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Chapter7.docxMain content
Chapter Introduction
Tropical rainforest in Malaysia, Asia
Core Case StudyAfrican Savanna
Learning Objectives
· LO 7.1Describe the African savanna in terms of habitat and types of species living there.
· LO 7.2Explain why much of the African savanna habitat is being degraded and destroyed.
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.
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.
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 7.2FExplain 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.
Critical Thinking:
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
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?
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.
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
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.
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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.
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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?
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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.
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.
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
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.
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.
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?
