Group formation stabilizes predator–prey dynamics

Super groups Many if not most predator and prey populations are puzzlingly stable, despite the fact that instability is expected based on the kinds of ecological interactions that typically occur. Fryxell et al . develop the hypothesis that one factor contributing to stability is reduced predator search efficiency caused by group formation by both predators and prey. Field data on lions and their large herbivore prey from Serengeti National Park suggest that group formation by both protagonists has an enormous impact on population dynamics, lending stability to what would otherwise be a highly unstable situation. Group formation by both predators and prey dramatically reduces predator search efficiency. Field data on lions and their large herbivore prey from Serengeti National Park suggest that group formation by both predators and prey has an enormous impact on Serengeti population dynamics, lending a stabilizing influence to what would be otherwise be a highly unstable situation. Theoretical ecology is largely founded on the principle of mass action, in which uncoordinated populations of predators and prey move in a random and well-mixed fashion across a featureless landscape. The conceptual core of this body of theory is the functional response, predicting the rate of prey consumption by individual predators as a function of predator and/or prey densities 1 , 2 , 3 , 4 , 5 . This assumption is seriously violated in many ecosystems in which predators and/or prey form social groups. Here we develop a new set of group-dependent functional responses to consider the ecological implications of sociality and apply the model to the Serengeti ecosystem. All of the prey species typically captured by Serengeti lions ( Panthera leo ) are gregarious, exhibiting nonlinear relationships between prey-group density and population density. The observed patterns of group formation profoundly reduce food intake rates below the levels expected under random mixing, having as strong an impact on intake rates as the seasonal migratory behaviour of the herbivores. A dynamical system model parameterized for the Serengeti ecosystem (using wildebeest ( Connochaetes taurinus ) as a well-studied example) shows that grouping strongly stabilizes interactions between lions and wildebeest. Our results suggest that social groups rather than individuals are the basic building blocks around which predator–prey interactions should be modelled and that group formation may provide the underly