebirds within human-dominated habitats. In such areas, bluebird populations have declined because most of their nesting trees have been cut down. Specially designed boxes have provided artificial nesting places for bluebirds. Their widespread use has allowed populations of this species to grow.

These and many other examples of people working together on projects to restore degraded ecosystems are applications of the biodiversity and win-win principles of sustainability. 

Figure 10.28

 lists some ways in which you can help sustain and expand the earth’s terrestrial biodiversity.

Figure 10.28

Individuals matter: Ways to help sustain terrestrial biodiversity.

Critical Thinking:

1. Which two of these actions do you think are the most important ones to take? Why? Which of these things do you already do?

Protecting the earth’s vital biodiversity and increasing our beneficial environmental impact will not be implemented without bottom-up political pressure on elected officials from individual citizens and groups. It also will require cooperation among scientists, engineers, and key people in government and the private sector. Individuals also need to “vote with their wallets” by buying only products and services that do not harm terrestrial biodiversity.

Big Ideas

· The economic value of the ecosystem services provided by the world’s ecosystems is far greater than the value of raw materials obtained from those systems.

· We can manage forests, grasslands, and nature reserves more effectively by protecting more land and by preventing overuse and degradation of these areas and the renewable resources they contain.

· We can sustain terrestrial biodiversity and ecosystem services and increase our beneficial environmental impact by protecting severely threatened areas and ecosystem services, restoring damaged ecosystems, and sharing with other species much of the land that we dominate.

· ying It All TogetherSustaining Costa Rica’s Biodiversity

·

·

Eduardo Rivero/ Shutterstock.com

· In this chapter, you learned how human activities are destroying or degrading much of the earth’s terrestrial biodiversity. You learned the importance of preserving what remains of diverse and highly endangered biodiversity hotspots and of sustaining the earth’s ecosystem services. You also saw how to reduce this destruction and degradation by using the earth’s resources more sustainably and by employing restoration ecology and reconciliation ecology. The 

Core Case Study introduced much of this by reporting on what Costa Rica is doing to protect and restore its precious biodiversity.

· Preserving terrestrial biodiversity and the ecosystem services it provides involves applying the three scientific principles of sustainability. First, it means respecting biodiversity and understanding the value of sustaining it. In addition, if we rely less on fossil fuels and more on direct solar energy and its indirect forms, such as wind and flowing water, we will generate less pollution. We will also interfere less with chemical cycling and other forms of natural capital that sustain biodiversity and our own lives and economies.

· Applying the three additional principles of sustainability will also help preserve biodiversity. By placing economic value on ecosystem services, we would acknowledge their importance by helping implement full-cost pricing. Working together to find win-win solutions to problems of environmental degradation benefits the earth and its people. Our actions can be guided by an ethical responsibility to sustain biodiversity and ecosystem services for current and future generations.

Chapter Review

Critical Thinking

1. Why do you think Costa Rica (Core Case Study) has set aside a much larger percentage of its land for biodiversity conservation than the United States has? Should the United States reserve more of its land for this purpose? Explain.

2. If we fail to protect a much larger percentage of the world’s remaining old-growth forests and tropical rain forests, what are three harmful effects that this failure is likely to have on any children and grandchildren you eventually might have?

3. In the early 1990s, Miguel Sanchez, a subsistence farmer in Costa Rica, was offered $600,000 by a hotel developer for a piece of land that he and his family had been using sustainably for many years. An area under rapid development surrounded the land, which contained an old-growth rain forest and a black sand beach. Sanchez refused the offer. Explain how Sanchez’s decision was an application of the ethical principle of sustainability. What would you have done if you were Sanchez? Explain.

4. Halting the destruction and degradation of tropical rain forests is a key to preserving the world’s biodiversity and slowing global climate change. Since this will benefit the entire world during this and future generations, should the United States and the world’s other more-developed nations pay tropical, less-developed countries to preserve their remaining tropical forests, as Norway and the United Kingdom have done? Explain. Do you think that the long-term economic and ecological benefits of doing this would outweigh the short-term economic costs? Why or why not?

5. Are you in favor of establishing more wilderness areas in the United States (or in the country where you live)? Explain. What might be some disadvantages of doing this?

6. You are a defense attorney arguing in court for preserving an old-growth forest that developers want to clear for a suburban development. Give your three strongest arguments for preserving this ecosystem. How would you counter the argument that preserving the forest would harm the economy by causing a loss of jobs in the timber industry?

7. Do you support or oppose the U.S. 2003 Healthy Forests Restoration Act (Section 10.2)? Why or why not?

8. It would cost about $76 billion a year to sustain the earth’s terrestrial biodiversity. Do you think we should spend this money? How might a decision not to make this investment affect you and any children or grandchildren you might have?

Chapter Review

Doing Environmental Science

1. Pick an area near where you live or go to school that hosts a variety of plants and animals. It could be a yard, an abandoned lot, a park, a forest, or some part of your campus. Visit this area at least three times and make a survey of the plants and animals that you find there, including any trees, shrubs, groundcover plants, insects, reptiles, amphibians, birds, and mammals. Also, take a small sample of the topsoil and find out what organisms are living there. (Be careful to get permission from whoever owns or manages the land before doing any digging.) Using guidebooks and other resources to help identify different species, record your findings, and categorize them into the general types of organisms listed above. Then do some research to find out about the ecosystem services that some or all of these organisms provide. Try to find and record five of these services. Finally, do some research to find a range of values that economists have assigned to these ecosystem services at the global level. Write a report summarizing your findings.

Chapter Review

Ecological Footprint Analysis

The table below compares five countries in terms of rain forest area and losses. Study the table and then answer the questions that follow.

1. What is the annual rate of tropical rain forest loss, as a percentage of total forest area, in each of the five countries? Answer by filling in the blank column in the table.

2. What is the annual rate of tropical deforestation collectively in all of the countries represented in the table?

3. According to the table, and assuming the rates of deforestation remain constant, which country’s tropical rain forest will be destroyed first?

4. Assuming the rate of deforestation in country C remains constant, how many years will it take for all of its tropical rain forests to be destroyed?

5. Assuming that a hectare ‍‍ of tropical rain forest absorbs 0.85 metric tons ‍‍ of carbon dioxide per year, what would be the total annual growth in the carbon footprint (carbon emitted but not absorbed by vegetation because of deforestation) in metric tons of carbon dioxide per year for each of the five countries in the table?

6.
7.

8. Below is where the book information came from Chapter 10

· 10.4b

Wilderness Areas

· 10.4c

Parks and Other Nature Reserves

· 10.4d

Designing and Managing Nature Reserves

· 10.4e

Sustaining Terrestrial Biodiversity: An Ecosystem Approach

· 10.4f

Protect Biodiversity Hotspots and Ecosystem Services

· 10.4g

Protecting Ecosystem Services

· 10.4h

Restoring Damaged Ecosystems

· 10.4i

Sharing Ecosystems with Other Species

· Tying It All Together

Sustaining Costa Rica’s Biodiversity

·
Chapter Review

·
Critical Thinking

·

Doing Environmental Science

·

Ecological Footprint Analysis

·

Chapter Introduction

·

Tropical Rain Forests Are Disappearing

· 3.1

Earth’s Life-Support System

· 3.1a

Earth’s Life-Support System Has Four Major Components

· 3.1b

Three Factors Sustain the Earth’s Life

· 3.2

Ecosystem Components

· 3.2a

Ecosystems Have Several Important Components

· 3.2b

Soil Is the Foundation of Life on Land

· 3.3

Energy in an Ecosystem

· 3.3a

Energy Flows through Ecosystems in Food Chains and Food Webs

· 3.3b
Some Ecosystems Produce Plant Matter Faster Than Others Do

· 3.4

Matter in an Ecosystem

· 3.4a
Nutrients Cycle Within and Among Ecosystems

· 3.4b

The Water Cycle

· 3.4c

The Carbon Cycle

· 3.4d

The Nitrogen Cycle

· 3.4e

The Phosphorus Cycle

· 3.5

How Do Scientists Study Ecosystems?

