Hybridization between a rare, native tiger salamander (Ambystoma californiense) and its introduced congener

Exotic species threaten native biodiversity through predation, competition, and habitat alteration, but also by hybridizing with native species. A lack of reproductive isolation between exotic and native species can lead to genetic swamping, loss of native genetic diversity, and, in rare or endangered species, extirpation or extinction. We examined hybridization between a declining native salamander, the California tiger salamander, Ambystoma californiense, and an introduced congener, A. tigrinum. Ambystoma californiense is restricted to central California where A. tigrinum has been deliberately introduced as fish bait. In the Salinas Valley, we sampled salamanders from four artificial ponds and two natural vernal pools. Based on mitochondrial DNA and two nuclear loci, we found that hybrids were present in all six ponds, and that these hybrids were viable and fertile. No potentially pure A. californiense were present in three of the six ponds, and only one pond had more than 8% possibly pure native animals. Despite a relatively ancient split and wide genetic divergence between these taxa, they are interbreeding and threatening the genetic purity of the native species. Our data also suggest that the extent of the genetic mixing depends on the breeding habitat. There is little evidence of barriers to gene exchange in the four artificial breeding ponds. However in the two vernal pools, we found significantly fewer larvae with hybrid genotypes and significantly more with pure parental genotypes than expected. Linkage disequilibria revealed positive associations between native alleles and genotypes, and negative associations between native and introduced alleles and genotypes in these two ponds. Despite rampant hybridization, these data provide evidence of some constraints on hybridization in the native breeding habitats. Our results suggest that habitat characteristics of native species should be exploited in management strategies to limit hybridization with exotics.