Predation and the evolution of gregariousness. II. An economic model for predator-prey interaction

An earlier paper presented models which allow the relative advantages of different prey dispersions to be assessed for two cases: when the prey's main defence is concealment, and when prey may evade the predator (Treisman 1975). With these as a basis, an economic model for predator-prey interaction is developed which takes explicit account of the conflicts between the need to maintain watch for a predator and the prey's other needs. The model embodies arguments based on statistical decision theory which allow factors such as the degree of predation threat, the watchfulness of the prey and the number of watchers to be related to the expected value of the prey's performance. The model provides a basis for predicting the optimal number of watchers or optimal group size under various conditions. The advantages of different group configurations are compared, and it is shown that animals which aggregate (and maintain a high sensory decision criterion) will have an advantage over solitary animals under certain conditions.