What Is an Ecosystem?

Transcription

1 Section 1 What Is an Ecosystem? Objectives Distinguish an ecosystem from a community. 12C Describe the diversity of a representative ecosystem. 7B 8B TAKS 3 Sequence the process of succession. 13A TAKS 3 Key Terms ecology habitat community ecosystem abiotic factor biotic factor biodiversity pioneer species succession primary succession secondary succession Interactions of Organisms and Their Environment It is easy to think of the environment as being around but not part of us something we always use, sometimes enjoy, and sometimes damage. But in fact, we are part of the environment along with all of Earth s other organisms. All of Earth s inhabitants are interwoven in a complex web of relationships, such as the one illustrated in Figure 1. To understand how the interactions of the parts can affect a whole system, think about how a computer operates. Removing one circuit from a computer can change or limit the interactions of the computer s many components in ways that influence the computer s overall operation. In a similar way, removing one species from our environment can have many consequences, not all of them easily predictable. In 1866, the German biologist Ernst Haeckel gave a name to the study of how organisms fit into their environment. He called this study ecology, which comes from the Greek words oikos, meaning house, or place where one lives, and logos, meaning study of. Ecology is the study of the interactions of living organisms with one another and with their physical environment (soil, water, climate, and so on). The place where a particular population of a species lives is its habitat. The many different species that live together in a habitat are called a community. An ecosystem, or ecological system, consists of a community and all the physical aspects of its habitat, such as the soil, water, and weather. The physical aspects of a habitat are called abiotic (ay bie AHT ihk) factors, and the organisms in a habitat are called biotic factors. Figure 1 Organisms interact within an ecosystem. Organisms within an ecosystem continually change and adjust. This plant species is dependent on the bat for its reproduction, and the bat uses part of the flower for food.

2 Diverse Communities in Ecosystems The number of species living within an ecosystem is a measure of its biodiversity. Consider a pine forest in the southeastern United States, such as the one shown in Figure 2. If you could fence in a square kilometer (0.4 mi 2 ) of this forest and then collect every organism, what would you expect to get? Which of the six kingdoms of organisms would be represented in your collection? Ecosystem Inhabitants Large animals in the forest might include a bear or a white-tailed deer. The woods also contain smaller mammals raccoons, foxes, squirrels, rabbits, and chipmunks. Snakes and toads often remain hidden among the leaves. Many birds can be found, including hawks, warblers, and sparrows. If the square kilometer included a lake, you might find catfish, bass, perch, a variety of turtles, and perhaps an alligator. There are pine trees, a variety of smaller trees, and shrubs. Beneath the trees, grasses and many kinds of flowers grow on the forest floor. The soil contains an immense number of worms. Hidden under the bark of trees and beneath the leaves covering the ground are many different species of insects and spiders, such as those shown in Figure 3. Many of the life-forms in the soil and water of a pine forest are too small to be seen without a microscope. Protists, which include algae and related microscopic eukaryotes, thrive in water. There may be billions of bacteria in a handful of soil. Figure 2 Pine forest. Pine forests like this one are common in the southeastern United States. Topic: Biodiversity Keyword: HX4020 Figure 3 Forest spider and insect The jumping spider is found in sunny, dry parts of the forest. The larvae of the stag beetle live in and eat decaying wood and bark. Jumping spider Male stag beetle 341

3 342 Figure 4 Forest fungi These fungi digest plants and other materials they find in the forest. Mushrooms are often found on moist forest floors. Shelf fungi grow on and digest trees. You might find many kinds of fungi growing on fallen trees and spreading as fine threads through the decaying material on the forest floor, as illustrated in Figure 4. Other fungi are found on the surface of trees or rocks as lichens. Lichens are associations between fungi and algae or cyanobacteria. If you were to remove every organism from your square kilometer, the nonliving surroundings that remain make up the abiotic factor. This would include the minerals, organic compounds, water, wind that blows over the Earth, rain, and sunlight. Topic: Biodiversity in Texas Keyword: HXX4002 Ecosystem Boundaries The physical boundaries of an ecosystem are not always obvious, and they depend on how the ecosystem is being studied. For example, a scientist might consider a single rotting log on the forest floor to be an ecosystem if he or she is interested only in the fungi and insects living in the log. Often individual fields, forests, or lakes are studied as isolated ecosystems. Of course, no location is ever totally isolated. Even oceanic islands get occasional migrant visitors, such as birds blown off course. Evaluating Biodiversity 1A 2B 2C 7A TAKS 1, TAKS 3 By making simple observations, you can draw some conclusions about biodiversity in an ecosystem. Materials note pad, pencil Procedure 1. CAUTION: Do not approach or touch any wild animals. Do not disturb plants. Prepare a list of biotic and abiotic factors that you observe around your home or in a nearby park. Analysis 1. Identify the habitat and community that you observed. 2. Calculate the number of different species as a percentage of the total number of organisms that you saw. 3. Rank the importance of biotic factors within the ecosystem you observed. 4. Infer what the relationships are between biotic factors and abiotic factors in the observed ecosystem.

