Safety in numbers: the dilution effect and other drivers of group life in the face of danger

Abstract

Animals can congregate in groups for many reasons, from reproductive assurance to improved foraging or predation efficiency, to avoiding themselves becoming the target of predation by other animals. It is the last category that is the focus of this review: group living as protection from predation. The drivers of group life in the face of danger are at the same time diverse and interlinked, with much potential for confusion between concepts. Here we review these concepts, using the dilution effect as a starting point. We construct a mathematical model that allows us to examine various features of the dilution effect and their connection to ecology. We also show the importance of including a time scale when modelling the dilution effect and how this translates into more realistic estimation of the fitness consequences of a diluted predation risk. The central role of the dilution effect in creating safety in numbers is underlined by showing how it may affect life-history evolution and result in the emergence of gregarious life-history strategies, even among sessile organisms limited in their abilities to exhibit behavioural responses to predation. Finally, we review the other central processes underpinning group protection from predation: the satiation effect, selfish herding, the confusion effect and group vigilance.