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

Theoretical and some empirical evidence suggest 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 this population dynamics is
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, i.e., implying group- 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 risk of local
extinction in these rodents and potentially, in other plurally breeding
and colonial species.