The fungal pathogen Batrachochytrium dendrobatidis (Bd) is responsible for amphibian declines worldwide, and understanding the host-pathogen relationship in the face of a changing climate is imperative for predicting disease outbreaks. Unpredictable climate will lead to variable hydroperiods for larval amphibians, and the stressors of low water levels may impact the future development of immune defenses. We hypothesized that the combined effects of pond drying during development and subsequent exposure to Bd after metamorphosis would result in reduced immunocompetence and body condition. To test this hypothesis, we exposed Northern leopard frog tadpoles, Rana pipiens, to three pond drying regimes in outdoor mesocosms: natural, moderate, and fast drying. Juvenile frogs were later repeatedly exposed to Bd in the laboratory to simulate natural pathogen exposure in the environment.
Results/Conclusions
Overall, animals from the natural drying treatment were larger at metamorphosis, and demonstrated a positive correlation between size and larval period, in contrast to medium and fast drying treatments. In addition, Bd exposure resulted in lower scaled mass index (SMI), a measure of body condition, and survival probability, across all treatments post-metamorphosis. Animals that experienced the fast drying treatment and were later exposed to Bd demonstrated the lowest SMI and survival over time, despite the absence of notable disease. Finally, while we found no effect of drying treatment or Bd exposure on measures of innate immune responses including bacterial killing ability and total spleen leucocyte counts, we did find a positive correlation between SMI and live peritoneal leucocyte count overall. This work demonstrates that a reduced hydroperiod and disease exposure can impact survival and have sublethal effects, but sustained impacts on immune function are unclear. Understanding these effects is the first step to predicting disease outbreaks in populations where disease is endemic.