Nowhere to hide: impact of a temperature-sensitive amphibian pathogen along an elevation gradient in the temperate zone

Amphibians are rapidly disappearing from habitats around the world and a major cause of these declines is the amphibian chytrid fungus, Batrachochytrium dendrobatidis ("Bd"). The growth rate of Bd is strongly temperature-dependent, and in areas where temperatures are well outside the envelope in which Bd has high growth rates, amphibians may be afforded a refuge from the effects of Bd. This possibility has received considerable empirical support in hot climates, but remains largely untested in cold climates. We conducted a five-year study of the impact of Bd on the declining Sierra Nevada yellow-legged frog (Rana sierrae) across an elevation/temperature gradient in Yosemite National Park using three approaches: (1) resurveys of all 285 R. sierrae populations to describe the landscape-scale patterns of Bd infection intensity, frog population size, and frog population persistence; (2) detailed description of seasonal patterns in temperatures and corresponding Bd infection intensities on R. sierrae; and (3) a frog translocation experiment in which infected R. sierrae from a single source lake were introduced into each of five lakes along an elevation gradient. We predicted that infection intensity should decrease with increasing elevation (i.e., decreasing temperature), and consequently frog survival, population size, and population persistence should increase with elevation. Results from resurveys indicated that frog population size increased with elevation but Bd infection intensity and frog population persistence were unrelated to elevation. Seasonal temperatures varied widely but had no significant effect on Bd infection intensity. Results from the translocation experiment indicated that Bd infection intensity and frog survival following translocation were also unrelated to water temperature. Therefore, contrary to the widely-accepted paradigm that cold environments should strongly limit effects of Bd on amphibians, we found little or no evidence of such limitation at even the highest elevations. Therefore, in temperate montane ecosystems it is unlikely that high elevations will provide amphibians with a refuge from Bd.