Trophic Level

Trophic Level

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.

Elliot Richmond

Bibliography

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth Publishing, 1990.

Johnson, George B. Biology: Visualizing Life. New York: Holt, Rinehart and Winston, Inc. 1998.