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Transcript
13.2 The Structure of Ecosystems E X P E C TAT I O N S Describe what is meant by the trophic structure of a community. Describe the ecosystem roles of producers, consumers, and decomposers. As mentioned in section 13.1, ecosystems contain both biotic and abiotic components. Each ecosystem consists of all the organisms in one or more communities, as well as the physical and chemical factors affecting them. The boundaries of ecosystems are not distinct — one ecosystem may overlap or exist inside another. There are terrestrial (land-based) and aquatic (water-based) ecosystems, and ecosystems that contain both land and water. An ecosystem can be small (such as the one shown in Figure 13.14), or as large as the biosphere — a global ecosystem. Figure 13.14 Small insects fall into the pitcher plant’s cup- shaped leaves, drown, and decompose. This plant, which typically lives in nitrogen-poor environments, extracts the nutrients it requires from the bodies of insects. The pitcher plant and its surroundings comprise a small ecosystem. 440 MHR • Unit 5 Population Dynamics Despite the wide range in sizes and types of ecosystems, the same basic processes take place in all of them. Two processes are particularly important: energy flow and chemical cycling. The proper functioning of these processes is vital to the survival of organisms in the ecosystem and to the integrity of the ecosystem itself. Trophic Structure When ecologists refer to the trophic structure of an ecosystem or community, they are describing the feeding relationships among its members. Each species is assigned to a specific trophic level in the structure, depending on its main source of nutrition. Most ecosystems have several trophic levels through which energy flows and chemicals (matter) cycle. The first (or lowest) trophic level consists of autotrophic organisms. Autotrophs are organisms that can make energy-rich organic molecules (such as glucose) from the raw materials available in the environment. They then break these “homemade” organic molecules down during cellular respiration to provide the energy that fuels the rest of their life processes. Photosynthetic autotrophs use the energy of the Sun to drive this manufacturing process. Almost all plants, as well as some types of protists (algae) and bacteria (cyanobacteria), are photosynthetic autotrophs. All organisms need energy to drive cellular processes. They must, therefore, have a source of organic molecules from which they can release this energy during cellular respiration. Autotrophs, which are at the first trophic level, produce organic molecules; this makes the first level the most important. Because the first trophic level supports all life at the higher levels, autotrophs are referred to as the primary producers in an ecosystem. This first trophic level provides all the potential energy required to drive the other levels in the ecosystem. All organisms in the trophic levels above this one are heterotrophs. Heterotrophs are unable to make the energy-rich molecules they need to fuel their life processes. Instead, they must obtain these molecules by consuming other organisms, either autotrophs or other heterotrophs. Therefore, they are referred to as consumers. Herbivores that eat autotrophs are termed primary consumers, since they are the first eaters in the chain. On land, insects, snails, grazing mammals, and birds and mammals that eat seeds and fruit are the most common herbivores. In aquatic ecosystems, this role is often taken by heterotrophic protists, various types of small invertebrate animals, and some species of fish. Carnivores that eat mainly herbivores are secondary consumers. Spiders, frogs, and insecteating birds are examples of secondary consumers. In most ecosystems, these secondary consumers are themselves eaten by other carnivores, which are known as tertiary consumers (the third set THINKING of eaters). There may also be higher levels of consumers above these. The members of another consumer group, often referred to as decomposers, obtain their energy-rich molecules by eating leftover or waste material derived from all the trophic levels, including the feces of living organisms, dead bodies, or body parts (for example, fallen leaves). Decomposers are very important to every ecosystem. Their role is to break large molecules (that were once part of living organisms) down into smaller ones and return them to the abiotic environment. Thus, decomposers return vitally important nutrient elements such as carbon and nitrogen to the soil and air. These materials can then be used again by autotrophs to make new energy-rich organic molecules. Decomposers are an ecosystem’s recyclers, ensuring that the biosphere’s limited supply of required nutrients is not lost. LAB Shrinking Polar Bears and Expanding Snow Geese Background An incredible variety of living things inhabits the biosphere. This biotic diversity reflects the abiotic diversity of Earth. Through the process of evolution, populations have evolved adaptations that enhance their survival and reproductive ability in diverse habitats. The result is the diversity of life. You Try It 1. Describe the ecological niches of the polar bear and the snow goose. Include diet, preferred habitat, time of peak activity during a day, yearly activities such as migration, and other relevant information. 2. Would you consider the polar bear to be a generalist or a specialist? What about the snow goose? Explain your answer. During the last 100 years, scientists have reported significant changes in the world’s ecosystems. While some of these changes seem to have been beneficial for some species, they seem to have had negative effects on others. For example, dramatic changes have been observed in populations of polar bears (Ursus maritimus) and lesser snow geese (Chen caerulescens) that breed in Canada’s arctic. Field surveys of polar bears have revealed that populations are declining, fewer cubs are born each year, and individual bears are smaller and weigh less than what has been considered typical for members of this species. In contrast, snow goose populations have tripled since 1968. Their numbers are so high that the feeding activity of these geese is causing substantial damage to the habitats where they and other species breed and overwinter. 