Energy Flow

When one organism eats another, molecules are metabolized and energy is transferred. As a result, energy flows through an ecosystem, moving from producers to consumers. One way to follow the pattern of energy flow is to group organisms in an ecosystem based on how they obtain energy. An organism's trophic (TRAHF-ik) level indicates the organism's position in a sequence of energy transfers. For example, all producers belong to the first trophic level. Herbivores belong to the second trophic level, and the predators belong to the third level. Most terrestrial ecosystems have only three or four trophic levels, whereas marine ecosystems often have more.

Food Chains and Food Webs

A food chain is a single pathway of feeding relationships among organisms in an ecosystem that results in energy transfer. A food chain may begin with grass, which is a primary producer. The chain may continue with a consumer of grass seeds—a meadow mouse. Next, a carnivorous snake may kill and eat the mouse. A hawk then may eat the snake, as shown in Figure 18-9.

The feeding relationships in an ecosystem are usually too complex to be represented by a single food chain. Many consumers eat more than one type of food. In addition, more than one species of consumer may feed on the same organism. Many food chains interlink, and a diagram of the feeding relationships among all the organisms in an ecosystem would resemble a web, as shown in Figure 18-10. For this reason, the interrelated food chains in an ecosystem are called a food web.

Energy Transfer trophic levels

Figure 18-11 represents the amount of 4 energy stored as organic material in each trophic level in an ecosystem. 3 The pyramid shape of the diagram indicates the low percentage of energy 2 transfer from one level to the next. On average, 10 percent of the total energy 1 consumed in one trophic level is incorporated into the organisms in the next.

Why is the percentage of energy transfer so low? One reason is that some of the organisms in a trophic level escape being eaten. They eventually die and become food for decomposers, but the energy contained in their bodies does not pass to a higher trophic level. Even when an organism is eaten, some of the molecules in its body will be in a form that the consumer cannot break down and use. For example, a cougar cannot extract energy from the antlers, hooves, and hair of a deer. Also, the energy used by prey for cellular respiration cannot be used by predators to synthesize new biomass. Finally, no transformation or transfer of energy is 100 percent efficient. Every time energy is transformed, such as during the reactions of metabolism, some energy is lost as heat.

Limitations of Trophic Levels

The low rate of energy transfer between trophic levels explains why ecosystems rarely contain more than a few trophic levels. Because only about 10 percent of the energy available at one trophic level is transferred to the next trophic level, there is not enough energy in the top trophic level to support more levels.

Organisms at the lowest trophic level are usually much more abundant than organisms at the highest level. In Africa, for example, you will see about 1,000 zebras, gazelles, and other herbivores for every lion or leopard you see, and there are far more grasses and shrubs than there are herbivores. Higher trophic levels contain less energy, so, they can support fewer individuals.

Large carnivores Small carnivores Herbivores

Producers figure 18-11

This diagram represents energy transfer through four trophic levels. The amount of energy transferred from one level to another can vary, so the structure shown can vary. What is always true, however, is that the top level is much smaller than the lowest level. Hence, energy-transfer diagrams are always roughly pyramid shaped.

Large carnivores Small carnivores Herbivores

Producers figure 18-11

This diagram represents energy transfer through four trophic levels. The amount of energy transferred from one level to another can vary, so the structure shown can vary. What is always true, however, is that the top level is much smaller than the lowest level. Hence, energy-transfer diagrams are always roughly pyramid shaped.

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