4 343 Change of Ecosystems over Time When a volcano forms a new island, a glacier recedes and exposes bare rock, or a fire burns all of the vegetation in an area, a new habitat is created. This change sets off a process of colonization and ecosystem development. The first organisms to live in a new habitat are small, fast-growing plants, called pioneer species. They may make the ground more hospitable for other species. Later waves of plant immigrants may then outcompete and replace the pioneer species. Succession A somewhat regular progression of species replacement is called succession. Succession that occurs where plants have not grown before is called primary succession. Succession that occurs in areas where there has been previous growth, such as in abandoned fields or forest clearings, is called secondary succession. It was once thought that the stages of succession were predictable and that succession always led to the same final community of organisms within any particular ecosystem. Ecologists now recognize that initial conditions and chance play roles in the process of succession. For example, if two species are in competition, a sudden change in the climate may favor the success of one species over the other. For this reason, no two successions are alike. Glacier Bay: an Example of Succession A good example of primary succession is a receding glacier because land is continually being exposed as the face of the glacier moves back. The glacier that composes much of the head of Glacier Bay, Alaska, has receded some 100 km (62 mi) over the last 200 years. Figure 5 shows the kinds of changes that have taken place as time passed. The most recently exposed areas are piles of rock and gravel that lack the usable nitrogen essential to plant and animal life. The seeds and spores of pioneer species are carried in by the wind. These include lichens, mosses, fireweed, willows, cottonwood, and Dryas, a sturdy plant with clumps about 30 cm (1 ft) across. At first all of these plants grow close to the ground, severely stunted by mineral deficiency, but Dryas eventually crowds out the other plants. After about 10 years, alder seeds blown in from distant sites take root. Alder roots have nitrogen-fixing nodules, so they are able to grow more rapidly than Dryas. Dead leaves and fallen branches from the alder trees add more usable nitrogen to the soil. The added nitrogen allows willows and cottonwoods to invade and grow with vigor. After about 30 years, dense thickets of alder, willow, and cottonwood shade and eventually kill the Dryas. Figure 5 Glacier Bay A receding glacier makes primary succession possible. Recently exposed land has few nutrients. Alders, grasses, and shrubs later take over from pioneer plants. As the amount of soil increases, spruce and hemlock trees become plentiful.

5 About 80 years after the glacier first exposes the land, Sitka spruce invades the thickets. Spruce trees use the nitrogen released by the alders and eventually form a dense forest. The spruce blocks the sunlight from the alders, and the alders then die, just as the Dryas did before them. After the spruce forest is established, hemlock trees begin to grow. Hemlocks are very shade tolerant and have a root system that competes well against spruce for soil nitrogen. Hemlock trees soon become dominant in the forest. This community of spruce and hemlock proves to be a very stable ecosystem from the perspective of human time scales, but it is not permanent. As local climates change, this forest ecosystem may change too. Modeling Succession 2A 2B You can create a small ecosystem and measure how organisms modify their environment. Materials 1 qt glass jar with a lid, one-half quart of pasteurized milk, ph strips TAKS 1 Procedure 1. Prepare a table like the one below. 2. Half fill a quart jar with pasteurized milk, and cover the jar loosely with a lid. Measure and record the ph. Place the jar in a 37 C incubator. 3. Check and record the ph of the milk with ph strips every day for seven days. As milk spoils, its ph changes. Different populations of microorganisms become established, alter substances in the milk, and then die off when conditions no longer favor their survival. 4. Record any visible changes in the milk each day. Analysis 1. Identify what happened to the ph of the milk as time passed. DATA TABLE 2. Infer what the change in ph means about the populations of microorganisms in the milk. 3. Critical Thinking Evaluating Results How does this model confirm the model of succession in Glacier Bay? Day ph Appearance Section 1 Review 344 Identify what components of an ecosystem are not part of a community. 12C Relate how gardening or agriculture affects succession. 7B 8B Differentiate primary succession from secondary succession. 13A Critical Thinking Applying Information Why do some ecosystems remain stable for centuries, while others undergo succession? TAKS Test Prep In the succession that occurs as a glacier recedes, alders can grow relatively rapidly because alders have 13A A nitrogen-fixing nodules. C no roots. B no need for minerals. D shade tolerance. 13A

