ABSTRACT
Biological production in ecosystems is classified in ecology according to the trophic position, or trophic level of the species studied. The trophic level of a species is a measure of the number of predator-prey interactions that separate it, on the average, from primary producers or detritus (dead organic matter). The term predator-prey, trophic or feeding interaction indicates one organism or population feeding on another to acquire energy, organic matter, and nutrients. Primary producers are assigned a trophic level of one since they represent the energetic and material basis of ecosystems (Chapters 13 and 15), and the same is done for detritus. Herbivores, which are the consumers feeding on primary producers, are assigned a trophic level equal to two since they are one trophic link away from primary production, while predators feeding on herbivores have a trophic level of three, and so on. The trophic level thus depends on an organism’s diet. Most organisms display a certain degree of omnivory, which means that they feed on different prey species or trophic levels, reflecting the fact that ecosystems are not simple chains of trophic interactions, but intricate food webs (Section 19.5). Thus, the trophic level of an organism will not be an integer number but will be calculated from the trophic levels of its prey items by weighting them on the amount that the corresponding preys represent in the predator’s diet:
TL DC TLi j
∑1 1
, (16.1)
Thus, the trophic level TLi of the predator i is one link away from the average of the trophic levels of its N preys, after weighting each of such trophic levels (TLj) on the fraction DCj,i of the predator’s diet represented by the given prey species j.
