A natural experiment in inbreeding depression in an isolated population of montane red fox

Background/Question/Methods

The reduced genetic diversity of small, bottlenecked populations renders them vulnerable to the effects of inbreeding depression. In many cases inbreeding depression may be the most immediate factor limiting growth in threatened populations, but difficulty in demonstrating its occurrence in the wild can hinder management efforts toward recovery. Sierra Nevada red fox (Vulpes vulpes necator) are an evolutionarily distinct subspecies that historically occurred throughout the subalpine zone in the Sierra Nevada and southern Cascade mountain ranges of California and Oregon. Today little is known about abundance in Oregon and only two isolated populations persist in California, each estimated to number fewer than 10 breeding pairs. In 2012 two nonnative male red fox of fur farm ancestry immigrated into an insular V.v.necator population in the central Sierra Nevada, providing a natural opportunity to test how an increase in genetic variance impacts reproductive fitness. Here we compared abundance and recruitment before and after arrival of the outbred males, using noninvasive genetic approaches with 33 microsatellite loci and a sex marker to identify individuals and reconstruct pedigrees on a 200 km²study area, during 2010–2013. 

Results/Conclusions

The number of offspring detected on the study area increased several fold following immigration of the two nonnative males. In two breeding seasons prior to the arrival of these individuals, 5 adult red fox and a single juvenile offspring were detected. In the breeding season following arrival of the nonnative males, 5 adult red fox and 6 offspring were detected. All offspring detected were of hybrid descent, with a male immigrant father and a native mother. Additionally, average observed heterozygosity of hybrid offspring was 0.77 ± 0.03, compared to an average observed heterozygosity of 0.46 ± 0.03 for V.v.necator. The increased reproductive output of outbred pairings is consistent with hybrid vigor associated with increased heterozygosity and masking of deleterious recessive alleles in the native population. Data from additional breeding seasons are needed to assess the fitness effects of outbreeding in future generations, but these findings emphasize the need to incorporate genetic restoration into conservation planning for the Sierra Nevada red fox.