6 Energy Flow in Ecosystems Movement of Energy Through Ecosystems Everything that organisms do in ecosystems running, breathing, burrowing, growing requires energy. The flow of energy is the most important factor that controls what kinds of organisms live in an ecosystem and how many organisms the ecosystem can support. In this section you will learn where organisms get their energy. Primary Energy Source Most life on Earth depends on photosynthetic organisms, which capture some of the sun s light energy and store it as chemical energy in organic molecules. These organic compounds are what we call food. The rate at which organic material is produced by photosynthetic organisms in an ecosystem is called primary productivity. Primary productivity determines the amount of energy available in an ecosystem. Most organisms in an ecosystem can be thought of as chemical machines driven by the energy captured in photosynthesis. Organisms that first capture energy, the producers, include plants, some kinds of bacteria, and algae. Producers make energy-storing molecules. All other organisms in an ecosystem are consumers. Consumers are those organisms that consume plants or other organisms to obtain the energy necessary to build their molecules. Trophic Levels Ecologists study how energy moves through an ecosystem by assigning organisms in that ecosystem to a specific level, called a trophic (TROHF ihk) level, in a graphic organizer based on the organism s source of energy. Energy moves from one trophic level to another, as illustrated in Figure 6. Section 2 Objectives Distinguish between producers and consumers. 8B Compare food webs with food chains. 12E TAKS 3 Describe why food chains are rarely longer than three or four links. 12E TAKS 3 Key Terms primary productivity producer consumer trophic level food chain herbivore carnivore omnivore detritivore decomposer food web energy pyramid biomass Figure 6 Trophic levels The sun is the ultimate source of energy for producers and all consumers. Sun Producer Consumer Consumer

7 Real Life Not all producers are photosynthetic. At the bottom of oceans near volcanic vents live bacteria that harvest energy from the reduced sulfur compounds ejected by these volcanic vents. Applying Information Where in their food chains do these bacteria lie? 12E TAKS 3 First Level The path of energy through the trophic levels of an ecosystem is called a food chain. An example is shown in Figure 7. The lowest trophic level of any ecosystem is occupied by the producers, such as plants, algae, and bacteria. Producers use the energy of the sun to build energy-rich carbohydrates. Many producers also absorb nitrogen gas and other key substances from the environment and incorporate them into their biological molecules. Second Level At the second trophic level are herbivores (HUHR beh vohrz), animals that eat plants or other primary producers. They are the primary consumers. Cows and horses are herbivores, as are caterpillars and some ducks. A herbivore must be able to break down a plant s molecules into usable compounds. However, the ability to digest cellulose is a chemical feat that only a few organisms have evolved. As you will recall, cellulose is a complex carbohydrate found in plants. Most herbivores rely on microorganisms, such as bacteria and protists, in their gut to help digest cellulose. Humans cannot digest cellulose because we lack these particular microorganisms. Third Level At the third trophic level are secondary consumers, animals that eat herbivores. These animals are called carnivores. Tigers, wolves, and snakes are carnivores. Some animals, such as bears, are both herbivores and carnivores; they are called omnivores (AHM nih vohrz). They use the simple sugars and starches stored in plants as food, but they cannot digest cellulose. In every ecosystem there is a special class of consumers called detritivores, which include worms and fungal and bacterial decomposers. Detritivores (deh TRIH tih vohrz) are organisms that obtain their energy from the organic wastes and dead bodies that are produced at Figure 7 Aquatic food chain This food chain shows one path of energy flow in an Antarctic ecosystem. Killer whale Algae Krill 346 Cod Leopard seal

