THE LIVING WORLD
Unit Eight. The Living Environment
The earth provides living organisms with much more than a place to stand or swim. Many chemicals cycle between our bodies and the physical environment around us. We live in a delicate balance with our physical surroundings, one easily disturbed by human activities. The collection of organisms that live in a place, and all the physical aspects of the environment that affect how they live, operate as a fundamental biological unit, the ecological system or ecosystem. This mountain meadow, in the High Sierras of California, is an ecosystem, and so is the desert of Death Valley. All of the earth’s surface—mountains and deserts and deep-sea floors—is teeming with life, although it may not always appear so. The same ecological principles apply to the organization of all the earth’s communities, both on land and in the sea, although the details may differ greatly. Ecology is the study of ecosystems and the organisms that live within them. In this chapter, we focus on principles that govern the functioning of communities of organisms living together, and on the physical and biological factors that determine why particular kinds of organisms can be found living in one place and not in another. A proper understanding of how ecosystems function will be critical to preserving the living world in this new century.
36.1. Energy Flows Through Ecosystems
The ecosystem is the most complex level of biological organization. Collectively, the organisms in ecosystems regulate the capture and expenditure of energy and the cycling of chemicals. All organisms depend on the ability of other organisms— plants, algae, and some bacteria—to recycle the basic components of life.
Ecologists, the scientists who study ecology, view the world as a patchwork quilt of different environments, all bordering on and interacting with one another. Consider for a moment a patch of forest, the sort of place a deer might live. Ecologists call the collection of creatures that live in a particular place a community—all the animals, plants, fungi, and microorganisms that live together in a forest, for example, are the forest community. Ecologists call the place where a community lives its habitat—the soil and the water flowing through it are key components of a forest habitat. The sum of these two, community and habitat, is an ecological system, or ecosystem. An ecosystem is a largely self-sustaining collection of organisms and their physical environment. An ecosystem can be as large as a forest or as small as a tidepool.
The Path of Energy: Who Eats Whom in Ecosystems
Energy flows into the biological world from the sun, which shines a constant beam of light on our earth. Life exists on earth because some of that continual flow of light energy can be captured and transformed into chemical energy through the process of photosynthesis and used to make organic molecules such as carbohydrates, nucleic acids, proteins, and fats. These organic molecules are what we call food. Living organisms use the energy in food to make new materials for growth, to repair damaged tissues, to reproduce, and to do myriad other things that require energy, like turning the pages of this text.
You can think of all the organisms in an ecosystem as chemical machines fueled by energy captured in photosynthesis. The organisms that first capture the energy, the producers, are plants, algae, and some bacteria, which produce their own energy-storing molecules by carrying out photosynthesis. They are also referred to as autotrophs. All other organisms in an ecosystem are consumers, obtaining their energy-storing molecules by consuming plants or other animals, and are referred to as heterotrophs. Ecologists assign every organism in an ecosystem to a trophic (or feeding) level, depending on the source of its energy. A trophic level is composed of those organisms within an ecosystem whose source of energy is the same number of consumption “steps” away from the sun. Thus, as shown in figure 36.1, a plant’s trophic level is 1, while herbivores (animals that graze on plants) are in trophic level 2, and carnivores (animals that eat these grazers) are in trophic level 3. Higher trophic levels exist for animals that eat higher on the food chain (like the top carnivore in figure 36.1). Food energy passes through an ecosystem from one trophic level to another. When the path is a simple linear progression, like the links of a chain, it is called a food chain. The chain ends with decomposers, who break down dead organisms, or their excretions, and return the organic matter to the soil.
Figure 36.1. Trophic levels within an ecosystem.
Ecologists assign all the members of a community to various trophic levels based on feeding relationships.
The lowest trophic level of any ecosystem is occupied by the producers—green plants in most land ecosystems (and, usually, algae in freshwater). Plants use the energy of the sun to build energy-rich sugar molecules. They also absorb carbon dioxide from the air, and nitrogen and other key substances from the soil, and use them to build biological molecules. It is important to realize that plants consume as well as produce. The roots of a plant, for example, do not carry out photosynthesis— there is no sunlight underground. Roots obtain their energy the same way you do, by using energy-storing molecules produced elsewhere (in this case, in the leaves of the plant).
At the second trophic level are herbivores, animals that eat plants. They are the primary consumers of ecosystems. Deer and zebras are herbivores, and so are rhinoceroses, chickens (primarily herbivores), and caterpillars. Most herbivores rely on “helpers” to aid in the digestion of cellulose, a structural material found in plants. A cow, for instance, has a thriving colony of bacteria in its gut that digests cellulose for it. So does a termite. Humans cannot digest cellulose because we lack these bacteria—that is why a cow can live on a diet of grass and you cannot.
