Tales of an 'invisible' life stage : survival and physiology among terrestrial juvenile ambystomatid salamanders
The vital rates (e.g., survival, growth, and reproduction) of distinct life stages within a species are known to influence the growth and persistence of populations. As such, studies describing stage-specific vital rates, and the factors that shape variation in these rates across species and populations, can help to improve our understanding of population dynamics and species distributions. Moreover, the insights gained from such studies can guide the prioritization of populations for conservation efforts and inform the selection of management strategies under predicted climate and land use changes. Yet life stage-specific vital rate estimates, and characterizations of the physiological responses that influence demographic rates under variable conditions, are lacking for most species. Amphibians are experiencing drastic population declines across the globe. As amphibians exhibit complex lifecycles, wherein unique life stages rely on divergent resources and habitat types, examinations of distinct life stages may be particularly critical for enhancing amphibian conservation efforts. Specifically, juveniles are known to play a critical role in amphibian population dynamics, but are relatively understudied compared with other life stages due to their small body size and often elusive life histories. The main objective of this dissertation was to elucidate survival rates, and physiological characteristics that contribute to survival, among terrestrial juveniles of multiple complex lifecycle pond-breeding salamanders in the genus Ambystoma. I first estimated juvenile survival rates among three ambystomatid species (A. annulatum, A. maculatum and A. texanum) by conducting an 11-month capture-mark recapture study within semi-natural enclosures. I found juvenile survival rates to be constant through time and comparable among species. These similarities indicated that vital rate estimates from congeneric, ecologically similar species can serve as robust place-holder information to examine the population dynamics of the many amphibian species for which stage-specific data are lacking. Next, I reared larvae of five species (A. annulatum, A. maculatum, A. opacum, A. talpoideum, and A. texanum) from populations along an ~200 km latitudinal gradient in Missouri, USA to metamorphosis under common conditions. By performing flow-through respirometry on juveniles, I found respiratory surface area water loss (RSAWL) and standard metabolic rates (SMR) to differ between species. Though SMR showed no relationship with locality, RSAWL was weakly positively correlated with latitude. This suggested that juvenile ambystomatids exposed to warmer average conditions at more southern latitudes, and thus a higher desiccation risk, may demonstrate the locally adaptive regulation of RSAWL compared with juveniles from northern populations. Given common rearing, it is likely that differences among species and populations had a genetic basis, and were not solely the result of phenotypic plasticity. I then conducted a replicated experiment to evaluate how juvenile RSAWL, SMR, and body mass might influence individual fitness by releasing A. maculatum and A. opacum juveniles from the previous study into the semi-natural enclosures for seven months of capture-mark-recapture. Examining known survival at three time points during the study, I found juveniles with higher initial body mass and/or lower SMR to have a higher likelihood of survival, particularly under warm initial conditions. There was no effect of RSAWL on survival. Acclimation experiments with surviving salamanders revealed that thermal tolerances and SMR demonstrated plastic responses to warming. Further, a simulation of juvenile survival following high temperatures suggested that the two study species may demonstrate diverging juvenile survival rates after being thermally challenged due to distinct acclimation strategies. Collectively, the results of these studies shed light on a key vital rate for ambystomatid population dynamics in a life stage that is difficult to observe. I compiled the findings of these and other ambystomatid studies to propose management objectives and strategies to conserve A. annulatum, an endemic species of the Ozark and Ouachita Mountains. This effort demonstrated the utility of life stage-specific demographic and physiological information for guiding the conservation of biodiversity.
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