Amphibians' Expansion to Record Elevations Influences Chytrid ( <scp> <i>Batrachochytrium dendrobatidis</i> </scp> ) Infection Dynamics

ABSTRACT The climate‐driven range expansion of host species could impact emerging infectious disease events through several mechanisms, with repercussions for conservation and public health. For instance, infection outcomes may be affected by the different responses of hosts and pathogens to new environments. Additionally, range expansions may create novel transmission opportunities as host movement patterns change. Here, we use an integrative approach to explore how the infection dynamics of Marbled four‐eyed frogs ( Pleurodema marmoratum ) with the pathogen Batrachochytrium dendrobatidis ( Bd ) have been impacted by their elevational range expansion in the Cordillera Vilcanota, Peru. With field surveys, we establish that range expansion created new opportunities for Bd transmission: P. marmoratum are now continuously distributed along a recently deglaciated mountain pass between populations separated by heavily glaciated mountains. With sequence data, we identify Bd from the Vilcanota as belonging to the lineage most frequently associated with amphibian declines ( Bd GPL) and find that it lacks genetic structure despite possessing abundant variation, consistent with high rates of local dispersal. With temperature loggers, we demonstrate that upslope expansion exposed frogs and Bd to challenging new thermal regimes. Finally, with size, mass, and Bd infection data from adult P. marmoratum , we find that the new elevations colonized may constrain infection intensities and influence sublethal costs of infection: at low elevations, infected frogs have lower body condition than uninfected frogs, while at high elevations, infected frogs have smaller bodies than uninfected frogs. Together, our results suggest that the climate‐driven range shifts of host species may influence pathogen transmission and infection outcomes.