Genetic introgression from introduced species into native populations is a growing challenge for biological conservation, and one that raises unique practical and ethical issues. Here, we describe the extent of introgression between native California tiger salamanders (Ambystoma californiense) and introduced barred tiger salamanders (A. tigrinum mavortium) relative to habitat, distance from introduction sites, and watershed boundaries. We used ancestry informative markers (AIMs) to characterize the degree of introgression at 85 sites within the range of A. californiense. Eight unlinked markers showed concordant patterns, indicating that different chromosomal segments are introgressing at similar rates. The current distribution of introduced alleles is largely contained in the Salinas Valley, California. Within it, the distribution of nonnative alleles was best explained at a broad geographic scale by the history of introductions, with limited introgression beyond 12 km from multiple independent release sites. The spatial transition from highly admixed to nearly pure native populations was abrupt, suggesting either cryptic barriers to dispersal or locally rapid displacement of natives by an advancing hybrid swarm. At a more ecological level, highly modified perennial breeding ponds had higher introduced allele frequencies than more natural seasonal ponds, suggesting greater invasion success in perennial breeding ponds. Management favoring natural habitat characteristics may substantially decrease the rate of spread of introduced alleles.
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