· 3.5a

Studying Ecosystems Directly

· 3.5b

Laboratory Research and Models

· 3.5c
Four Laws of Ecology

· Tying It All Together

Tropical Rain Forests and Sustainability

·

Chapter Review

·

Critical Thinking

·

Doing Environmental Science

·

Data Analysis

· Chapter Introduction

· Cleared area of tropical rainforest in Uganda (Africa)

·
·

·

Prill/ Shutterstock.com

ore Case StudyTropical Rain Forests Are Disappearing
Learning Objective

· LO 3.1State three reasons why we need to care about the ongoing loss of rain forests.

Tropical rain forests are found near the earth’s equator and contain an amazing variety of life. These lush forests are warm year round and have high humidity because it rains almost daily. Rain forests cover only 7% of the earth’s land but contain up to half of the world’s known plant and animal species found on land. The diversity of species in these forests makes them excellent natural laboratories in which to study 

ecosystems

—communities of organisms that interact with one another and with the physical environment of matter and energy in which they live.

To date, human activities have destroyed or degraded more than half of the earth’s rain forests (see chapter-opening 

photo

). People continue clearing the forests to grow crops, graze cattle, and build settlements (

Figure 3.1

). Ecologists warn that without protection, most of these ecologically important forests will be gone or severely degraded by the end of this century.

Figure 3.1

Natural capital degradation: Satellite image of the loss of tropical rain forest, cleared for farming, cattle grazing, and settlements, near the Bolivian city of Santa Cruz between June 1975 (left) and May 2003 (right). This is the latest available view of the area but forest degradation has continued since 2003.

Left: United Nations Environment Programme; United Nations Environment Programme

Why should we care that tropical rain forests are disappearing? Scientists give three reasons. First, clearing these forests causes the extinction of many plant and animal species by destroying the habitats where they live. The loss of key species in these forests can have a ripple effect that leads to the extinction of other species they help support.

Second, destroying these forests contributes to atmospheric warming and speeds up climate change. How does this occur? Eliminating large areas of trees faster than they can grow back means that there are fewer plants using photosynthesis to remove some of the excess carbon dioxide  emitted into the atmosphere when carbon-containing fossil fuels are burned. The resulting increased levels of  in the atmosphere contributes to atmospheric warming and climate change, which you will learn more about in 

Chapter 19

.

Third, large-scale loss of tropical rain forests can change regional weather patterns in ways that can prevent the regrowth of rain forests in cleared or degraded areas. When this irreversible ecological tipping point is reached, tropical rain forests in such areas become drier and less-diverse tropical grasslands.

In this chapter, you will learn how tropical rain forests and other ecosystems work, how human activities are affecting them, and how we can help sustain them.

3.1Earth’s Life-Support System

· LO 3.1ADescribe the four main systems, or spheres, that make up the earth’s life-support system.

· LO 3.1BExplain how the flow of energy from the sun and the greenhouse effect are connected within the biosphere.

· LO 3.1CExplain the roles that solar energy, nutrient cycling, and gravity play in sustaining life.

· 3.1aEarth’s Life-Support System Has Four Major Components

· The earth’s life-support system consists of four main systems (

Figure 3.2

) that interact with one another. They are the atmosphere (air), the hydrosphere (water), the geosphere (rock, soil, and sediment), and the biosphere (living things).

· Figure 3.2

· Natural capital: The earth consists of a land sphere (geosphere), an air sphere (atmosphere), a water sphere (hydrosphere), and a life sphere (biosphere).

·

·

The 

atmosphere

 is a spherical mass of air surrounding the earth’s surface that is held by gravity. Its innermost layer, the troposphere, extends about 19 kilometers (12 miles) above sea level at the equator and about 6 kilometers (4 miles) above the earth’s North and South Poles. The troposphere contains the air we breathe. It is 78% nitrogen  and 21% oxygen . The remaining 1% of air is mostly water vapor, carbon dioxide, and methane. The troposphere is the layer in which the earth’s weather occurs and where life can survive.

· The 

stratosphere

 is the atmospheric layer above the troposphere. It reaches 17 to 50 kilometers (12–31 miles) above the earth’s surface. The lower stratosphere, called the ozone layer, contains enough ozone  gas to filter out about 95% of the sun’s harmful ultraviolet (UV) radiation. It acts as a global sunscreen that allows life to exist on the earth’s surface.

· The 

hydrosphere

 contains all of the water on or near the earth’s surface. It is found as water vapor in the atmosphere, as liquid water on the surface and underground, and as ice—polar ice, icebergs, glaciers, and ice in frozen soil-layers called permafrost. Salty oceans that cover about 71% of the earth’s surface contain 97% of the planet’s water and support almost half of the world’s species. About 2.5% of the earth’s water is freshwater and three-fourths of that is ice.

· The 

geosphere

 contains the earth’s rocks, minerals, and soil. It consists of an intensely hot core, a thick mantle of very hot rock, and a thin outer crust of rock and soil. The crust’s upper portion contains soil chemicals or nutrients that organisms need to live, grow, and reproduce. It also contains nonrenewable fossil fuels—coal, oil, and natural gas—and minerals that we extract and use.

· The 

biosphere

 consists of the parts of the atmosphere, hydrosphere, and geosphere where life is found. If the earth were the size of an apple, the biosphere would be no thicker than the apple’s skin.

3.1bThree Factors Sustain the Earth’s Life

Life on the earth depends on three interconnected factors:

1. One-way flow of high-quality energy from the sun. The sun’s energy supports plant growth, which provides energy for plants and animals, in keeping with the solar energy principle of sustainability. As solar energy interacts with carbon dioxide , water vapor, and several other gases in the troposphere, it warms the troposphere—a process known as the 

greenhouse effect

 (

Figure 3.3

). Without this natural process, the earth would be too cold to support most of the forms of life we find here today.

2. Cycling of nutrients through parts of the biosphere. Nutrients are chemicals that organisms need to survive. Because the earth does not get significant inputs of matter from space, its fixed supply of nutrients must be recycled to support life. This is in keeping with the chemical cycling principle of sustainability.

3. Gravity allows the planet to hold on to its atmosphere and enables the movement and cycling of chemicals through air, water, soil, and organisms.

Figure 3.3

Greenhouse Earth. High-quality solar energy flows from the sun to the earth. It is degraded to lower-quality energy (mostly heat) as it interacts with the earth’s air, water, soil, and life forms, and eventually some of it returns to space. Certain gases in the earth’s atmosphere retain enough of the sun’s incoming energy as heat to warm the planet as a result of the greenhouse effect.

National Geographic Visual Atlas of the World. Washington, DC: National Geographic Society, 2008.

3.2Ecosystem Components

· LO 3.2AExplain the relationships among the biosphere, ecosystems, communities, populations, and organisms using an organism of your choice (which could even be you).

· LO 3.2BList two consumer organisms that feed on a common producer organism of your choice.

