Geographic variation of stress across range limits in the terrestrial salamander Plethodon metcalfi
Organismal stress responses increase fitness over short timescales by altering energy allocation regimes away from processes such as reproduction and towards survival. Over longer periods chronic stress may increase the allostatic load of an organism and become detrimental. As the climate warms and the likelihood of encountering abiotic stressors increases, balancing physiological stress responses with competing energy demands may mediate organismal responses to climate change. Therefore, we might expect range contractions in areas where increases in background stress are observed. Terrestrial and lungless salamanders of the genus Plethodon rely on cool and wet microhabitats to maintain their moist skin, which makes them vulnerable to small changes in their abiotic environment. Combined with predictions of substantial habitat loss in the near future, there is an urgent need to understand salamander physiology from an ecological perspective. To investigate how stress varies with natural hydrothermal gradients across species range limits we collected salamanders from five sites across the elevational and latitudinal distribution of Plethodon metcalfi. We sampled from randomly generated GPS points across all sites between late May and early August 2014. We obtained blood samples and analyzed leukocyte ratios as a metric of stress when compared to fine-scale temperature and humidity data.
The thermal environment salamanders experience on the night of collection positively correlates to leukocyte ratios. Vapor pressure deficit (VPD) and aspect also correlate with leukocyte ratios where stress increased with drier conditions. In the Southern Appalachian Mountains thermal and hydric conditions covary where lower elevations are typically warmer and drier compared to high elevation sites. Increases in stress with hydrothermal gradients suggest a more physiological taxing environment at low elevations and low latitudes. The increased physiological demands of low elevation and low latitude may lead to decreased fitness and local extinctions in those areas. To further test this hypothesis we will investigate the impacts of multiple stressors combinations to determine how stressors interact to create observed responses in nature. Future studies will also investigate the timescale over which specific stressors act through the use of multiple stress metrics. Findings will be incorporated in a mechanistic model to improve energy budget and potential activity time predictions. Incorporation of testable field data into model predictions will increase our abilities to forecast future species range dynamics in changing climates.