A trophic level consists of organisms that get their energy from a similar source. Each step in a food chain is a trophic level. A food chain is a series of organisms each eating or decomposing the preceding organism in the chain. For example, in a lake phytoplankton are eaten by zooplankton and zooplankton are eaten by small fish. A food web is similar to a food chain, but in a food web there are many interconnected and interacting food chains. In a typical food chain, a producer or autotroph is the source of solar energy that powers the ecosystem . For example, in a grazing food web a herbivore eats living plant tissue (the producer) and is eaten in turn by an array of carnivores and omnivores. In contrast, a detrivore harvests energy from dead organic material and provides energy for a separate food chain.
There are communities of organisms surrounding deep-ocean hydrothermal vents that obtain their energy from bacteria (known as chemotrophs or chemosynthetic autotrophs ) that harvest heat energy and store it in chemical bonds. These communities are the rare exception. Photosynthesis is the ultimate source of energy for every other ecosystem on our planet. Producers (autotrophs, or photosynthesizing autotrophs ) use photosynthesis to harvest energy from the sun. All other organisms obtain their energy, directly or indirectly, from the autotrophs. All of the other organisms in the food chain are consumers (known as heterotrophs). The primary consumers eat the producers. Secondary consumers eat the primary consumers, and so on. For example, in a grassland ecosystem, grass is the producer. Grasshoppers are primary consumers. Shrews that prey on the grasshoppers are secondary consumers. Owls, hawks, and snakes prey on the shrews, so they are tertiary consumers. Of course, hawks also prey on snakes and grasshoppers, so the connections get complicated and are usually described as a web of relationships or a food web.
A food chain involves a transfer of matter and energy from organism to organism. As energy is transferred through the food chain or food web, some energy is converted to waste heat at each transfer. The quantity of energy lost is so great that food chains rarely involve more than four or five steps from consumer to top predator. Each level in a food chain or food web is known as a trophic level, a group of organisms that all consume the same general types of food in a food web or a food chain. In a typical food web, all producers belong to the first trophic level and all herbivores (primary consumers) belong to the second trophic level. Using the same grassland as an example, the second trophic level would be all of the herbivores that eat the grass. This group can include a wide variety of different organisms. In the original grasslands of the central United States, the second trophic level included grasshoppers, rabbits, voles and other small rodents, prairie dogs, and American bison. Since they all eat the same grass, they are all at the same trophic level, despite their differences in size, reproductive habits, or any other factors.
The second trophic level in an ecosystem is relatively easy to identify, because the organisms in this level all obtain their energy directly from the autotrophs at the first level. After the second level in a food web, the situation becomes progressively more complex. Many organisms obtain energy from several different sources at different trophic levels. For example, foxes are opportunistic omnivores. They will eat fruits, small herbivores, and small carnivores. Likewise, many birds eat seeds and fruits in one season and switch to eating insects in a different season.
Another problem in classifying trophic level arises because the energy available at a given trophic level includes many different forms. Plant tissue includes wood, nectar, pollen, seeds, leaves, and fruit. No animal eats all of these different forms of plant tissue. Animals with very different ecological characteristics exploit these various tissues. Termites eat wood; fruit bats eat fruit. Termites offer an additional complication, because it is the organisms in the termite's gut that digest the cellulose who are the actual second-level consumers.
How energy flows through an ecosystem depends on the nature of the producers at the first trophic level. These producers support the entire ecosystem, so their abundance and energy content per kilogram determine the overall energy flow and biomass of the ecosystem. Organisms that live on land expend much of their energy in building supporting structures. These supporting structures are not generally available as an energy source to consumers. For example, in a forest, both matter and energy accumulate in the form of wood fibers that cannot be eaten by most animals. On the other hand, grasses have little supporting structure. The herbivores that consume grass are able to eat all of the above ground parts of the plant. With the aid of the specialized bacteria in their guts, grassland herbivores are able to harvest more energy per kilogram of plant material present.
A grassland ecosystem—or any ecosystem—can be represented by an energy pyramid. The base of the pyramid is the community of producers, including various species of grass. The primary consumers, grasshoppers, rodents, rabbits, and bison make up the second level of the pyramid— herbivores. The third level of the pyramid is the secondary consumers, predators that prey on the herbivores. The producers far outnumber the herbivores who far outnumber the carnivores, so the grassland pyramid has a broad base and a narrow tip.
In aquatic environments away from the shoreline, the situation is reversed. The primary photosynthesizing organisms are algae. Small herbivores (grazers) consume the entire organism and harvest almost all of the energy. These grazers decimate the algae population , keeping it relatively small. In this ecosystem, most of the energy (and matter) is stored in the second trophic level—the grazers. However, the high reproductive rate and short life cycle of the algae keep the population at a level sufficient to support the grazers.
see also Biomass; Food Web.
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Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth Publishing, 1990.
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One way of analyzing the biological relationships within an ecosystem is to describe who eats whom within the system, also called a functional analysis. Each feeding level in an ecosystem is called a trophic level. In the grasslands , for example, plants are considered primary producers, forming the first trophic level. The second trophic level consists of primary consumers, such as deer, mice, seed and fruit-eating birds, and other animals, depending completely on the primary producers for their food. Carnivores and predators, such as hawks, are the secondary consumers. Often, the same species may fit into several categories. Bears, for example, are considered both primary and secondary consumers because they feed on plant matter as well as meat. Bacteria and fungi that decompose dead organic matter are called the decomposers . Thus, on the basis of food source and feeding behavior, a complex food chain/web exists within any ecosystem and every species belongs to one or more of several trophic levels.
In environmental science , the concept of trophic levels is often used to assess the potential for transfer of pollutants through an ecosystem. Since each trophic level is dependent on all the other levels, positive or negative changes in the composition or abundance of any one trophic level will ultimately affect all other levels. In ecosystems that normally have stable, complex trophic levels within the food web, pollution can lead to fluctuations and simplification of the trophic levels. Contaminants that are taken up by plants from the soil may be transferred to primary and secondary consumers through their feeding patterns. This is known as trophic level transfer.
A classic example of trophic level transfer was the release of DDT in the environment . DDT is an insecticide commonly used in the United States during the 1950s and early 1960s. DDT ran off treated fields into lakes and rivers, where it accumulated in the fatty tissues of primary consumers such as fish and shellfish. The chemicals were then transferred into secondary consumers, such as eagles, which fed on the primary consumers. While the concentrations of DDT were rarely high enough to kill the birds, it did cause them to lay eggs with thin shells. The thin eggshells led to decreased hatching success and thus caused a decline in the eagle population.
Trophic level analysis is a commonly used method of environmental assessment. A pollutant or disturbance is assessed in terms of its effects on each trophic level. If significant amounts of nutrients are brought into a lake receiving fertilizer runoff from fields, a spurt in the growth of algae (primary producers) in the lake may be triggered. However the increased growth might actually be dominated by certain algal groups, such as blue-green algae, which may not constitute desirable food sources for the zooplankton and fish (primary consumers) which normally depend on algae for food. In this case, even though the environmental conditions might appear to stimulate increased growth in one trophic level, the nature of the change does not necessarily prove advantageous to other trophic levels within the same system.
[Usha Vedagiri ]
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