“Matrix models of hierarchical demography: Linking group- and population-level dynamics in cooperative breeders”
Andrew W. Bateman, Arpat Ozgul, Martin Krkošek, and Tim Clutton-Brock (Aug 2018)
Group matrix models shed light on population dynamics, Allee effects in meerkats and evolution of cooperative breeding
For animal species that form social groups, living together can affect individuals’ chances of survival and reproduction. Whether the effects are positive or negative, an individual’s prospects tend to be limited within its group of birth, and many individuals leave in an effort to establish new breeding groups. As a result, understanding changes in population size for social species – several of which, such as African wild dogs and southern resident killer whales, are endangered – requires understanding of what goes on within groups and how individuals fare when they strike out on their own.
New work by a team of researchers from Canada, the UK, and Switzerland combines theory and data to shed light on how these considerations play out for meerkats, a species of mongoose native to southern Africa. While past work has theorized that larger meerkat groups may produce more descendants each year than smaller groups (a phenomenon called the “Allee effect”), this new work indicates the opposite. Interestingly, dominant breeding females in larger groups do appear to produce more daughters that go on to breed. Overall, however, population growth seems to be maximized when meerkats form groups of intermediate size.
This work presents a new way of analyzing data from social animals. The techniques used may prove valuable in exploring the evolution of sociality and in understanding fluctuations in populations of endangered or threatened species.
For highly social species, population dynamics depend on hierarchical demography that links local processes, group dynamics, and population growth. Here, we describe a stage-structured matrix model of hierarchical demography, which provides a framework for understanding social influences on population change. Our approach accounts for dispersal and affords insight into population dynamics at multiple scales. The method has close parallels to integral projection models, but focuses on a discrete characteristic (group size). Using detailed long-term records for meerkats (Suricata suricatta), we apply our model to explore patterns of local density dependence and implications of group size for group and population growth. Taking into account dispersers, the model predicts a per-capita growth rate for social groups that declines with group size. It predicts that larger social groups should produce a greater number of new breeding groups; thus dominant breeding females (responsible for most reproduction) are likely to be more productive in larger groups. Considering the potential for future population growth, larger groups have the highest reproductive value, but per-capita reproductive value is maximized for individuals in smaller groups. Across a plausible range of dispersal conditions, meerkats' long-run population growth rate is maximized when individuals form groups of intermediate size.