· LO 3.2CDescribe the diet of a detritus feeder and a decomposer and explain their role in chemical cycling.

· LO 3.2DWrite the chemical equations that represent photosynthesis and aerobic respiration.

· LO 3.2EList six components of soil.

· LO 3.2FExplain how topsoil supports terrestrial life.

3.2aEcosystems Have Several Important Components

Ecology is the science that focuses on how organisms interact with one another and with their nonliving physical environment of matter and energy. Scientists classify matter into levels of organization ranging from atoms to galaxies. Ecologists study five levels of matter—the biosphere, ecosystems, 

communities

, 

populations

, and 

organisms

—all shown and defined in 

Figure 3.4

.

Figure 3.4

Ecology focuses on the top five of these levels of the organization of matter in nature.

The biosphere and its ecosystems are made up of living (biotic) and nonliving (abiotic) components (

Figure 3.5

). Living components include plants, animals, and microbes. Nonliving components include water, air, nutrients, rocks, heat, and solar energy.

Figure 3.5

Key living (biotic) and nonliving (abiotic) components of an ecosystem in a field.

Ecologists assign each organism in an ecosystem to a feeding level, or 

trophic level

, based on its source of nutrients. Organisms are classified as producers and consumers based on whether they make (produce) or find (consume) their food.

Producers

 are organisms, such as green plants, that make the nutrients they need from compounds and energy obtained from their environment. In the process known as 

photosynthesis

, green plants capture solar energy that falls on their leaves. They use it to combine carbon dioxide and water to form carbohydrates such as glucose , which they store as a source of chemical energy. In the process, they emit oxygen  gas into the atmosphere. This oxygen keeps us and most other animal species alive. The following chemical reaction summarizes the overall process of photosynthesis.

About 2.8 billion years ago, producer organisms called cyanobacteria, most of them floating on the surface of the ocean, started carrying out photosynthesis and adding oxygen to the atmosphere. After several hundred million years, oxygen levels reached about 21%—high enough to keep humans and other oxygen-breathing animals alive.

Learning from Nature

Scientists hope to make a molecular-sized solar cell by mimicking how a leaf uses photosynthesis to capture solar energy. These artificial leaf films might be used to coat the roofs, windows, or walls of a building and provide electricity for most homes and other buildings.

On land, most producers are green plants such as trees and grasses. In freshwater and ocean ecosystems, algae and aquatic plants growing near shorelines are the major producers. In open water, the dominant producers are phytoplankton—mostly microscopic organisms that float or drift in the water.

Some producer bacteria live in dark and extremely hot water around fissures on the ocean floor. Their source of energy is heat from the earth’s interior, or geothermal energy. They are an exception to the solar energy principle of sustainability.

The other organisms in an ecosystem are 

consumers

 that cannot make their food. They get the nutrients they need by feeding on other producers, or other consumers or on the wastes and remains of producers and consumers.

There are several types of consumers. 

Primary consumers

, or 

herbivores

 (plant eaters), are animals that eat mostly green plants. Examples are caterpillars, giraffes, and zooplankton (tiny sea animals that feed on phytoplankton). 

Carnivores

 (meat eaters) are animals that feed on the flesh of other animals. Some carnivores, including spiders, lions (

Figure 3.6

), and most small fishes, are 

secondary consumers

 that feed on the flesh of herbivores. Other carnivores such as tigers, hawks, and killer whales (orcas) are 

tertiary

 (or higher-level) consumers that feed on the flesh of herbivores and other carnivores. Some of these relationships are shown in Figure 3.5. 

Omnivores

 such as pigs, rats, and humans eat both plants and animals.

Figure 3.6

This lioness (a carnivore) is feeding on a freshly killer zebra (an herbivore) in Kenya, Africa.

nelik/ Shutterstock.com

Critical Thinking

1. When you ate your most recent meal, were you an herbivore, a carnivore, or an omnivore?

Decomposers are consumers that get nourishment by breaking down (decomposing) the wastes or remains of plants and animals. These nutrients return to the soil, water, and air for reuse by producers. Most decomposers are bacteria and fungi, such as molds and mushrooms. Other consumers, called 

detritus feeders

, or detritivores, feed on the wastes or dead bodies (detritus) of other organisms. Examples are earthworms, soil insects, hyenas, and vultures (

Figure 3.7

).

Figure 3.7

The vultures and Marabou storks, eating the carcass of an animal that was killed by another animal, are detritivores.

javarman/ Shutterstock.com

Detritivores and decomposers can transform a fallen tree trunk into simple inorganic molecules that plants can absorb as nutrients (

Figure 3.8

). In natural ecosystems, the wastes and dead bodies of organisms are resources for other organisms in keeping with the chemical cycling principle of sustainability. Without decomposers and detritivores, many of which are microscopic organisms, the planet’s land surfaces would be buried in plant and animal wastes, dead animal bodies, and garbage.

Figure 3.8

Various detritivores and decomposers (mostly fungi and bacteria) can “feed on” or digest parts of a log and eventually convert its complex organic chemicals into simpler inorganic nutrients that can be taken up by producers.

Producers, consumers, and decomposers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells, this energy is released by 

aerobic respiration

, which uses oxygen to convert glucose and other organic compounds back into carbon dioxide and water. The overall chemical reaction for the aerobic respiration is shown in the following equation:

Some decomposers, such as yeast and some bacteria get the energy they need by breaking down glucose (or other organic compounds) in the absence of oxygen. This form of cellular respiration is called 

anaerobic respiration

, or 

fermentation

. Instead of carbon dioxide and water, the products of this process are compounds such as methane gas , ethyl alcohol , acetic acid (, the key component of vinegar), and hydrogen sulfide (, a highly poisonous gas that smells like rotten eggs). Note that all organisms get their energy from aerobic or aerobic respiration, but only plants carry out photosynthesis.

To summarize, ecosystems and the biosphere are sustained by the one-way energy flow from the sun and the nutrient cycling of key materials—in keeping with two of the scientific principles of sustainability (

Figure 3.9

).

Figure 3.9

Natural capital: The main components of an ecosystem are energy, chemicals, and organisms. Nutrient cycling and the flow of energy—first from the sun, then through organisms, and finally into the environment as low-quality heat—link these components.

3.2bSoil Is the Foundation of Life on Land

Soil

 is a complex mixture of rock pieces and particles, mineral nutrients, decaying organic matter, water, air, and living organisms that support plant life, which supports animal life. Life on land depends on roughly 15 centimeters (6 inches) of topsoil. The minerals that make up your muscles, bones, and most other parts of your body come almost entirely from soil.

Soil is one of the most important components of the earth’s natural capital. It purifies water and supplies most of the nutrients needed for plant growth. Through aerobic respiration, organisms living in soil remove some of the carbon dioxide in the atmosphere and store it as organic carbon compounds, thereby helping to control the earth’s climate.

Soil formation begins when physical, chemical, and biological processes called 

weathering

 break down bedrock into small pieces. Various forms of plant and animal life begin living on the weathered particles. Their wastes and decaying bodies add organic matter and minerals to the slowly forming soil. Decomposers and detritivores break down fallen leaves and wood (Figure 3.8) and add organic matter and nutrients to the soil. Air (mostly nitrogen and oxygen) and water occupy pores or spaces between soil particles. Over hundreds to thousands of years, various types of life build up distinct layers of mineral and organic matter on a soil’s original bedrock.