8 all trophic levels. Bacteria and fungi are known as decomposers because they cause decay. Decomposition of bodies and wastes releases nutrients back into the environment to be recycled by other organisms. Many ecosystems contain a fourth trophic level composed of those carnivores that consume other carnivores. They are called tertiary consumers, or top carnivores. A hawk that eats a snake is a tertiary consumer. Very rarely do ecosystems contain more than four trophic levels. In most ecosystems, energy does not follow simple straight paths because individual animals often feed at several trophic levels. This creates a complicated, interconnected group of food chains called a food web, as illustrated in Figure 8. Topic: Food Chains and Webs Keyword: HX4085 Figure 8 Aquatic food web This food web shows a more complete picture of the feeding relationships in an Antarctic ecosystem. Killer whale Crabeater seal Elephant seal Leopard seal Adelie penguin Cod Squid Krill Algae Small animals and protists

9 348 The word ecosystem is from the Greek words oikos, meaning house, and systematos, meaning to place together. Knowing this information makes it easier to remember that an ecosystem includes a community of living things as well as all physical aspects of its environment. Topic: Energy Pyramids Keyword: HX4069 Loss of Energy in a Food Chain A deer browsing on leaves is acquiring energy. Potential energy is stored in the chemical bonds within the molecules of the leaves. Some of this energy is transformed to other forms of potential energy, such as fat. Some of it aids in the deer s mechanical work, such as running, breathing, and eating more leaves. But almost half of the energy is lost to the environment as heat. Energy Transfer During every transfer of energy within an ecosystem, energy is lost as heat. Although heat can be used to do work (as in a steam engine), it is generally not a useful source of energy in biological systems. Thus, the amount of useful energy available to do work decreases as energy passes through an ecosystem. The loss of useful energy limits the number of trophic levels an ecosystem can support. When a plant harvests energy from sunlight, it stores in chemical bonds only about one-half of the energy it is able to capture. When a herbivore uses plant molecules to make its own molecules, only about 10 percent of the energy in the plant ends up in the herbivore s molecules. And when a carnivore eats the herbivore, about 90 percent of the energy is lost in making carnivore molecules. At each trophic level, the energy stored by the organisms in a level is about one-tenth of that stored by the organisms in the level below. The Pyramid of Energy Ecologists often illustrate the flow of energy through ecosystems with an energy pyramid. An energy pyramid is a diagram in which each trophic level is represented by a block, and the blocks are stacked on top of one another, with the lowest trophic level on the bottom. The width of each block is determined by the amount of energy stored in the organisms at that trophic level. Because the energy stored by the organisms at each trophic level is about onetenth the energy stored by the organisms in the level below, the diagram takes the shape of a pyramid, as shown in Figure 9. Figure 9 Trophic levels of a terrestrial ecosystem In this simple ecosystem, each trophic level contains about 90 percent less energy than the level below it. Top carnivore Carnivore Herbivores Producers

10 Limitations of Trophic Levels Most terrestrial ecosystems involve only three or, on rare instances, four levels. Too much energy is lost at each level to allow more levels. For example, a large human population could not survive by eating lions captured on the Serengeti Plain of Africa because there are too few lions to make this possible. The amount of grass in that ecosystem cannot support enough zebras to maintain a large enough population of lions to feed lion-eating humans. In other words, the number of trophic levels that can be maintained in an ecosystem is limited by the loss of potential energy. Humans are omnivores, and unlike lions, we can choose to eat either meat or plants. As illustrated in Figure 10, about 10 kg (22 lb) of grain are needed to build about 1 kg (2.2 lb) of human tissue if the grain is directly ingested by a human. If a cow eats the grain and a human eats the cow, then about 100 kg (220 lb) of grain are needed to build about 1 kg (2.2 lb) of human tissue. Also, the number of individuals in a trophic level may not be an accurate indicator of the amount of energy in that level. Some organisms are much bigger than others and therefore use more energy. Because Figure 10 It takes a certain amount of grain to produce enough bread to provide one person with a certain amount of energy. of this, the number of organisms often does not form a pyramid when one compares different trophic levels. For instance, caterpillars and other insect herbivores greatly outnumber the trees they feed on. To better determine the amount of energy present in trophic levels, ecologists measure biomass. Biomass is the dry weight of tissue and other organic matter found in a specific ecosystem. Each higher level on the pyramid contains only 10 percent of the biomass found in the trophic level below it. Section 2 Review Energy efficiency in food consumption Adding a trophic level to a food chain increases the energy demand by consumers a factor of about 10. It takes 10 times more grain to feed one cow to make enough beef to provide one person with the same amount of energy. Explain how producers differ from consumers. Analyze the flow of energy through a food chain that contains four tropic levels, one of which is a carnivore. 9D Construct a food web, and explain the interactions of the organisms that compose it. List the reasons why food chains do not tend to exceed four links. 12E 12E 8B Critical Thinking Justifying an Argument Explain why scientists believe that most animals would become extinct if all plants died. 9D TAKS Test Prep Which series shows a correct 12E path of energy flow in a marine food chain? A krill cod algae B cod leopard seal krill C leopard seal algae krill D algae krill cod 349

