Fragile coexistence of a global chytrid pathogen with amphibian populations is mediated by environment and demography

Unravelling the multiple interacting drivers of host pathogen co-existence is crucial in understanding how an apparently stable state of endemism may shift towards an epidemic and lead to biodiversity loss. Here, we investigate the apparent co-existence of the global amphibian pathogen *Batrachochytrium dendrobatidis* (Bd) with *Bombina variegata* populations in the Netherlands over a seven-year period. We used a multi-season mark-recapture data set and assessed potential drivers of co-existence (individual condition, environmental mediation and demographic compensation) at the individual and population level. We show that even in a situation with a clear cost incurred by endemic Bd, population sizes remain largely stable. Current environmental conditions and an over-dispersed pathogen load likely stabilize disease dynamics, but as higher temperatures increase infection probability, changing environmental conditions, for example a climate change-driven rise in temperature, could unbalance the current fragile host-pathogen equilibrium. Understanding the proximate mechanisms of such environmental mediation and of site-specific differences in infection dynamics can provide vital information for mitigation actions.