Most mature soils contain several horizontal layers or horizons. A cross-sectional view of the horizons of a soil is called a 

soil profile

 (

Figure 3.10

, right). The major horizons in a mature soil are O (leaf litter), A (topsoil), B (subsoil), and C (weathered parent material), which build up over the parent material. Each layer has a distinct texture, composition, and thickness that vary with the soils formed in different climates and biomes such as deserts, grasslands, and forests (

Figure 3.11

). Soil forms faster in wet, warm climates.

Figure 3.10

Natural capital: Generalized soil formation and soil profile.

Critical Thinking:

1. What role do you think the tree in this figure plays in soil formation? How might the soil formation process change if the tree were removed?

Figure 3.11

Natural capital: Soil profiles of the principal soil types typically found in five types of terrestrial ecosystems.

The roots of most plants and the majority of a soil’s organic matter are found in the soil’s two upper layers: the O-horizon of leaf litter and the A-horizon of topsoil. In a fertile soil, these two layers teem with bacteria, fungi, earthworms, and numerous small insects, all interacting by feeding on and decomposing one another. The leaf litter and topsoil layers are also habitats for larger animals such as snails, reptiles, amphibians, and burrowing animals such as moles.

Every handful of topsoil contains billions of bacteria and other decomposer organisms. They break down some of the soil’s complex organic compounds into a mixture of the partially decomposed bodies of dead plants and animals, called humus. A fertile soil that produces high crop yields has a thick topsoil layer with a lot of humus mixed with minerals from weathered rock.

Moisture in topsoil dissolves nutrients needed for plant growth. The resulting solution is drawn up by the roots of the plants and transported through their stems into the leaves of plants. This movement of nutrients from the topsoil to plant leaves and to insects and other animals that eat the leaves is part of the chemical cycling process essential for the earth’s life. Healthy soils retain more water and help reduce the severity of drought by releasing some of the water into the atmosphere. 

Figure 3.12

 shows the movement of plant nutrients in soils.

Figure 3.12

Pathways of plant nutrients in soils.

The color of topsoil indicates how useful it is for growing crops or other plants. Black or dark brown topsoil is rich in nitrogen and organic matter. A gray, bright yellow, or red topsoil is low in organic matter and needs the addition of nitrogen to support most crops.

The B-horizon (subsoil) and the C-horizon (parent material) contain most of a soil’s inorganic matter, mostly broken-down rock, consisting of various mixtures of sand, silt, clay, and gravel. The C-horizon sits on the soil’s parent material, which is often bedrock.

Soils can include particles of three different sizes: very small clay particles, medium-size silt particles, and larger sand particles. The relative amounts of these different sizes and types of these mineral particles, the composition of organic materials, and the amount of space between the particles determine the texture of a soil. A soil’s texture affects how rapidly water flows through it (

Figure 3.13

).

Figure 3.13

Natural capital: The size, shape, and degree of clumping of soil particles determine the number and volume of spaces for air and water within a soil. Water can flow more easily through soils with more spaces (left) than through soils with fewer spaces (right).

Soil is a renewable resource but it is renewed very slowly and becomes a nonrenewable resource if we deplete it faster than nature can replenish it. The formation of just 2.5 centimeters (1 inch) of topsoil can take hundreds to thousands of years. Removing plant cover from soil exposes its topsoil to erosion by water and wind. This explains why protecting and renewing topsoil is a key to sustainability. You will learn more about soil erosion and soil conservation in 

Chapter 12

.

3.3aEnergy Flows through Ecosystems in Food Chains and Food Webs

Chemical energy, stored as nutrients in the bodies and wastes of organisms, flows through ecosystems from one trophic (feeding) level to another in food chains and food webs. A sequence of organisms with each one serving as a source of nutrients or energy for the next level of organisms is called a 

food chain

 (

Figure 3.14

).

Figure 3.14

In a food chain, chemical energy in nutrients flows through various trophic levels.

Critical Thinking:

1. Think about what you ate for breakfast. At what level or levels on a food chain were you eating?

Every use and transfer of energy by organisms from one feeding level to another involves a loss of some high-quality energy to the environment as low-quality energy in the form of heat, as required with the second law of thermodynamics. A graphic display of the energy loss at each trophic level is called a 

pyramid of energy flow

. 

Figure 3.15

 illustrates this energy loss for a food chain, assuming a 90% energy loss for each level of the chain.

Figure 3.15

Generalized pyramid of energy flow showing the decrease in usable chemical energy available at each succeeding trophic level in a food chain or food web. This model assumes that with each transfer from one trophic level to another, there is a 90% loss of usable energy to the environment in the form of low-quality heat. (Calories and joules are used to measure energy. .)

Critical Thinking:

1. Why is a vegetarian diet more energy efficient than a meat-based diet?

Connections

Energy Flow and Feeding People

Energy flow pyramids explain why the earth could support more people if they all ate at a low trophic level by consuming grains, vegetables, and fruits directly rather than passing such crops through another trophic level and eating the meat of herbivores such as cattle, pigs, sheep, and chickens. About two-thirds of the world’s people survive primarily by eating wheat, rice, and corn at the first trophic level, mostly because they cannot afford to eat much meat.

Ecologists can estimate the number of organisms feeding at each trophic level. Here is a hypothetical example: 100,000 blades of grass (producer) might support 30 rabbits (herbivore), which might support 1 fox (carnivore). Ecologists also measure 

biomass

—the total mass of organisms in each trophic level—as illustrated by this hypothetical example: 1,000 kilograms (2,200 pounds) of producers might provide 100 kilograms (220 pounds) of food for herbivores, which might provide 10 kilograms (22 pounds) of food for carnivores, which might supply a top carnivore with 1 kilogram (2.2 pounds) of food.

In natural ecosystems, most consumers feed on more than one type of organism, and most organisms are eaten or decomposed by more than one type of consumer. Because of this, organisms in most ecosystems form a complex network of interconnected food chains called a 

food web

. Food chains and food webs show how producers, consumers, and decomposers are connected to one another as energy flows through trophic levels in an ecosystem. 

Figure 3.16

 shows an aquatic food web and 

Figure 3.17

 shows a terrestrial food web.

Figure 3.16

A greatly simplified aquatic food web found in the southern hemisphere. The shaded middle area shows a simple food chain that is part of these complex interacting feeding relationships. Many more participants in the web, including an array of decomposer and detritus feeder organisms, are not shown here.

Figure 3.17

Greatly simplified terrestrial food web found in a temperate desert ecosystem. The shaded middle area shows a simple food chain that is part of these complex interacting feeding relationships. Many more participants in the web, including an array of decomposer and detritus feeder organisms, are not shown.

Learning from Nature

There is no waste in nature because the wastes or remains of one organism provide food for another. Scientists and engineers study food webs to learn how to reduce or eliminate food waste and some of the other wastes we produce.

3.3bSome Ecosystems Produce Plant Matter Faster Than Others Do

Scientists measure the rates at which ecosystems produce chemical energy to compare ecosystems and understand how they interact. 

Gross primary productivity (GPP)

 is the rate at which an ecosystem’s producers (such as plants and phytoplankton) convert solar energy into chemical energy stored in compounds found in their tissues. It is usually measured in terms of energy production per unit area over a given time span, such as kilocalories per square meter per year . To stay alive, grow, and reproduce, producers must use some of their stored chemical energy for their own aerobic respiration.

Net primary productivity (NPP)

 is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration. NPP measures how fast producers can make the chemical energy that is stored in their tissues and that is potentially available to the consumers in an ecosystem.