At the third trophic level are animals that eat herbivores, called carnivores (meat-eaters). They are the secondary consumers of ecosystems. Tigers and wolves are carnivores, and so are mosquitoes and blue jays. Some animals, like bears and humans, eat both plants and animals and are called omnivores. They use the simple sugars and starches stored in plants as food and not the cellulose. Many complex ecosystems contain a fourth trophic level, composed of animals that consume other carnivores. They are called tertiary consumers, or top carnivores. A weasel that eats a blue jay is a tertiary consumer. Only rarely do ecosystems contain more than four trophic levels, for reasons we will discuss later.
In every ecosystem there is a special class of consumers that include detritivores, organisms that eat dead organisms (also referred to as scavengers) and decomposers, organisms that break down organic substances making the nutrients available to other organisms. They obtain their energy from all trophic levels. Worms, crabs, and vultures are examples of detritivores. Bacteria and fungi are the principal decomposers in land ecosystems.
How much energy passes through an ecosystem? Primary productivity is the total amount of light energy converted by photosynthetic organisms into organic compounds in a given area per unit of time. An ecosystem’s net primary productivity is the total amount of energy fixed by photosynthesis per unit of time, minus that which is expended by photosynthetic organisms to fuel metabolic activities. In short, it is the energy stored in organic compounds that is available to heterotrophs. The total weight of all ecosystem organisms, called the ecosystem’s biomass, increases as a result of the ecosystem’s net productivity. Some ecosystems, such as cattail swamps, which are wetlands, have a high net primary productivity. Others, such as tropical rain forests, also have a relatively high net primary productivity, but a rain forest has a much larger biomass than a wetlands area. Consequently, a rain forest’s net primary productivity is much lower in relation to its biomass.
When a plant uses the energy from sunlight to make structural molecules such as cellulose, it loses a lot of the energy as heat. In fact, only about half of the energy captured by the plant ends up stored in its molecules. The other half of the energy is lost. This is the first of many such losses as the energy passes through the ecosystem. When the energy flow through an ecosystem is measured at each trophic level, we find that 80% to 95% of the energy available at one trophic level is not transferred to the next. In other words, only 5% to 20% of the available energy passes from one trophic level to the next. For example, the amount of energy that ends up in the beetle’s body in figure 36.2 is approximately only 17% of the energy present in the plant molecules it eats. Similarly, when a carnivore eats the herbivore, a comparable amount of energy is lost from the amount of energy present in the herbivore’s molecules. This is why food chains generally consist of only three or four steps. So much energy is lost at each step that very little usable energy remains in the system after it has been incorporated into the bodies of organisms at four successive trophic levels.
Figure 36.2. How heterotrophs use food energy.
A heterotroph assimilates only a fraction of the energy it consumes. For example, if a "bite" is composed of 500 Joules of energy (1 Joule = 0.239 calories), about 50%, 250 J, is lost in feces; about 33%, 165 J, is used to fuel cellular respiration; and about 17%, 85 J, is converted into consumer biomass. Only this 85 J is available to the next trophic level.
Lamont Cole of Cornell University studied the flow of energy in a freshwater ecosystem in Cayuga Lake in upstate New York. In figure 36.3, each block represents the energy obtained by a different trophic level, with the producers, the algae and cyanobacteria, being the largest block. He calculated that about 150 of each 1,000 calories of potential energy fixed by algae and cyanobacteria are transferred into the bodies of animal plankton (small heterotrophs). Of these, about 30 calories are incorporated into the bodies of a type of small fish called a smelt, the principal secondary consumers of the system. If humans eat the smelt, they gain about 6 of the 1,000 calories that originally entered the system. If trout eat the smelt and humans eat the trout, humans gain only about 1.2 of the 1,000 calories. Thus, in most ecosystems, the path of energy is not a simple linear one because individual animals often feed at several trophic levels. This creates a more complicated path of energy flow called a food web, as shown in figure 36.4.
Figure 36.3. Energy loss in an ecosystem.
In a classic study of Cayuga Lake in New York, the path of energy was measured precisely at all points in the food web.
Figure 36.4. A food web.
A food web is much more complicated than a linear food chain. The path of energy passes from one trophic level to another and back again in complex ways.
Key Learning Outcome 36.1. Energy moves through ecosystems from producers, to herbivores, to carnivores, and finally to detritivores and decomposers, which consume the dead bodies of all the others. Much energy is lost at each stage of a food chain.