3. To which trophic levels do the polar bear and snow goose belong? Why do you think these changes are happening? In completing this assignment, you will draw on your knowledge of the nature of ecology and evolution. You might choose to work on one of these two species, and then compare your findings with those of students who worked on the other species. 7. How might changes that currently appear to be occurring in Earth’s climate affect the future evolution of each of these species? 4. Describe the feeding, sensory, and locomotory adaptations that improve the ability of each species to survive and reproduce in its habitat and niche. What features allow each species to cope with environmental stresses (for example, temperature or moisture extremes) in its habitat? 5. Draw a map illustrating the approximate range of each species. 6. Find out as much as you can about the evolutionary history of these species. What other species are they most closely related to? When might they have appeared as a species? 8. Estimate what chance each of these species has for surviving for the next 100 years. Does either face possible extinction? Why or why not? Chapter 13 Ecological Principles • MHR 441 BIO FACT Some types of bacteria use energy derived from breaking the chemical bonds in hydrogen sulfide molecules to form the organic molecules they use as food. This type of autotrophic food production is called chemosynthesis. It is quite rare and occurs in some very unusual environments, including ocean depths of 2500 m. At these depths there is no light and very little oxygen. In addition, hot magma from Earth’s molten core escapes to superheat the surrounding water. may eat plant parts (fruits and seeds) and grasshoppers. In fact, many organisms, including humans, are referred to as omnivores because they eat plants, animals, and other types of organisms. In other words, they are both primary and higherlevel consumers. The result is that the feeding relationships in most ecosystems form complex food webs, rather than simple food chains. Figure 13.16 illustrates a food web. In the Thinking Lab on page 441, you explored how communities are structured and the specific roles individuals can take within a community. The next section will show you how energy flows through ecosystems. Quaternary consumers carnivore carnivore Some unusual marine organisms live near undersea vents off the coast of British Columbia. These organisms include tube worms, limpets, and palm worms. Tertiary consumers carnivore ELECTRONIC LEARNING PARTNER Your Electronic Learning Partner has animation clips that will enhance your understanding of deep-sea vent communities. Secondary consumers carnivore Food Chains and Webs The trophic structure determines the route taken by the energy and matter (chemical elements) contained in food as it moves through an ecosystem. Food is transferred from primary producers to primary consumers and then to secondary consumers along a pathway referred to as a food chain (see Figure 13.15). In reality, few ecosystems are so simple that they consist of only a single, unbranched food chain. More commonly, many species may feed on a single species below themselves, while at the same time a single upper-level consumer species may eat many different species below itself. In addition, organisms may eat individuals from two or more levels. The hawk shown in Figure 13.15 may eat mice, grasshoppers, and snakes, while the mouse 442 MHR • Unit 5 Population Dynamics carnivore carnivore Primary consumers zooplankton herbivore Primary producers plant Terrestrial food chain phytoplankton Marine food chain Figure 13.15 Terrestrial and aquatic communities contain different species, but can have the same overall trophic structure. “Plankton” is a general term referring to small aquatic animals and protists. What distinguishes the two types of plankton? wastes and dead organisms Tertiary and secondary consumers Secondary and primary consumers Primary consumers Primary producers Decomposers (plants, algae, cyanobacteria) (bacteria, fungi, certain animals) Figure 13.16 A simplified food web. There are many other species that feed with or on the ones shown in the diagram. Which species eat organisms on more than one trophic level? SECTION REVIEW 1. K/U Explain why some ecosystems can support highly complex food webs while others can support only relatively simple ones. 2. Draw a chart to illustrate a food web that could be found in a typical pond ecosystem (show at least three distinct trophic levels). Identify the organisms and label each trophic level. 3. 4. 5. K/U Explain why autotrophs rather than decomposers occupy the lowest level of a food chain. 6. K/U Could photosynthetic producers exist in the absence of consumers in an ecosystem? Explain your answer. 7. K/U Describe the typical energy sources that decomposers rely on in an aquatic ecosystem (such as a pond or lake). 8. K/U Describe the difference between a food chain and a food web. Which is more realistic in its depiction of what actually exists in nature? 9. K/U Explain why producer organisms that live deep below the surface of Earth’s oceans rely on chemosynthesis rather than photosynthesis to manufacture high-energy food molecules. C K/U Explain how the same species can occupy more than one trophic level within the same food web. Explain how this type of ecological interaction can enhance the stability of a food web. K/U Describe the types of biotic and abiotic factors that can lead to the collapse of a food web in an ecosystem. Chapter 13 Ecological Principles • MHR 443