11 350 Section 3 Cycling of Materials in Ecosystems Objectives Summarize the role of plants in the water cycle. 12A Analyze the flow of energy through the carbon cycle. 12A Identify the role of bacteria in the nitrogen cycle. 11D 12A Key Terms biogeochemical cycle ground water transpiration nitrogen fixation Figure 11 Trees and the carbon cycle. Approximately 500 million tons of carbon were taken up as a result of forest regrowth in the Northern Hemisphere between 1980 and Biogeochemical Cycles Humans throw away tons of garbage every year as unwanted, unneeded, and unusable. Nature, however, does not throw anything away. Most energy flows through the Earth s ecosystems from the sun to producers to consumers. The physical parts of the ecosystems, however, cycle constantly. Carbon atoms, for example, are passed from one organism to another in a great circle of use. Producers are eaten by herbivores, herbivores are eaten by carnivores, and carnivores are eaten by top carnivores. Eventually the top carnivores die and decay; their carbon atoms then become part of the soil to feed the producers in a long and complex cycle that reuses this important element. Carbon is not the only element that is constantly recycled in this way. Other recycled elements include many of the inorganic (noncarbon) substances that make up the soil, water, and air, such as nitrogen, sulfur, calcium, and phosphorus. All materials that cycle through living organisms are important in maintaining the health of ecosystems, but four substances are particularly important: water, carbon, nitrogen, and phosphorus. All organisms require carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur in relatively large quantities. They require other elements, such as magnesium, sodium, calcium, and iron, in smaller amounts. Some elements, such as cobalt and manganese, are required in trace amounts. The paths of water, carbon, nitrogen, and phosphorus pass from the nonliving environment to living organisms, such as the trees in Figure 11, and then back to the nonliving environment. These paths form closed circles, or cycles, called biogeochemical (bie oh jee oh KEHM ih kuhl) cycles. In each biogeochemical cycle, a pathway forms when a substance enters living organisms such as trees from the atmosphere, water, or soil; stays for a time in the living organism; then returns to the nonliving environment. Ecologists refer to such substances as cycling within an ecosystem between a living reservoir (an organism that lives in the ecosystem) and a nonliving reservoir. In almost all biogeochemical cycles, there is much less of the substance in the living reservoir than in the nonliving reservoir.

12 351 The Water Cycle Of all the nonliving components of an ecosystem, water has the greatest influence on the ecosystem s inhabitants. In the nonliving portion of the water cycle, water vapor in the atmosphere condenses and falls to the Earth s surface as rain or snow. Some of this water seeps into the soil and becomes part of the ground water, which is water retained beneath the surface of the Earth. Most of the remaining water that falls to the Earth does not remain at the surface. Instead, heated by the sun, it reenters the atmosphere by evaporation. The path of water within an ecosystem is shown in Figure 12. In the living portion of the water cycle, much water is taken up by the roots of plants. After passing through a plant, the water moves into the atmosphere by evaporating from the leaves, a process called transpiration. Transpiration is also a sun-driven process. The sun heats the Earth s atmosphere, creating wind currents that draw moisture from the tiny openings in the leaves of plants. In aquatic ecosystems (lakes, rivers, and oceans), the nonliving portion of the water cycle is the most important. In terrestrial ecosystems, the nonliving and living parts of the water cycle both play important roles. In thickly vegetated ecosystems, such as tropical rain forests, more than 90 percent of the moisture in the ecosystem passes through plants and is transpired from their leaves. In a very real sense, plants in rain forests create their own rain. Moisture travels from plants to the atmosphere and falls back to the Earth as rain. Topic: Water Cycle Keyword: HX4188 Figure 12 Water cycle This diagram shows the major steps in the water cycle. Precipitation Water vapor (clouds) Transpiration Runoff Evaporation Evaporation Lake Ocean Ground water Percolation into soil

