For animal species that form social groups, living together can have a strong effect on individuals’ chances of survival and reproduction, and ultimately on how population sizes change over time. New work, led by myself in collaboration with 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.
While meerkats, a species of mongoose native to southern Africa, may seem far removed from Raincoast’s work in BC, they share many characteristics with a much more familiar species: killer whales.
Both killer whales and meerkats are social mammals, living and breeding in family groups. Both species are matrilineal, with females usually reproducing in their group of birth. Both species share food: mostly invertebrates, in the case of meerkats, and Chinook salmon, in the case of resident killer whales. Both species’ survival and reproductive success are affected by other group members.
These new techniques may prove useful for understanding trends in resident killer whale populations. Tweet This!
Whether the effects of group living are positive or negative, an individual’s prospects tend to be limited within its birth group, and eventually many individuals disperse to establish new breeding groups. As a result, understanding changes in population size for social species – several of which, such as Southern Resident killer whales, are endangered – requires understanding what goes on within groups and how individuals fare when they strike out on their own.
This research presents new tools for analysis, aimed at improving our understanding of fluctuations in the population sizes of social species.
In the context of meerkats, past work had theorised that larger groups might produce more descendants each year than smaller groups. This phenomenon is called the Allee effect. My new work indicates the opposite.
The new techniques we have developed combine such group-level patterns to paint a picture of a population as a whole. While dominant breeding females in larger meerkat groups do appear to produce more daughters each year that go on to breed, smaller groups produce proportionately more descendants over the short term. Ultimately, it appears that population growth is highest when individuals form groups of intermediate size. These results help to bridge the gap between observed group trends, past theory concerning Allee effects, and the benefit that group living must provide, for it to exist at all.
These new techniques may prove useful for understanding trends in resident killer whale populations. While Northern Residents have increased in number to more than 250 over the last 40 years, the Southern Resident population has stagnated, now numbering only 74 individuals.
Some of my ongoing work will analyse patterns of Northern Resident killer whale births and deaths, within the context of social groups. Taking an approach similar to that used with meerkats, insights gained will be able to shed light on whether different social patterns in Northern and Southern Resident killer whales might contribute to their different population trends.
Bateman, A. W., Ozgul, A., Krkošek, M., & Clutton-brock, T. H. 2018. Matrix models of hierarchical demography: Linking group- and population-level dynamics in cooperative breeders. The American Naturalist 2018 192:2, 188-203
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.
A version of this article first appeared at the Applied Conservation Lab at UVic.