Ecology and Evolution of Geographic Range Size Variation in North American Plethodontid Salamanders: Perspectives from Thermal Physiology

Species exhibit remarkable variation in geographic range size. Understanding the causes of this variation is fundamental to the fields of ecology and evolution, and is central to understanding how species will respond to rapid climate change. Using eastern North America's species-rich salamander fauna, I explore whether seasonal variation in temperature and climatic tolerance evolution underlie geographic range size variation (as per the climate variability hypothesis). First, I determined critical thermal maximum (CTMax) and critical thermal minimum (CTMin) temperatures of 18 salamander species. I then tested for relationships between thermal tolerances, seasonality, and geographic range size. Localities with greater annual temperature ranges (seasonality) were found to have individuals with broader thermal tolerances, and correspondingly species with larger latitudinal extents. Intraspecific tests, however, found only one of six wide-ranging species to relate thermal tolerances to environmental temperature changes across the range. Next, I estimated acclimation ability of salamanders to see if species with larger distributions have greater physiological plasticity. Salamanders were acclimated to 14 and 22°C and results of a phylogenetically controlled MCMCglmm model indicated that there are significant differences in temperature adjusted standard metabolic rates (SMRs) of species with wide versus narrow latitudinal extents. Wide-ranging species showed a slight increase in SMR after acclimation, whereas narrow-ranging species showed a statistically significant drop in SMR. These results indicate that wide-ranging species have a greater thermal acclimation capacity than narrow-ranging species. Finally, using GIS-based climate data I included all available locality points to estimate species-level thermal niche breadths. I found that CTMax and CTMin of species are strongly correlated with the maximum and minimum temperatures that occur within their geographic ranges. I also found that species' thermal tolerance breadths (CTMax -- CTMin) are highly correlated with estimates of their thermal niche breadths. My general finding that wide-ranging species have broader physiological tolerances than narrow-ranging species supports key predictions of the climate variability hypothesis and the role of seasonality in the evolution of physiological traits. It also highlights the potential vulnerability of narrow-ranging montane salamanders.