13 352 Topic: Carbon Cycle Keyword: HX4031 Figure 13 Carbon cycle This diagram shows the major steps of the carbon cycle. The Carbon Cycle Carbon also cycles between the nonliving environment and living organisms. You can follow the carbon cycle in Figure 13. Carbon dioxide in the air or dissolved in water is used by photosynthesizing plants, algae, and bacteria as a raw material to build organic molecules. Carbon atoms may return to the pool of carbon dioxide in the air and water in three ways. 1. Respiration. Nearly all living organisms, including plants, engage in cellular respiration. They use oxygen to oxidize organic molecules during cellular respiration, and carbon dioxide is a byproduct of this reaction. 2. Combustion. Carbon also returns to the atmosphere through combustion, or burning. The carbon contained in wood may stay there for many years, returning to the atmosphere only when the wood is burned. Sometimes carbon can be locked away beneath the Earth for thousands or even millions of years. The remains of organisms that become buried in sediments may be gradually transformed by heat and pressure into fossil fuels coal, oil, and natural gas. The carbon is released when the fossil fuels are burned. 3. Erosion. Marine organisms use carbon dioxide dissolved in sea water to make calcium carbonate shells. Over millions of years, the shells of the dead organisms form sediments, which form limestone. As the limestone becomes exposed and erodes, the carbon becomes available to other organisms. Carbon dioxide (CO2) in atmosphere Photosynthesis Cellular respiration Combustion Dissolved CO 2 in water Death and decomposition Marine plankton remains Limestone Fossil fuels

14 353 The Phosphorus and Nitrogen Cycles Organisms need nitrogen and phosphorus to build proteins and nucleic acids. Phosphorus is an essential part of both ATP and DNA. Phosphorus is usually present in soil and rock as calcium phosphate, which dissolves in water to form phosphate ions, PO 3-4. This phosphate is absorbed by the roots of plants and used to build organic molecules. Animals that eat the plants reuse the organic phosphorus. The atmosphere is 79 percent nitrogen gas, N 2. However, most organisms are unable to use it in this form. The two nitrogen atoms in a molecule of nitrogen gas are connected by a strong triple covalent bond that is very difficult to break. However, a few bacteria have enzymes that can break it, and they bind nitrogen atoms to hydrogen to form ammonia, NH 3. The process of combining nitrogen with hydrogen to form ammonia is called nitrogen fixation. Nitrogen-fixing bacteria live in the soil and are also found within swellings, or nodules, on the roots of beans, alder trees, and a few other kinds of plants. The nitrogen cycle, diagramed in Figure 14, is a complex process with four important stages. 1. Assimilation is the absorption and incorporation of nitrogen into plant and animal compounds. 2. Ammonification is the production of ammonia by bacteria during the decay of nitrogen-containing urea (found in urine). 3. Nitrification is the production of nitrate from ammonia. 4. Denitrification is the conversion of nitrate to nitrogen gas. Plants Atmospheric nitrogen (N 2 ) Animals Reviewing Information Using your own words, write four sentences, each one describing one of the four biogeochemical cycles. Figure 14 Nitrogen cycle Bacteria carry out many of the important steps in the nitrogen cycle, including the conversion of atmospheric nitrogen into a usable form, ammonia. Denitrification Denitrifying bacteria Nitrates (NO 3 ) Assimilation Death Waste (urine and feces) Decomposers Ammonification Death Nitrogen fixation Nitrogen-fixing bacteria in plant roots Nitrifying bacteria Nitrification Ammonia (NH 3 ) Nitrogen fixation Nitrogen-fixing bacteria in soil

