Climate change,chytridiomycosis or condition: an experimental test of amphibian survival

Variation in climatic temperature is frequently cited as playing a powerful role in driving host/pathogen dynamics, and as a consequence future patterns of disease are predicted to change owing to global warming. However, the preponderance of data is correlative and few quantitative tests exist that dissect the relative effects of changing temperature on host and pathogen responses. In this study, we experimentally tested the effect overwintering temperature had on the susceptibility of a vertebrate ectothermic host, the common toad Bufo bufo, to infection and mortality caused by Batrachochytrium dendrobatidis (Bd). We show that a ‘poor’ (warmer) overwintering regime increases the probability of infection, supporting the concept that the dynamics of infection are altered by changing environmental temperature profiles. We also show once established, the proliferation of Bd in the host was better in toadlets that experienced a ‘good’ (colder) winter. We find no relationship between survival and the different overwintering regimes or any consistent deleterious influence of disease on survival. Instead, survival is overwhelmingly predicted by measurements of condition (mass, mass lost over winter) that were not determined by overwintering temperature. We conclude that the survival of a cohort of overwintering toadlets is primarily driven by patterns of growth during the larval period rather than winter temperature or infectious disease.