Inferring Process from Pattern: Trophic Level Abundances and Imbedded Interactions

Classical predator—prey models do not predict the positive relationships between the abundances of organisms on adjacent trophic levels seen in natural communities. Modifications that can relieve this disparity can be categorized as mechanisms of "predator damping," which can be expressed in a simple, analytical form as ratio dependence. It has been suggested that the strength of predator damping can be estimated from the shape of the relationship between prey and predator biomass using ratio—dependent models. I present a theoretical argument showing why this approach should only be applied when predator and prey are the top two trophic levels in an ecosystem. Using published data from temperate lakes that include large—scale fish manipulations, I also show that the relationship between phytoplankton biomass and Daphnia biomass suggests much weaker damping when higher trophic levels (fish) are absent than when fish are present. Thus, inferring the strength of predator damping using patterns of abundance in ecosystems is of dubious validity when interactions are imbedded within simple food chains.