A restricted hybrid zone between native and introduced red fox (Vulpes vulpes) populations suggests reproductive barriers and competitive exclusion

Abstract

Introduced species can threaten native taxa in multiple ways, including competition and hybridization, which can reduce fitness, alter ecological niches or swamp native genomes. Encroachment and hybridization by introduced species also provide opportunities to study the dynamics of invasiveness and hybridization during early stages following contact. We used 33 microsatellites, 51 single nucleotide polymorphisms and a mtDNA marker to characterize the extent and spatial pattern of encroachment and hybridization between a native, endemic subspecies of red fox (Vulpes vulpes patwin) and an introduced red fox population composed of highly admixed, phylogenetically divergent stock, resulting from a century of domestication. Both nuclear and mtDNA markers indicated that hybridization was primarily restricted to a narrow zone where the two populations came into contact. Although a few introgressed genotypes were detected in the interior of the native range, we found no immigrant foxes or F(1) or F(2) hybrids there, suggesting native foxes excluded introduced individuals. We speculate that the observed interbreeding at the periphery was facilitated by low densities. In total, 98% of mtDNA haplotypes in the native range were native and 96% of the nuclear ancestry was estimated to be native. Although the introduced range had expanded fivefold over the past four decades, native and non-native haplotypes from museum samples collected in and near the native range three decades earlier showed a similar geographic distribution as today, suggesting that the native range and hybrid zone were relatively stable. We hypothesize that the monogamous mating system of red foxes and other wild canids may enhance their resistance to hybridization because of greater fitness consequences associated with mate discrimination.

A Preliminary Range-Wide Distribution Model for the Sacramento Valley Red Fox

“…When active burrows or aboveground sightings of neonatal pups were located, we recorded locations with a global positioning unit. As described in detail previously in a genetic study that utilized the same specimens as the present study (Sacks et al 2011), fresh scats were collected from near den entrances for genetic analysis (mitochondrial cytochrome b and D loop sequencing, microsatellites) to confirm species and population of origin. Likewise, road kills also were genetically assigned previously (Sacks et al 2011) to assess population of origin.…”

“…As described in detail previously in a genetic study that utilized the same specimens as the present study (Sacks et al 2011), fresh scats were collected from near den entrances for genetic analysis (mitochondrial cytochrome b and D loop sequencing, microsatellites) to confirm species and population of origin. Likewise, road kills also were genetically assigned previously (Sacks et al 2011) to assess population of origin. Because some den sites and road-killed foxes on the periphery of the native range were determined to reflect hybrids between native and nonnative red foxes, in the present study, we used only dens and road-killed foxes in our analysis for which genetic assignment had previously indicated pure native ancestry (Sacks et al 2011).…”

“…Likewise, road kills also were genetically assigned previously (Sacks et al 2011) to assess population of origin. Because some den sites and road-killed foxes on the periphery of the native range were determined to reflect hybrids between native and nonnative red foxes, in the present study, we used only dens and road-killed foxes in our analysis for which genetic assignment had previously indicated pure native ancestry (Sacks et al 2011). …”

“…Moreover, the genetic integrity of Sacramento Valley red foxes has been maintained largely intact despite introductions in the 1900s and subsequent spread of feral red foxes (originating from fur farms), which currently occupy most other low-elevation regions of California . Thus, despite the superficial similarities in the appearance of native and nonnative lowland red foxes and low levels of hybridization, native red foxes in the Sacramento Valley have maintained their genetic distinctiveness, possibly as a consequence of social reproductive barriers (Sacks et al 2011). At present, however, little is known about the status of the subspecies, including its census population size, threats to its persistence, or basic habitat use (Sacks et al 2010a).…”

“…In 2007, we initiated a study to determine the range extent and genetic integrity of Sacramento Valley red foxes relative to an adjacent red fox population comprised of individuals of known nonnative ancestry (Moore 2009;Sacks et al 2010aSacks et al , 2010bSacks et al , 2011Converse 2012). As part of our efforts, we documented the presence of reproductive den sites and red foxes killed by vehicles (hereafter, ''road kills'') of the native subspecies, which can be used to describe and map habitats supporting foxes.…”