Gross primary productivity is similar to the rate at which you make money, or the number of dollars you earn per year. Net primary productivity is similar to the amount of money earned per year that you can spend after subtracting your expenses such as the costs of transportation, clothes, food, and supplies.

Terrestrial ecosystems and aquatic life zones differ in their NPP as illustrated in 

Figure 3.18

. Despite its low NPP, the open ocean produces more of the earth’s biomass per year than any other ecosystem or life zone. This happens because oceans cover 71% of the earth’s surface and contain huge numbers of phytoplankton and other producers.

Figure 3.18

Estimated annual average net primary productivity in major life zones and ecosystems expressed as kilocalories of energy produced per square meter per year .

Question:

1. What are the three most productive and the three least productive systems?

(Compiled by the authors using data from R. H. Whittaker, Communities and Ecosystems, 2nd ed., New York: Macmillan, 1975.)

Tropical rain forests have a high net primary productivity (NPP) because they have a large number and variety of producer trees and other plants to support a large number of consumers. When such forests are cleared (Core Case Study) or burned to make way for crops or for grazing cattle, they suffer a sharp drop in net primary productivity and lose plant and animal species.

Only the plant matter represented by NPP is available as nutrients for consumers. Thus, the planet’s NPP ultimately limits the number of consumers (including humans) that can survive on the earth. This is one of nature’s important lessons.

3.4aNutrients Cycle Within and Among Ecosystems

The elements and compounds that make up nutrients move continually through air, water, soil, rock, and living organisms within ecosystems in cycles called 

nutrient cycles

, or 

biogeochemical cycles

 (life-earth-chemical cycles). They represent the chemical cycling principle of sustainability in action. These cycles are driven directly or indirectly by incoming solar energy and by the earth’s gravity. They include the hydrologic (water), carbon, nitrogen, and phosphorus cycles. Human activities are altering these important components of the earth’s natural capital (

Figure 1.3

).

Connections

Nutrient Cycles and Life

Nutrient cycles connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of an oak leaf, a dinosaur’s skin, or a layer of limestone rock. Your grandmother, George Washington, or a hunter–gatherer who lived 25,000 years ago may have inhaled some of the nitrogen  molecules you just inhaled.

3.4bThe Water Cycle

Water  is an amazing substance (

Science Focus 3.1

) that is essential for life on the earth. The 

hydrologic cycle

, or the 

water cycle

, collects, purifies, and distributes the earth’s fixed supply of water, as shown in 

Figure 3.19

.

Figure 3.19

Natural capital: Simplified model of the water cycle, or hydrologic cycle, in which water circulates in various physical forms within the biosphere. The red arrows and boxes identify major effects of human activities on this cycle.

Critical Thinking:

1. What are three ways in which your lifestyle directly or indirectly affects the hydrologic cycle?

Science Focus 3.1

Water’s Unique Properties

Water  is a remarkable substance with a unique combination of properties:

· Water exists as a liquid over a wide temperature range because of forces of attraction between its molecules. If liquid water had a much narrower range of temperatures between freezing and boiling, the oceans would probably have frozen solid or boiled away long ago.

· Liquid water changes temperature slowly because it can store a large amount of heat without a large change in its own temperature. This helps protect living organisms from temperature changes, moderates the earth’s climate, and makes water an excellent coolant for car engines and power plants.

· It takes a large amount of energy to evaporate water because of the attractive forces between its molecules. Water absorbs large amounts of heat as it changes into water vapor and releases this heat as the vapor condenses back to liquid water. This helps to distribute heat throughout the world and to determine regional and local climates. It also makes evaporation a cooling process—explaining why you feel cooler when perspiration evaporates from your skin.

· Liquid water can dissolve more compounds than other liquids. Water carries dissolved nutrients into the tissues of living organisms, flushes waste products out of those tissues, serves as an all-purpose cleanser, and helps to remove and dilute the water-soluble wastes of civilization. This property also means that water-soluble wastes can easily pollute water.

· Water filters out wavelengths of the sun’s ultraviolet (UV) radiation that would harm some aquatic organisms. This allows life to exist in the upper layer of aquatic systems.

· Unlike most liquids, water expands when it freezes. This means that ice floats on water because it has a lower density (mass per unit of volume) than liquid water has. Otherwise, lakes and streams in cold climates would freeze solid from the bottom up and loose most of their aquatic life. Because water expands on freezing, it can break pipes, crack a car’s engine block (if it does not contain antifreeze), break up pavement, and fracture rocks (which helps form soil).

Critical Thinking

1. Pick two of the properties listed above and, for each property, explain how life on the earth would be different if the property did not exist.

The sun provides the energy needed to power the water cycle. Incoming solar energy causes evaporation—the conversion of some of the liquid water in the earth’s oceans, lakes, rivers, soil, and plants to vapor. Most water vapor rises into the atmosphere, where it condenses into droplets in clouds. Gravity then draws the water back to the earth’s surface as precipitation such as rain, snow, or sleet.

Most precipitation falling on terrestrial ecosystems becomes 

surface runoff

—water that flows into streams, rivers, lakes, wetlands, and the ocean. Some of the water evaporates back into the atmosphere, while some seeps into the upper layers of soils and is used by plants,

Water that seeps deeper into the soil is known as 

groundwater

. Groundwater collects in 

aquifers

, which are underground layers of water-bearing rock. Some precipitation is converted to ice that is stored in glaciers.

0.024%

Percentage of the earth’s freshwater supply that is available to humans and other species

Only about 0.024% of the earth’s vast water supply is available to humans and other species as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest of the planet’s water is too salty, is too deep underground to extract at affordable prices, or is stored as ice in glaciers. Because water is good at dissolving many different compounds, it can easily be polluted. However, natural processes in the water cycle can purify water, which makes the cycle a vital ecosystem service.

Human activities alter the water cycle in three major ways (see the red arrows and boxes in 
Figure 3.19
). First, people sometimes withdraw freshwater from rivers, lakes, and aquifers at rates faster than natural processes can replace it. As a result, some aquifers are being depleted and some rivers no longer flow to the ocean.

Second, people clear vegetation from land for agriculture, mining, road building, and other activities, and cover much of the land with buildings, concrete, and asphalt. This increases water runoff and reduces infiltration that would normally recharge groundwater supplies.

Third, people drain and fill wetlands for farming and urban development. Left undisturbed, wetlands provide the ecosystem service of flood control. Wetlands act like sponges to absorb and hold overflows of water from drenching rains or rapidly melting snow.

3.4cThe Carbon Cycle

Carbon is the basic building block of the carbohydrates, fats, proteins, DNA, and other organic compounds required for life. Various compounds of carbon circulate through the biosphere, the atmosphere, and parts of the hydrosphere, in the 

carbon cycle

 shown in 

Figure 3.20

.

Figure 3.20

Natural capital: Simplified model showing the circulation of various chemical forms of carbon in the global carbon cycle. Red arrows show major harmful impacts of human activities on this cycle. (Yellow box sizes do not show relative reservoir sizes.)

Critical Thinking:

1. What are three ways in which you directly or indirectly affect the carbon cycle?

A key component of the carbon cycle is carbon dioxide  gas. It makes up about 0.040% of the volume of the troposphere. Carbon dioxide (along with water vapor in the water cycle) affects the temperature of the atmosphere through the greenhouse effect (Figure 3.3) and thus plays a major role in determining the earth’s climate. If the carbon cycle removes too much  from the atmosphere, the atmosphere will cool, and if it generates too much , the atmosphere will get warmer. Thus, even slight changes in this cycle caused by natural or human factors can affect the earth’s climate, which helps determine the types of life that can exist in various places, as you will learn in 

Chapter 7

.

