TROPHIC CASCADES AND TROPHIC TRICKLES IN PELAGIC FOOD WEBS

Prey-dependent models, with the predation rate (per predator) a functi on 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 fo od chain to make a three-level food chain will lead to increases in th e population size of the primary producers, and the addition of nutrie nts to three level chains will lead to increases in the population num bers at only the first and third trophic levels. In contrast, ratio-de pendent models, with the predation rate (per predator) dependent on th e ratio of predator numbers to prey, predict that additions of top pre dators 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. Surpris ingly, recent meta-analyses show that freshwater pelagic food web patt erns match neither prey-dependent models (in pelagic webs, 'prey' are phytoplankton, and 'predators' are zooplankton), nor ratio-dependent m odels. In this paper we use a modification of the prey-dependent model -incorporating strong interference within the zooplankton trophic leve l-that does yield patter ns matching those found in nature. This zoopl ankton interference model corresponds to a more reticulate food web th an in the linear, prey-dependent model, which lacks zooplankton interf erence. We thus reconcile data with a new model, and make the testable prediction that the strength of trophic cascades will depend on the d egree of heterogeneity in the zooplankton level of the food chain.