Trophic levels…

This entry is part 2 of 1 in the series Ecology

I mentioned previously that plants are the first step in nearly all food chains. A food chain or food web describes the flow of energy between the organisms in a habitat. Each step in the food chain represents a different ‘trophic level’ (trophic is a word that refers to nutrition).

The first level contains the primary producers (or just producers) and usually consists of green plants. The second level has the primary consumers (herbivores); you then have secondary & tertiary consumers above that. At the final trophic level sits the top carnivore. It is unusual for a food chain to have more than four or five levels. There are a number of possible reasons for this.

No feeding system is 100% efficient, there is always waste so not all the energy is passed on at each stage.
Often organisms get larger, and better armed, as the trophic levels increase.
Communities cannot usually support a large number of different carnivores.

You can also imagine the same idea in terms of the numbers of organisms at each trophic level. A large number of plants are needed to support a population of herbivores. They in turn can support a smaller number of secondary consumers which can again support even fewer tertiary consumers.

If you imagine a food chain sustained by an oak tree then it might be easier to imagine it as a pyramid of biomass rather than numbers of individual organisms. Biomass means the mass of biological material/living things.

The group of organisms that I have so far omitted are the decomposers. Bacteria and fungi break down all the organisms in a food chain after death. This releases nutrients such as nitrate back into the soil and carbon dioxide into the atmosphere.