Precipitation paradox: drought suppresses amphibian pathogen yet intensifies transmission and disrupts protective skin microbiome

Shifting precipitation regimes driven by global climate change are altering host behavior and host-symbiont relationships, both of which may compromise resilience to disease. In Brazil's Atlantic Forest, a biodiversity hotspot facing severe droughts and increased rainfall variability linked to climate and land-use change, prior research identified drought as a key factor disrupting the skin microbiome, contributing to the die-off of pumpkin toadlets due to the waterborne fungal pathogen Batrachochytrium dendrobatidis (Bd). Because observational studies often cannot distinguish moisture's direct effect on Bd growth from increased amphibian activity during wet breeding seasons, we tested the impact of drought conditions on host behavior, Bd disease dynamics, and the composition and predicted function of cutaneous bacterial communities using experimental field enclosures. Each enclosure housed ecologically realistic densities of the micro-endemic pumpkin toadlet Brachycephalus pitanga. We subjected half of the enclosures to drought conditions using translucent tarp coverings. We identified individual toadlets by their unique markings and collected skin swabs every two weeks for three months. Then, we implemented molecular techniques to quantify Bd loads and characterize skin bacterial diversity and composition over time. Our results showed that drought suppressed Bd, but increased host use of wet microenvironments and shifts in the protective skin microbiome partially offset this effect. This study provides valuable insights into the cascading effects of climate change on host behavior, host-symbiont interactions, and disease dynamics, emphasizing the importance of understanding how changing environmental conditions affect disease susceptibility and survival of vulnerable hosts.