Understanding extinction risk and resilience in an extremely small population facing climate and ecosystem change

Abstract

AbstractSan Nicolas Island (SNI) foxes historically had the highest densities of the six subspecies of Channel Island fox, four of which were listed as endangered in the 1990s. As an island species, SNI foxes are inherently vulnerable because they evolved in isolation from many potential threats, and their small range features relatively simple and degraded habitats, limiting their population size and heightening susceptibility to climate‐driven habitat changes. In the past decade, the SNI fox population has decreased by nearly half, spurring concern. Our analyses of 18 yr of trapping data suggest that both drought and density dependence contributed to this decline. Density dependence in survival acted on older individuals and its strength doubled after recent drought‐induced habitat changes. Annual precipitation was a strong driver of pup production, but density dependence moderated precipitation benefits. Using a stochastic model with island‐specific drivers of demography, we developed a risk isocline tool to predict 50‐yr quasi‐extinction risk based on population size and mortality. Predicted extinction risk for SNI foxes is currently <5% and expected to decrease. Compared to the previously listed subspecies, risk isoclines indicate the SNI subspecies is more resistant to quasi‐extinction at moderate population sizes and mortality rates, but quasi‐extinction risk does not decline as rapidly at higher population sizes or lower mortality rates. Thus, limiting human‐caused mortality and monitoring for novel threats such as exotic disease remain important. The ability of SNI foxes to increase pup production following high‐rainfall low‐density years is a key feature stabilizing their dynamics. Habitat‐ and location‐specific dynamics produce substantial portfolio effects, whereby asynchrony in growth rates stabilizes overall population numbers, and restoration efforts that increase food resources and habitat diversity will further enhance the resilience of this population. Although the SNI fox appears to be rebounding, its recent decline may foreshadow future dynamics and serves as a cautionary tale for stewards of island species facing unprecedented climatic conditions, especially those inhabiting small and less diverse systems. Finally, our analyses highlight limitations in applying risk analyses from one population to related species, or even the same species in different habitat settings.