The spatial distribution of foragers and food patches can influence antipredator vigilance

Abstract

Antipredator vigilance is a major component of defenses against predators for many prey species. For group foragers, such vigilance is predicted by models to decrease with group size reflecting better predator detection ability and risk dilution in larger groups. Influential vigilance models for group foragers have made simplifying and often quite restrictive assumptions. Prey species, for instance, are expected to search for resources in groups of fixed sizes although frequent changes in group sizes often occur while foraging. Groups of prey in the same area are also assumed to be attacked independently, but predators could sequentially target several local groups after a failed attempt. I propose a framework in which prey animals can form groups by joining feeding neighbors and also adjust their vigilance in these groups of varying sizes. Predators can attack one of the many groups that occur in the same area and can also target groups of specific sizes. I used a genetic algorithm approach to simultaneously tackle joining and vigilance choices by prey individuals. I show that joining tendencies and the effect of group size on vigilance can vary with forager population size, the spatial distribution of resources, and predator attack tactics. The modeling framework adopted here generates several novel predictions about vigilance and joining tendencies for group foragers, and highlights the importance of considering the availability and vulnerability of prey groups in the same habitat when predicting antipredator vigilance.