A general theory for the dynamics of social populations: Within-group density dependence and between-group processes.

Abstract

Despite the importance of population structures throughout ecology, relatively little theoretical attention has been paid to understanding the implications of social groups for population dynamics. The dynamics of socially structured populations differ substantially from those of unstructured or metapopulation-structured populations, because social groups themselves may split, fuse, and compete. These "between-group processes" have been suggested to be important drivers of the dynamics of socially-structured populations, but no general theoretical framework exists that can handle various density-dependent between-group processes within a single model. Here, we develop a general framework for the dynamics of socially-structured populations that considers births, deaths, migration, group extinction, fissions, fusions, and between-group competition within a single model. Both logistic growth and an Allee effect are considered for within-group density dependence. We show that the effect of various between-group processes is mediated by their influence on the stable distribution of group sizes, with the ultimate impact on the population determined by the interaction between the altered group size distribution and within-group density dependence. The group level is important to the dynamics of the entire population, since it drives extinction risk, impacts population growth rates, and leads to the emergence of population-level density dependence (even if birth and death rates depend only on group size and not population size). We conclude with a series of case studies that illustrate different ways that age, sex, and class structure impact the dynamics of social populations. In sum, our results make clear the importance of within-group density dependence, between-group dynamics, and the interactions between them for the population dynamics of social species and provide a general, flexible framework for modeling social populations.