Hydroperiod Impacts on the Fall-Breeding Ambystomatid Salamanders Ambystoma annulatum and Ambystoma opacum

Wetland hydroperiod affects the ecology and evolution of numerous freshwater organisms. Pond-breeding amphibians are particularly affected by hydroperiod, with their life histories and distributional patterns often resulting from how long ponds remain inundated. Substantial variability exists among species in their responsiveness to fluctuations in hydroperiod, making taxon-specific investigations needed to understand its general impacts. We tested whether two fall-breeding salamanders, Ambystoma annulatum and A. opacum, each of which have long (> 6 month) larval period durations, respond to variation in hydroperiod length. We manipulated hydroperiod in outdoor experimental mesocosms under three scenarios (short hydroperiod, medium hydroperiod, and constant water levels), focusing specifically on how hydroperiod affected their life history traits (survival, size, and time to metamorphosis). We found that in the shortened hydroperiod treatments, few individuals of either species completed metamorphosis and would have died, whereas nearly all surviving individuals underwent metamorphosis under longer hydroperiod treatments. After correcting for differences in survival, body size at metamorphosis was greater in constant hydroperiod treatments for only A. annulatum. Larval periods were on average longer in constant hydroperiods for both species. The relationship between size and time to metamorphosis was positive for both A. annulatum and A. opacum, with limited differences between hydroperiod treatments in this relationship for either species. Overall, these results indicate these fall-breeding species are indeed affected by hydroperiod variation, but in different ways, depending on the life history response. These diverse responses highlight the complexities of building generalized responses of amphibians to a shared stressor like hydroperiod variation, as taxon- and trait-specific responses appear to be common. Future work should consider other aspects of hydroperiod as a structuring abiotic force (e.g., onset of drying), or the life history traits of amphibians (e.g., their breeding phenology), to better understand the ecology and evolution of pond-breeding amphibians, as well as provide insights into management actions for their conservation.