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Conflict between ratio-dependent and prey-dependent views of trophic in- teractions finds its simplest resolution in the comparison of steady-state responses of trophic chains 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 -O, 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 uni- formly 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).

Recent papers opposing ratio dependence focus on four main criticisms: (1) the empirical evidence we present is insufficient or biased, (2) ratio-dependent models exhibit pathological behavior, (3) ratio dependence lacks a logical or mechanistic base, and (4) more general models incorporate both prey and ratio dependence and there is no need for either of the two simplifications. We review these arguments in the light of empirical evidence from field and experimental studies. We argue that (1) empirical evidence shows that most natural systems are closer to ratio dependence than to prey dependence, (2) pathological dynamics in a mathematical sense is not only realistic, but the lack of such dynamics in prey-dependent models actually makes them pathological in a biological sense, (3) the mechanistic base of ratio dependence is (direct and indirect) interference and resource sharing, and (4) although more general models (with extra parameters) can never fit natural patterns worse than either prey- or ratio-dependent models, there are theoretical, practical, and pedagogical reasons for attempting to find simpler models that can capture the essential dynamics of natural systems.

Ratio-dependent and prey-dependent models of trophic interactions make very different predictions about the steady-state (equilibrial) properties of ecosystems, such as the response of equilibrium abundance and biomass of organisms in each trophic level to increased primary productivity or nutrient input. Prey-dependent theory predicts alter- nating positive, negative, and zero responses of trophic levels to increased productivity, whereas ratio-dependent theory predicts proportional increase in all trophic level bio- masses. We analyzed data on the nutrient input and the biomasses of phytoplankton, zooplankton, and fish across lakes to distinguish between ratio-dependent and prey-de- pendent models. The results show parallel increases in all trophic levels as a result of increased nutrient input, demonstrating that natural systems are closer to ratio dependence than to prey dependence.