Carbon is cycled through the biosphere by a combination of photosynthesis by producers, which removes  from the air and water, and aerobic respiration by producers, consumers, and decomposers that add  to the atmosphere. Typically,  remains in the atmosphere for 100 years or more. Some of the  in the atmosphere dissolves in the ocean. In the ocean, decomposers release carbon that is stored as insoluble carbonate minerals and rocks in bottom sediment for long periods.

Over millions of years, some of the carbon in deeply buried deposits of dead plant matter and algae have been converted into carbon-containing fossil fuels such as coal, oil, and natural gas (

Figure 3.20). Within a few hundred years, we have extracted and burned huge quantities of fossil fuels that took millions of years to form, which explains why fossil fuels are classified as nonrenewable resources. This has added large quantities of  to the atmosphere and altered the carbon cycle (see red arrows in Figure 3.20). In effect, we have been adding  to the atmosphere faster than the carbon cycle can recycle it.

As a result, levels of  in the atmosphere have been rising sharply since about 1960. There is considerable scientific evidence that this disruption of the carbon cycle is helping to warm the atmosphere and change the earth’s climate. The oceans remove some of this  from the atmosphere but as a result, the acidity of ocean waters is rising. This ocean acidification is bad news for organisms that are adapted to less-acidic ocean waters and is a serious and growing global environmental problem.

Another way in which we alter the carbon cycle is by clearing carbon-absorbing vegetation from forests, especially tropical forests (

Figure 3.1), faster than they can grow back (Core Case Study). This reduces the ability of the carbon cycle to remove excess  from the atmosphere and contributes to climate change, which we discuss in Chapter 19.

3.4dThe Nitrogen Cycle

Nitrogen gas  makes up 78% of the volume of the lower atmosphere and is a crucial component of proteins, many vitamins, and DNA. However,  in the atmosphere cannot be absorbed and used directly as a nutrient by plants or other organisms. It becomes a plant nutrient only as a component of nitrogen-containing ammonia , ammonium ions , and nitrate ions .

These chemical forms of nitrogen are created in the 

nitrogen cycle

 (

Figure 3.21

) by lightning, which converts  to , and by specialized bacteria in topsoil. Other bacteria in topsoil and in the bottom sediments of aquatic systems convert  to  and nitrate ions  that are taken up by the roots of plants. Plants then use these forms of nitrogen to produce various proteins, nucleic acids, and vitamins. Animals that eat plants consume these nitrogen-containing compounds, as do detritus feeders and decomposers. Bacteria in waterlogged soil and bottom sediments of lakes, oceans, swamps, and bogs convert nitrogen compounds into nitrogen gas , which is released to the atmosphere to begin the nitrogen cycle again.

Figure 3.21

Natural capital: Simplified model showing the circulation of various chemical forms of nitrogen in the nitrogen cycle, with major harmful human impacts shown by the red arrows. (Yellow box sizes do not show relative reservoir sizes.)

Critical Thinking:

1. What are two ways in which the carbon cycle and the nitrogen cycle are linked?

Humans intervene in the nitrogen cycle in several ways (see red arrows in 

Figure 3.21
). When we burn gasoline and other fuels, the resulting high temperatures convert some of the  and  in air to nitric oxide (NO). In the atmosphere, NO can be converted to nitrogen dioxide gas  and nitric acid vapor , which can return to the earth’s surface as damaging acid deposition, commonly called acid rain. Acid rain damages stone buildings and statues. It can also kill forests and other plant ecosystems, and wipe out life in ponds and lakes.

We remove large amounts of  from the atmosphere and combine it with  to make ammonia  and ammonium ions  used to make fertilizers. In addition, we add the greenhouse gas nitrous oxide  to the atmosphere through the action of anaerobic bacteria on nitrogen-containing fertilizer or animal manure applied to the soil. This greenhouse gas can warm the atmosphere and take part in reactions that deplete stratospheric ozone, which keeps most of the sun’s harmful ultraviolet (UV) radiation from reaching the earth’s surface.

People also alter the nitrogen cycle in aquatic ecosystems by adding excess nitrates . The nitrates contaminate bodies of water through agricultural runoff of fertilizers, animal manure, and discharges from municipal sewage treatment systems. This plant nutrient can cause excessive growth of algae that can disrupt aquatic systems. Our nitrogen inputs into the environment have risen sharply and are projected to continue rising (

Figure 3.22

).

Figure 3.22

Global trends in the inputs of nitrogen into the atmosphere from human activities, with projections to 2050.

Data Analysis:

1. By what percentage did the overall nitrogen input increase between 1960 and 2000? By what percentage is it projected to increase between 2000 and 2050?

Compiled by the authors using data from the Millennium Ecosystem Assessment and the Fertilizer Industry Association

3.4eThe Phosphorus Cycle

Phosphorus (P) is an element that is essential for living things. It is necessary for the production of DNA and cell membranes, and is important for the formation of bones and teeth.

The cyclic movement of phosphorus (P) through water, the earth’s crust, and living organisms is called the 

phosphorus cycle

 (

Figure 3.23

). Most phosphorus compounds in this cycle contain phosphate ions , which are an important plant nutrient. Phosphorus does not cycle through the atmosphere because few of its compounds exist as a gas. Phosphorus also cycles more slowly than water, carbon, and nitrogen.

Figure 3.23

Natural capital: Simplified model showing the circulation of various chemical forms of phosphorus (mostly phosphates) in the phosphorus cycle, with major harmful human impacts shown by the red arrows. (Yellow box sizes do not show relative reservoir sizes.)

Critical Thinking:

1. What are two ways in which the phosphorus cycle and the nitrogen cycle are linked?

As water runs over exposed rocks, it slowly erodes inorganic compounds that contain phosphate ions. Water carries these ions into the soil, where they are absorbed by the roots of plants and by other producers. Phosphate compounds are then transferred by food webs from producers to consumers and eventually to detritus feeders and decomposers.

Much of the phosphate that erodes from rocks is carried into rivers, streams, and the ocean, where phosphates can be deposited as marine sediments and remain trapped for millions of years. Over time, geological processes can uplift and expose these seafloor deposits, from which phosphate can be eroded and re-enter the phosphorus cycle.

Most soils contain little phosphate, which often limits plant growth. For this reason, people often fertilize soil by adding phosphorus as phosphate salts mined from the ground. Lack of phosphorus also limits the growth of producer populations in many freshwater streams and lakes. This is because phosphate salts are only slightly soluble in water and do not release many phosphate ions to producers in aquatic systems.

Human activities, including the removal of large amounts of phosphate from the earth to make fertilizer, disrupt the phosphorus cycle (see red arrows in Figure 3.23). Clearing tropical forests (Core Case Study) exposes and erodes the topsoil, which reduces phosphate levels in tropical soils.

Eroded topsoil and fertilizer washed from fertilized crop fields, lawns, and golf courses carry large quantities of phosphate ions into streams, lakes, and oceans. There they stimulate the growth of producers such as algae and various aquatic plants, which can upset chemical cycling and other processes in bodies of water. According to a number of scientific studies, we are disrupting the phosphorus cycle because our inputs of phosphorus into the environment (primarily for use as fertilizer) have exceeded the planet’s environmental limit for phosphorus (

Science Focus 3.2

).

