Predator Interference across Trophic Chains

Conflict between ratio—dependent views of trophic interactions finds its simplest resolution in the comparison of steady—state responses of trophic chain to increases in primary productivity. We analyze data on trophic abundances of lake ecosystems with a general trophic—chain model that is intermediate between prey dependence and ratio dependence, and can describe either of the two extremes, depending on parameter values. We estimate, for all trophic levels, the set of interference coefficients that predicts the observed statistical relationship between biomasses of adjacent trophic levels. This constitutes a test of two competing theories, since the model we use is a prey—dependent model if all interference coefficients are °0, and a ratio—dependent model if they are °1. The results show that the observed relationships among trophic levels in lake ecosystems are compatible with a ratio—dependent parameterization of this model, with interference distributed relatively evenly across trophic chains, and are incompatible with a pure prey dependent parameterization. We suggest that approximate ratio dependence applied uniformly across trophic chains is a more plausible description of lake ecosystems than either pure prey dependence (non—interference) or "concentrated interference" (prey dependence at some trophic levels and partial interference in others).