Community effects of tiger salamander hybridization in central California

Background/Question/Methods: Hybridization between the native California tiger salamander (Ambystoma californiense) and the introduced barred tiger salamander (Ambystoma tigrinum mavortium) from Texas began 50 years ago when non-native animals introduced to ponds by bait fishermen began interbreeding with native animals. Hybrid tiger salamanders now occur across fifteen to twenty percent of the range of A. californiense. We examined the ecological effects of hybridization on interactions between tiger salamander larvae of differing genetic composition, and their interactions with California newt larvae (Taricha torosa), and Pacific chorus frog tadpoles (Pseudacris regilla). We conducted two mesocosm experiments and one field enclosure experiment to investigate the impact of early generation hybrids (lab-crossed F1, F2, and backcrosses to californiense and mavortium), wild individuals of hybrid ancestry (from natural ponds in the hybrid zone) and both parental species on species interactions.

Results/Conclusions: Wild hybrid tiger salamanders significantly reduce recruitment of Taricha torosa and Pseudacris regilla through predation. Some early generation hybrids also negatively influenced Taricha and Pseudacris. Reductions in Taricha/Pseudacris recruitment varied predictably, but in a non-additive way, with early generation hybrid genotype: backcrosses to mavortium and F1’s exhibited ecological effects similar in magnitude to pure mavortium and wild hybrids, while backcrosses to californiense and F2’s had effects similar to pure californiense. Absolute growth and density-dependent competitive effects between tiger salamanders also differed between californiense and wild hybrid genotypes: wild hybrids grew larger and exhibited stronger negative density-dependence than native californiense. Wild hybrids also infrequently cannibalized native tiger salamanders. Our field enclosure experiment, which focused solely on interactions between hybrid and native tiger salamanders, generally supported mesocosm results. Large and significant size differences between wild hybrid and native tiger salamander larvae in enclosures exceeded differences found in mesocosms, however density-dependent effects were reduced. These findings suggest that tiger salamander hybridization has the potential to influence recruitment of other native amphibians. Evidence for accelerated growth of wild hybrid tiger salamanders and occasional cannibalism on native californiense suggests that ecological mechanisms may contribute directly to the success and spread of hybrid Ambystoma in central California.