The Ecology, Evolution, and Management of Recent Non-Native Hybridization of the Endangered California Tiger Salamander (Ambystoma californiense)
The introduction of non-native species is a growing threat to biodiversity worldwide. Hybridization between invasive and endangered species severely complicates the management and conservation of threatened taxa. In these situations, it is necessary to understand the forces that drive hybrid success on the landscape in order to employ the most efficient and effective strategies to preserve native diversity. In this thesis, I present three studies that target specific aspects of hybridization between the endangered California tiger salamander (Ambystoma californiense) and the introduced barred tiger salamander (Ambystoma mavortium). In the first chapter, I use a Critical Thermal Maximum (CTMax) assay to show that hybrid salamanders can function at greater temperatures than either parental species. Complementary analysis of gene expression uncovered extensive transgressive segregation in F1 hybrids, which may explain this enhanced thermal ability. Increased temperature tolerance in hybrid salamanders may contribute to their success in California. The final two chapters evaluate a potential method for reducing the success of hybrid salamanders in the wild. Previous work has suggested that breeding pond duration (hydroperiod) may confer fitness differences between hybrid and native salamanders. In the second chapter I constructed an array of large, semi-natural experimental ponds to test the effect of hydroperiod on larval survival and mass at metamorphosis. I demonstrate that both hybrid and native larvae benefit from increased pond duration, though hybrids benefit substantially more from each additional day of pond duration. While there were no hydroperiod treatments where native larvae outperformed hybrids, shortened hydroperiods significantly reduced hybrid advantage. In the third chapter, I use data from Chapter 2 to modify a recently developed demographic model for CTS to estimate the effects that hydroperiod manipulation might have on population persistence and hybrid success. Through demographic simulations, I demonstrate that the short hydroperiod treatments do not support a stable CTS population, and do not increase population resistance to hybrid colonization. It appears that healthy native populations near their carrying capacity, supported by long hydroperiod ponds, represent the best tool for deterring hybrid expansion. Conversely, reducing the hydroperiod of primarily non-native ponds may reduce the success of hybrids, decreasing the proliferation of non-native genotypes. Managing non-native hybrids is a difficult, but important conservation priority given the increased performance of hybrids in both temperature tolerance and larval success. Integrative strategies that include targeted hydroperiod management may represent the best strategy for maintaining native CTS diversity in California.