15 The growth of plants in ecosystems is often limited by the availability of nitrate and ammonia in the soil. Today most of the ammonia and nitrate that farmers add to soil is produced chemically in factories, rather than by bacterial nitrogen fixation. Genetic engineers are trying to place nitrogen-fixing genes from bacteria into the chromosomes of crop plants. If these attempts are successful, the plants themselves will be able to fix nitrogen, thus eliminating the need for nitrogen-supplying fertilizers. Some farmers adjust their farming methods to increase natural recycling of nitrogen. Sustainable Agriculture In an ecosystem, decomposers return mineral nutrients to the soil. However, when the plants are harvested and shipped away, there is a net loss of nutrients from the soil where the plants were growing. The amount of organic matter in the soil also decreases, making the soil less able to hold water and more likely to erode. What is Sustainable Agriculture? Sustainable agriculture refers to farming that remains productive and profitable through practices that help replenish the soil s nutrients, reduce erosion, and control weeds and insect pests. Use of Cover Crops After harvest, farmers can plant cover crops, such as rye, clover, or vetch, instead of letting the ground lie bare. Cover crops keep the soil from compacting and washing away, and they help the soil absorb water. They also provide a habitat for beneficial insects, slow the growth of weeds, and keep the ground from overheating. When cover crops are plowed under, as illustrated in the figure at right, they return nutrients to the soil. Rotational Grazing Farmers who raise cattle and sheep can divide their pastures into several grazing areas. By rotating their livestock from one area to another, they can prevent the animals from overgrazing the pasture. This allows the plants on which the animals feed to live longer and be more productive. Water quality improves as the pasture vegetation becomes denser. Animals distribute manure more evenly with rotational grazing than they do in feed lots or unmanaged pastures. There are many other methods used in sustainable agriculture. Farmers must determine which methods work best for their crops, soil conditions, and climate. Topic: Sustainable Agriculture Keyword: HX4170 Section 3 Review Identify the role of energy in the part of the water cycle in which plants transfer water to the atmosphere. 12A Analyze the carbon cycle s relationship to the flow of energy. 9D 12A Describe how bacteria participate in the nitrogen cycle. 9D 11D 12A Critical Thinking Defend the argument that nutrients can cycle but energy cannot. 9D 12A TAKS Test Prep Which component of the carbon cycle removes carbon dioxide from the atmosphere? 12A A combustion C erosion B cellular respiration D photosynthesis

16 355 Key Concepts Key Terms 1 What Is an Ecosystem? Ecology is the study of how organisms interact with each other and with their environment. A community of organisms and their nonliving environment constitute an ecosystem. Ecosystems contain diverse organisms. Ecosystems change through the process of succession. Succession on a newly formed habitat is primary succession. Secondary succession occurs on a habitat that has previously supported growth. Section 1 ecology (340) habitat (340) community (340) ecosystem (340) abiotic factor (340) biotic factor (340) biodiversity (341) pioneer species (343) succession (343) primary succession (343) secondary succession (343) 2 Energy Flow in Ecosystems Energy moves through ecosystems in food chains, passing from photosynthesizers (producers) to herbivores (consumers) to carnivores (consumers), creating a food web. Energy transfers between trophic levels transfer only 10 percent of the energy in a trophic level to the next level. Most terrestrial ecosystems have only three or four trophic levels because energy transfers between trophic levels are inefficient. 3 Cycling of Materials in Ecosystems Minerals and other materials cycle within ecosystems among organisms and between organisms and the physical environment. In the water cycle, water falls as precipitation and either evaporates from bodies of water, is stored in ground water, or cycles through plants and then evaporates. Carbon enters the living portion of the carbon cycle through photosynthesis. Organisms release carbon through cellular respiration. Carbon trapped in rocks and fossil fuels is released by erosion and burning. Bacteria fix atmospheric nitrogen, thus making ammonia available to other organisms. Phosphorus is cycled through plants into animals Section 2 primary productivity (345) producer (345) consumer (345) trophic level (345) food chain (346) herbivore (346) carnivore (346) omnivore (346) detritivore (346) decomposer (347) food web (347) energy pyramid (348) biomass (349) Section 3 biogeochemical cycle (350) ground water (351) transpiration (351) nitrogen fixation (353) BIOLOGY Unit 7 Ecosystem Dynamics Use Topics 1, 3 6 in this unit to review the key concepts and terms in this chapter.

