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.
