The relationship between group size and population abundance in foraging birds and ungulates

Abstract

Group foraging is a commonly observed phenomenon across various taxa that has been shown to improve an individual's fitness. There are tradeoffs to foraging in groups, with benefits such as reducing predation risk and costs such as increased competition within and between groups. The combination of these factors determines the optimal group size. While these factors are known to be density dependent, group size itself has not been explicitly tested for density dependence. Here I use an ODE model to show that the optimal group size in a population can exhibit density dependence. My model predicts that the optimal group size displays a power-law relationship with abundance, with a coefficient that depends on how predators respond to changes in their prey abundance. I then tested this model using existing empirical data and found that results were consistent across both the empirical and theoretical results. Previous work has shown that power-law group formation can stabilize predator-prey dynamics. Here I identify the key driving factors behind this pattern of group formation. My work contributes to a growing body of work that groups, rather than individuals, play a primary role in the population dynamics of group-forming fauna. This also illustrates how an individual's behavior, namely the decision to join a group, can scale up to be a significant driver of population dynamics in birds and ungulates.