17 Using Key Terms 1. A mountain lion is a(n) 12B a. omnivore. c. detritivore. b. herbivore. d. carnivore. 2. Plants return water to the atmosphere by a. assimilation. c. succession. 12A b. transpiration. d. nitrification. 3. An organism that obtains energy from organic wastes and dead bodies is a(n) 12B a. carnivore. c. detritivore. b. omnivore. d. herbivore. 4. The process by which materials pass between the nonliving environment and living organisms is called a(n) 12A a. biogeochemical cycle. b. energy pyramid. c. food web. d. primary succession. 5. For each pair of terms, explain the 8B differences in their meanings. a. biodiversity, biomass b. ecosystem, community c. producers, consumers Understanding Key Ideas 6. Ecosystems differ from a community in that ecosystems usually contain a. several climates. b. several communities. c. only one habitat. d. only one food web. 7. What critical role is played by fungi and bacteria in any ecosystem? 11D a. primary production b. decomposition c. boundary setting d. physical weathering 8. Which sequence shows the correct order of succession at Glacier Bay, Alaska? 13A a. alder, Dryas, hemlock b. Dryas, hemlock, alder c. Dryas, alder, Sitka spruce d. mosses, hemlock, Sitka spruce 9. How much energy is available at the third trophic level of an energy pyramid if 1,000 kcal is available in the first level? a. 1,000 kcal c. 10 kcal b. 100 kcal d. 1 kcal 10. Which role is not performed by bacteria in the nitrogen cycle? 12A a. fixing nitrogen b. changing urea to ammonia c. turning nitrates into nitrogen gas d. changing nitrates to ammonia 11. How would the food web below be affected if the plants were eliminated? 12E a. Birds and mice would starve. b. The food web would collapse. c. The herbivores would change trophic levels. d. Nothing would happen. 12. Humans, raccoons, and bears are omnivores. What adaptive advantage might this feeding strategy provide? 13. After harvesting, a farmer could either plow the remaining cornstalks into the field or burn them. Which option is best for sustainable agriculture? Explain your answer. 11D 14. Relate photosynthesis to the nitrogen cycle. (Hint: See Chapter 5, Section 2.) 12A 15. Concept Mapping Make a concept map that describes the flow of energy through an ecosystem. Try to include the following terms in your map: trophic level, food web, food chain, producer, consumer, carnivore, detritivore, and herbivore. 2C 3E 9D 12E 7B 12D

18 Critical Thinking 16. Critiquing Scientific Explanations Ecologists once referred to stable ecosystems as a final or climax community. Now most ecologists say that no ecosystem can truly have a final end point. Explain why ecologists changed their viewpoint. 3A 3C 17. Inferring Relationships Analyze the flow of energy between an ecosystem and one of its top carnivores, such as a hawk. 9D 18. Applying Information Is nitrogen cycling or carbon cycling more important to a pioneer species during primary succession? Explain your answer. 12A 19. Predicting Results Describe the probable effects on an ecosystem if all decomposers were to die. 12E Alternative Assessment 20. Identifying Functions Obtain photocopies of nature paintings by American painters such as John James Audubon or Edward Hicks. Choose three animals, and write a report that compares the animals, the ecosystems in which they live, their roles in biogeochemical cycles, and the trophic level they occupy. 8B 12A 12E 21. Career Connection Ecologist Use library or Internet resources to research the educational background necessary to become an ecologist. Describe degrees or training that is recommended for this career. Summarize the employment outlook for this field. 3D 22. Interactive Tutor Unit 7 Ecosystem Dynamics Write a report summarizing how artificial ecosystems used in the management and treatment of waste water and pollutants, can demonstrate succession. 11D TAKS Test Prep The chart below shows the monthly variation in atmospheric carbon dioxide concentration over a deciduous forest. Use the chart and your knowledge of science to answer questions 1 3. Carbon dioxide concentration (parts per million) Atmospheric Carbon Dioxide Variation Jan. March May July Sept. Month Nov. 1. During which of the following months is the rate of photosynthesis greatest? 2C 12A A May C January B March D September 2. If the data were obtained from the atmosphere over an evergreen forest, the curve likely would 2C 12A F rise from February to May and fall from August to November. G vary less throughout the year. H rise steadily from January to December. J fall steadily from January to December. 3. If the y-axis of a graph displayed the rate of transpiration of a deciduous forest, the curve likely would 2C 12A A rise from February to May and fall from August to November. B vary little throughout the year. C rise steadily from January to December. D fall steadily from January to December. Test Carefully read questions that ask for the one choice that is not correct. Often these questions include words such as is not, except, or all but.