Science Focus 3.2

Planetary Boundaries

For most of the past 10,000–12,000 years, humans have been living in an era called the Holocene. During this era, we have enjoyed a favorable climate and other environmental conditions. This general stability allowed the human population to grow, develop agriculture, and take over a large share of the earth’s land and other resources (

Figure 1.9

).

Most geologists contend that we are still living in the Holocene era, but some scientists disagree. According to them, when the Industrial Revolution began (around 1750) we entered an era called the Anthropocene (the era of man or humans). In this new era, our ecological footprints have expanded significantly (Figure 1.9 and 

Figure 1.10

) and are changing and stressing the earth’s life-support system, especially since 1950.

In 2015, an international team of 18 leading researchers in their fields, led by Will Steffen and Johan Rockstrom of the Stockholm Resilience Centre, published a paper estimating how close we are to exceeding several major planetary boundaries, or ecological tipping points, because of human activities (

Figure 3.A

). They warn that exceeding them could change how the planet operates and could trigger abrupt and long-lasting or irreversible environmental changes. This could seriously degrade the earth’s life-support system and our economies.

Figure 3.A

Planetary boundaries for ten major components of the earth’s life-support system. A team of scientists estimated that human activities have exceeded the boundary limits for three systems (shown in red) and are close to the limits for five other systems (shown in orange). There is not enough information to evaluate the other two systems (shown in white).

(Compiled by the authors using data from Johan Rockström, Paul Crutzen, and James Hansen, et al., 2009, “Planetary Boundaries: Exploring the Safe Operating Space for Humanity,” Ecology and Society, vol. 14, no. 2, p. 32.); Photo: Sailorr/ Shutterstock.com

The researchers estimated that we have exceeded or nearly exceeded several boundaries, including disruption of the nitrogen and phosphorus cycles, mostly from greatly increased use of fertilizers to produce food; biodiversity loss from replacing biologically diverse forests and grasslands with simplified fields of single crops; land system change from agriculture and urban development; and climate change from disrupting the carbon cycle, mostly by overloading it with carbon dioxide produced by the burning of fossil fuels.

However, there is an urgent need for more research to verify these findings and fill in the missing data on these planetary boundaries. This would help scientists to further evaluate how close we are to exceeding them and how exceeding them could affect humans, other species, and the earth’s life-support systems. Regardless of what we call the era we are living in, such information, combined with taking action to live more sustainably, could help us to avoid exceeding such boundaries by shrinking our ecological footprints while expanding our beneficial environmental impacts.

Critical Thinking

1. Select one of the planetary boundaries shown in 

Figure 3.A and think about how exceeding that boundary might speed our exceeding one or more other boundaries.

Learning from Nature

Scientists study the water, carbon, nitrogen, and phosphorus cycles to help us learn how to recycle the wastes we create.

3.5aStudying Ecosystems Directly

Ecologists and other scientists use several approaches to increase their scientific understanding of ecosystems. These approaches include field and laboratory research and mathematical and other types of models.

Field research involves going into forests and other natural settings to study ecosystems. Ecologists use a variety of methods for field research. They include collecting water and soil samples, identifying and studying the species in an area, observing feeding behaviors, and using global positioning system (GPS) to track the movements of animals. Most of what we know about ecosystems has come from such research (

Individuals Matter 3.1

). GREEN CAREER: Ecologist

Individuals Matter 3.1

Thomas E. Lovejoy—Forest Researcher and Biodiversity Educator

Luiz Rampelotto/ZUMAPRESS/Newscom

For several decades, conservation biologist and National Geographic Explorer Thomas E. Lovejoy has played a major role in educating scientists and the public about the need to understand and protect tropical forests. He has carried out research in the Amazon forests of Brazil since 1965, which focused on estimating the minimum area necessary for sustaining biodiversity in national parks and biological reserves in tropical forests. In 1980, he coined the term biological diversity.

Lovejoy served as the principal adviser for the popular and widely acclaimed public television series Nature. He has also written numerous articles and books on issues related to conserving biodiversity. In addition to teaching environmental science and policy at George Mason University, he has held several prominent posts, including director of the World Wildlife Fund’s conservation program, president of the Society for Conservation Biology, and executive director of the U.N. Environment Programme (UNEP). In 2012, he was awarded the Blue Planet Prize for his efforts to understand and sustain the earth’s biodiversity.

Scientists also use a variety of methods to study tropical forests (Core Case Study). Some erect construction cranes to reach the canopies. This, along with climbing trees and installing rope walkways between treetops, helps them identify and observe the diversity of species living or feeding in these treetop habitats.

Learning from Nature

Scientists are developing a robot modeled on the inchworm, which works its way up tree trunks by using sensors to feel for surfaces that allow for good grip. These tree-climbing robots could carry equipment up into trees for forest researchers.

Ecologists carry out controlled experiments by isolating and changing a variable in part of an area and comparing the results with nearby unchanged areas. You learned about a classic example of this in the 

Core Case Study of 

Chapter 2

.

Scientists also use aircraft and satellites equipped with sophisticated cameras and remote sensing devices to scan and collect data on the earth’s surface. They use geographic information system (GIS) software to capture, store, analyze, and display such data. For example, GIS software can convert digital satellite images into global, regional, and local maps. These maps show variations in vegetation, gross primary productivity, soil erosion, deforestation, air pollution emissions, water usage, drought, flooding, pest outbreaks, and other variables.

Some researchers attach tiny radio transmitters to animals and use global positioning systems (GPSs) to track where and how far animals go. This technology is important for studying endangered species. Scientists also study nature by using cell phone cameras and mounting time-lapse cameras or video cameras on small drones and on stationary objects such as trees to capture images of wildlife. GREEN CAREERS: GIS Analyst; Remote Sensing Analyst

3.5bLaboratory Research and Models

Ecologists supplement their field research by conducting laboratory research. In laboratories, scientists create, simplified systems in containers such as culture tubes, bottles, aquariums, and greenhouses, and in indoor and outdoor chambers. In these structures, they control temperature, light, , humidity, and other variables.

These systems make it easier for scientists to carry out controlled experiments. Laboratory experiments are often faster and less costly than similar experiments in the field. However, scientists must consider how well their scientific observations and measurements in simplified, controlled systems in laboratory conditions reflect what takes place under the more complex and often-changing conditions found in nature.

Since the late 1960s, ecologists have developed mathematical models that simulate ecosystems, and they run the models on high-speed supercomputers. The models help them understand large and complex systems, such as lakes, oceans, forests, and the earth’s climate, that cannot be adequately studied and modeled in field or laboratory research. GREEN CAREER: Ecosystem modeler

Ecologists call for greatly increased research on the condition of the world’s ecosystems to see how they are changing and how well they can adapt to projected changing environmental conditions during this century. This would help scientists develop strategies for preventing or slowing their degradation.

.5cFour Laws of Ecology

Here are four basic principles or laws of ecology proposed by ecologist Barry Commoner in 1971.

·
Everything is connected to everything else. (Interdependence

) Humans and other species are connected to and dependent on other species and on the earth for their survival. The challenge of ecology and environmental science is to identify these connections in nature and learn which ones are the most important.

