| The small size of critically endangered populations causes them to be inherently volatile. Minor perturbations can rapidly shift demography and genetic composition, potentially outpacing the capacity of conservation research and management to detect and respond. Here we summarize seven years of noninvasive genetic sampling of an insular population of Sierra Nevada red fox (Vulpes vulpes necator), estimated at <10 breeding pairs, in the central Sierra Nevada. We used noninvasive fecal DNA approaches to monitor genetic and demographic aspects of the population during 2010-2017, including genetic assignment, pedigree reconstruction, and temporal and spatial capture-recapture density estimation. Notably, at the start of our study we observed no evidence of nonnative introgression over the past century. Yet in 2012, we began detecting emigration of nonnative foxes originating from the Great Basin region and subsequent interbreeding with native foxes. This unexpected series of events afforded an opportunity to observe how fundamental characteristics of the population shifted following outbreeding, including observed vital rates, density, genetic composition and diversity, and distribution limits. Our results highlight the dynamism of small populations and the importance of maintaining long-term monitoring programs. |