Effects of predation and nutrient enrichment on a food web with edible and inedible prey

Summary 1. Trophic heterogeneity, or differences in edibility or resource consumption among species within a single trophic level, is widespread in natural food webs. Here, we test simple food web models that incorporate trophic heterogenity and that make robust predictions regarding food web responses to nutrient enrichment. To test these predictions, we assembled simple food webs consisting of an inedible alga, a mixed assemblage of bacteria, and a protist bacterivore in laboratory microcosms of contrasting nutrient concentrations. 2. Several results were consistent with model predictions. First, increasing nutrient concentration caused an increase in the abundance of the inedible alga, but only in the presence of the bacterivore. Secondly, nutrient enrichment increased the abundance of bacteria, but only in the absence of their bacterivore. Last, nutrient enrichment had no effect on abundance of the bacterivore. 3. Two results were not consistent with model predictions. First, at low nutrient concentrations, the presence of the bacterivore increased the abundance of bacteria. Secondly, although the abundances of the bacterivore and bacteria were positively correlated, some of the lowest abundances of both occurred in the high nutrient treatment. Thus, while our results were generally consistent with several simple food web models, additional explanations are required for selected food web responses.