·

Everything must go somewhere. (There is no “away.”)
 There is no “away” to which we can throw our wastes. Because of the law of conservation of matter, the atoms in the earth’s mater can neither be created nor destroyed. Matter can be transformed into different chemicals and materials but will always be around in some form because the atoms that makeup all matter cannot be destroyed (law of conservation of matter, 

Chapter 2

). There is no waste in nature because the wastes of organisms are recycled and become resources (nutrients) for other organisms based on the chemical (nutrient) recycling principle of sustainability.

·
There is no free lunch. (Everything costs something.)
 Everything that we do to our life-support system has an environmental cost and often a financial one. Many people treat natural resources such as clean air, clean water, wildlife, and public lands (such as protected wilderness areas) as “free” resources that anyone can use. The cumulative effect of large numbers of people using such resources can pollute, degrade, or deplete them, and this is the tragedy of the commons (

Chapter 1

). Then governments and taxpayers end up paying the bills for expensive environmental cleanup, restoration, and wildlife protection that could have been prevented.

·
Nature knows best.
 Over billions of years, nature has experienced catastrophic and long-lasting environmental changes, including five mass extinctions of the earth’s species. Despite these events, nature has sustained a variety of life on the earth for billions of years. Biomimicry (Core Case Study, 
Chapter 1
) is the scientific effort to identify and copy these lessons from nature.

Observing these four laws would help us avoid going beyond ecological tipping points that could cause severe environmental degradation and economic disruption. Examples of such tipping points include:

· Severe disruption of key chemical cycles

· Significant reduction of life-sustaining biodiversity from excessive losses of species, the natural ecosystems where they live, and the ecosystem services these ecosystems provide

· Climate change from increasing levels of  and other greenhouse gases emitted into the atmosphere by burning carbon containing fossil fuels and other human activities

· Ocean acidification and disruption of marine ecosystems caused by absorption of some of the  emitted into the atmosphere, mostly from burning fossil fuels

· Ozone depletion—the reduction of ozone in the stratosphere that protects life on land from the sun’s harmful ultraviolet (UV) radiation, caused by chemicals emitted into the atmosphere through human activities

· Human consumption of water faster than it can be renewed by the water cycle

· Increased air and water pollution resulting from failure to enact and enforce pollution control laws and regulations.

The scientific challenge is to

1. identify the levels of such tipping points and the projected effects of exceeding them on ecosystems, human well-being, and human economies, and

2. develop strategies for not exceeding estimated tipping point levels.

Big Ideas

· Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.

· Some organisms produce the nutrients they need, others survive by consuming other organisms, and still others live on the wastes and remains of organisms while recycling nutrients that are used again by producer organisms.

· Human activities are altering the chemical cycling of nutrients and the flow of energy through food chains and webs in ecosystems.

.5cFour Laws of Ecology

Here are four basic principles or laws of ecology proposed by ecologist Barry Commoner in 1971.

·
Everything is connected to everything else. (Interdependence
) Humans and other species are connected to and dependent on other species and on the earth for their survival. The challenge of ecology and environmental science is to identify these connections in nature and learn which ones are the most important.

·
Everything must go somewhere. (There is no “away.”)
 There is no “away” to which we can throw our wastes. Because of the law of conservation of matter, the atoms in the earth’s mater can neither be created nor destroyed. Matter can be transformed into different chemicals and materials but will always be around in some form because the atoms that makeup all matter cannot be destroyed (law of conservation of matter, 
Chapter 2
). There is no waste in nature because the wastes of organisms are recycled and become resources (nutrients) for other organisms based on the chemical (nutrient) recycling principle of sustainability.

·
There is no free lunch. (Everything costs something.)
 Everything that we do to our life-support system has an environmental cost and often a financial one. Many people treat natural resources such as clean air, clean water, wildlife, and public lands (such as protected wilderness areas) as “free” resources that anyone can use. The cumulative effect of large numbers of people using such resources can pollute, degrade, or deplete them, and this is the tragedy of the commons (
Chapter 1
). Then governments and taxpayers end up paying the bills for expensive environmental cleanup, restoration, and wildlife protection that could have been prevented.
·
Nature knows best.
 Over billions of years, nature has experienced catastrophic and long-lasting environmental changes, including five mass extinctions of the earth’s species. Despite these events, nature has sustained a variety of life on the earth for billions of years. Biomimicry (Core Case Study, 
Chapter 1
) is the scientific effort to identify and copy these lessons from nature.
Observing these four laws would help us avoid going beyond ecological tipping points that could cause severe environmental degradation and economic disruption. Examples of such tipping points include:
· Severe disruption of key chemical cycles
· Significant reduction of life-sustaining biodiversity from excessive losses of species, the natural ecosystems where they live, and the ecosystem services these ecosystems provide
· Climate change from increasing levels of  and other greenhouse gases emitted into the atmosphere by burning carbon containing fossil fuels and other human activities
· Ocean acidification and disruption of marine ecosystems caused by absorption of some of the  emitted into the atmosphere, mostly from burning fossil fuels
· Ozone depletion—the reduction of ozone in the stratosphere that protects life on land from the sun’s harmful ultraviolet (UV) radiation, caused by chemicals emitted into the atmosphere through human activities
· Human consumption of water faster than it can be renewed by the water cycle
· Increased air and water pollution resulting from failure to enact and enforce pollution control laws and regulations.
The scientific challenge is to
1. identify the levels of such tipping points and the projected effects of exceeding them on ecosystems, human well-being, and human economies, and
2. develop strategies for not exceeding estimated tipping point levels.
Big Ideas
· Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.
· Some organisms produce the nutrients they need, others survive by consuming other organisms, and still others live on the wastes and remains of organisms while recycling nutrients that are used again by producer organisms.
· Human activities are altering the chemical cycling of nutrients and the flow of energy through food chains and webs in ecosystems.
Critical Thinking

1. How would you explain the importance of tropical rain forests (Core Case Study) to people who think that such forests have no connection to their lives?

2. Explain

1. why the flow of energy through the biosphere depends on the cycling of nutrients, and

2. why the cycling of nutrients depends on gravity.

3. Explain why microbes are important. What are two ways in which they benefit your health or lifestyle? Write a brief description of what you think would happen to you if microbes were eliminated from the earth.

4. Make a list of the foods you ate for lunch or dinner today. Trace each type of food back to a particular producer species. Describe the sequence of feeding levels that led to your feeding.

5. Use the second law of thermodynamics (see Chapter 2) to explain why many poor people in less-developed countries live on a mostly vegetarian diet.

6. List three ways in which your life and the lives of any children or grandchildren you might eventually have would be affected if human activities continue to modify the water cycle.

7. What would happen to an ecosystem if

1. all of its decomposers and detritus feeders were eliminated,

2. all of its producers were eliminated, and

3. all of its insects were eliminated? Could an ecosystem exist with producers and decomposers but no consumers? Explain.

8. For each of the proposed four laws of ecology listed near the end of this chapter, find one way in which observing or breaking the law is connected to one or more of the seven possible tipping points. Explain each case.

9. Doing Environmental Science

10. Visit a nearby terrestrial ecosystem and identify its major producers, primary and secondary consumers, detritus feeders, and decomposers. Take notes and describe at least one example of each of these types of organisms. Make a simple sketch showing how these organisms might be related to each other or to other organisms in a food chain or food web. Think of two ways in which this food web or chain could be disrupted. Write a report summarizing your research and conclusions.

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Chapter Introduction

Endangered green sea turtle and diver over a coral reef.

Khoroshunova Olga/ Shutterstock.com

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Core

The Jellyfish Invasion

Learning Objectives

· LO 11.1Describe a jellyfish bloom and the problems it can cause.