Host size influences the effects of four isolates of an amphibian chytrid fungus.

Understanding factors that influence host-pathogen interactions is key to predicting outbreaks in natural systems experiencing environmental change. Many amphibian population declines have been attributed to an amphibian chytrid fungus, (). While this fungus is widespread, not all positive populations have been associated with declines, which could be attributed to differences in pathogen virulence or host susceptibility. In a laboratory experiment, we examined the effects of isolate origin, two from areas with -associated amphibian population declines (El Copé, Panama, and California, USA) and two from areas without -related population declines (Ohio and Maine, USA), on the terrestrial growth and survival of American toad () metamorphs reared in larval environments with low or high intraspecific density. We predicted that (1) isolates from areas experiencing declines would have greater negative effects than isolates from areas without declines, and (2) across all isolates, growth and survival of smaller toads from high-density larval conditions would be reduced by exposure compared to larger toads from low-density larval conditions. Our results showed that terrestrial survival was reduced for smaller toads exposed to with variation in the response to different isolates, suggesting that smaller size increased susceptibility to . Toads exposed to gained less mass, which varied by isolate. isolates from areas with population declines, however, did not have more negative effects than isolates from areas without recorded declines. Most strikingly, our study supports that host condition, measured by size, can be indicative of the negative effects of exposure. Further, isolates' impact may vary in ways not predictable from place of origin or occurrence of disease-related population declines. This research suggests that amphibian populations outside of areas experiencing -associated declines could be impacted by this pathogen and that the size of individuals could influence the magnitude of 's impact.