Population dynamics of a communally rearing mammal is driven by population-level but not group-level Allee effects.

Abstract

Theoretical and some empirical evidence suggest that the population dynamics of cooperative breeders (i.e. species with groups including non-reproductive individuals that raise the offspring of dominant breeders) are more likely to exhibit Allee effects at the level of social groups rather than at the population level. However, the extent to which these population dynamics are similar in species where breeding is plural, and group members communally rear their offspring remains unclear. Such species may still be subject to demographic Allee effects at the population-level. Using a 15-year dataset, we examined population and group-level dynamics of communal rearing and colonial Octodon degus to determine whether population- and group-level Allee effects influence population dynamics. We tested whether these effects are contingent on food availability, and whether group size is decoupled from population density, that is implying group-level but not population-level Allee effects. We recorded (i) population-level Allee effects on per capita population growth rate (i.e. demographic) and on per female fecundity rate (i.e. component), (ii) no group-level Allee effects on group per female fecundity, and (iii) that Allee effects detected are more likely whenever food availability is scarce. We further verified that group size is coupled to population density (iv). Our study highlighted how food-mediated cooperation through a colonial setting underlies Allee effects at the population level, and that group-living does not buffer degus against population-level Allee effects. Thus, our findings provide a plausible mechanism underpinning the risk of local extinction in these rodents and potentially in other plurally breeding and colonial species.