Applying stochastic and Bayesian integral projection modeling to amphibian population viability analysis.

Integral projection models (IPMs) can estimate the population dynamics of species for which both discrete life stages and continuous variables influence demographic rates. Stochastic IPMs for imperiled species, in turn, can facilitate population viability analyses (PVAs) to guide conservation decision-making. Biphasic amphibians are globally distributed, often highly imperiled, and ecologically well suited to the IPM approach. Herein, we present a stochastic size- and stage-structured IPM for a biphasic amphibian, the U.S. federally threatened California tiger salamander (CTS) (Ambystoma californiense). This Bayesian model reveals that CTS population dynamics show greatest elasticity to changes in juvenile and metamorph growth and that populations are likely to experience rapid growth at low density. We integrated this IPM with climatic drivers of CTS demography to develop a PVA and examined CTS extinction risk under the primary threats of habitat loss and climate change. The PVA indicated that long-term viability is possible with surprisingly high (20%-50%) terrestrial mortality but simultaneously identified likely minimum terrestrial buffer requirements of 600-1000 m while accounting for numerous parameter uncertainties through the Bayesian framework. These analyses underscore the value of stochastic and Bayesian IPMs for understanding both climate-dependent taxa and those with cryptic life histories (e.g., biphasic amphibians) in service of ecological discovery and biodiversity conservation. In addition to providing guidance for CTS recovery, the contributed IPM and PVA supply a framework for applying these tools to investigations of ecologically similar species.

Divergent physiological acclimation responses to warming between two co-occurring salamander species and implications for terrestrial survival.

Small differences in physiological responses are known to influence demographic rates such as survival. We tested for differences in the physiological acclimation responses of two closely-related salamander species that often co-occur, Ambystoma maculatum and A. opacum. Specifically, we measured changes in critical thermal maxima (CTmax), standard metabolic rates (SMRs), and respiratory surface area water loss (RSAWL) following exposure to three temperature treatments under laboratory conditions. While the magnitude of RSAWL and CTmax acclimation responses to warming did not differ between the study species, SMR was maintained across acclimation temperatures among A. maculatum, but declined among A. opacum acclimated to warmer temperatures. Metabolic compensation may facilitate maintained A. maculatum activity levels during warm periods following the relatively cool spring breeding season. In contrast, metabolic suppression may allow A. opacum to conserve energy when exposed to surface conditions during fall breeding and nest guarding. We simulated how these different SMR responses would likely alter post-metamorphic survival in our study species using previously collected data representing six weeks under relatively warm seminatural conditions. Our simulation indicated that, following warming and under identical study conditions, metabolic compensation may allow juvenile A. maculatum to maintain survival likelihoods, whereas metabolic depression may cause juvenile A. opacum to experience increased survivorship. These findings underscore that comparable physiological responses among ecologically similar, sympatric species cannot be assumed. Further, results of this study suggest that metabolic responses may play an important role in amphibian species persistence as temperatures increase due to habitat modification and climate change.

Inter- and intraspecific variation in juvenile metabolism and water loss among five biphasic amphibian species.

Population persistence is informed by the ability of individuals to cope with local abiotic conditions, which is commonly mediated by physiological traits. Among biphasic amphibians, juveniles-which are infrequently studied but play a key role in amphibian population dynamics-are the first life stage to experience terrestrial conditions following the aquatic larval stage. To illuminate phenotypic variation that may allow juveniles to survive the physiological challenges presented by this transition, we examined respiratory surface area water loss (RSAWL) and standard metabolic rates (SMR) among juveniles reared under common larval conditions for five salamander species (Ambystoma annulatum, A. maculatum, A. opacum, A. talpoideum, and A. texanum) collected across ~ 200 km of latitude in Missouri, USA. We found that SMR described 34% of variation in RSAWL, suggesting that physiological water conservation may be limited by energetic regulation among these species, and vice versa. On average, species differed in juvenile SMR and residual values of RSAWL (corrected for body size/shape) by 0.04 mL [Formula: see text] and 0.16, respectively, possibly because of distinct species ecologies. For example, A. annulatum had higher SMR and RSAWL compared to broadly distributed study species, potentially associated with a relatively narrow range of environmental conditions experienced across the small geographic distribution of A. annulatum. Latitude correlated negatively with temperature and precipitation, and positively with RSAWL, suggesting that variation in RSAWL may be adaptive to local conditions. We provide evidence that species differences likely have a genetic basis, reflecting selection favoring species divergence to effectively use distinct microhabitats.