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Trophic cascades and trophic trickles in pelagic food webs

Prey–dependent models, with the predation rate (per predator) a function of prey numbers alone, predict the existence of a trophic cascade. In a trophic cascade, the addition of a top predator to a two–level food chain to make a three–level food chain will lead to increases in the population size of the primary producers, and the addition of nutrients to three–level chains will lead to increases in the population numbers at only the first and third trophic levels. In contrast, ratio–dependent models, with the predation rate (per predator) dependent on the ratio of predator numbers to prey, predict that additions of top predators will not increase the population sizes of the primary producers, and that the addition of nutrients to a three–level food chain will lead to increases in population numbers at all trophic levels. Surprisingly, recent meta–analyses show that freshwater pelagic food web patterns match neither prey–dependent models (in pelagic webs, ‘prey’ are phytoplankton, and ‘predators’ are zooplankton), nor ratio–dependent models. In this paper we use a modification of the prey–dependent model, incorporating strong interference within the zooplankton trophic level, that does yield patterns matching those found in nature. This zooplankton interference model corresponds to a more reticulate food web than in the linear, prey–dependent model, which lacks zooplankton interference. We thus reconcile data with a new model, and make the testable prediction that the strength of trophic cascades will depend on the degree of heterogeneity in the zooplankton level of the food chain.