Evidence for an Allee effect in a declining fur seal population

Allee effects play an important role in the dynamics of many populations
and can increase the risk of local extinction. However, some authors have
questioned the weight of evidence for Allee effects in wild populations.
We therefore exploited a natural experiment provided by two adjacent
breeding colonies of contrasting density to investigate the potential for
Allee effects in an Antarctic fur seal (Arctocephalus gazella) population
that is declining in response to climate-change induced reductions in food
availability. Biometric time-series data were collected from 25 pups per
colony during two consecutive breeding seasons, the first of which was
amongst the worst on record in terms of breeding female numbers, pup birth
weights and foraging trip durations. In previous decades when population
densities were higher, pup mortality was consistently negatively
density-dependent, with rates of trauma and starvation scaling positively
with density. However, we found the opposite, with higher pup mortality at
low density and the majority of deaths attributable to predation. In
parallel, body condition was also depressed at low density, particularly
in the poor-quality season. Our findings shed light on Allee effects in
wild populations and highlight a potential emerging role of predators in
the ongoing decline of a pinniped species.