Localized transmission of an aquatic pathogen drives hidden epidemics and population collapse in a terrestrial host

Understanding fine-scale spatial variation in infection risk is central to epidemiology, disease ecology and conservation, yet its causes and consequences remain poorly understood. Here we investigate the dynamics of infection with the aquatic fungus Batrachochytrium dendrobatidis (Bd) in several populations of the fully terrestrial Darwin's frog (Rhinoderma darwinii) across southern Chile. Using high-resolution spatial capture-recapture data, long-term demographic monitoring and a spatial individual-based model parameterized with empirical estimates, we show that Bd infection in this system exhibits pronounced spatial heterogeneity at scales of only metres. This fine-scale clustering arises from localized transmission of an aquatic pathogen in a terrestrial system, driven by spatial proximity between infected and susceptible individuals. Such transmission generates clustered epidemics and can drive rapid subpopulation collapse in this species, with declines of up to 98% within a year. These epidemics can remain undetected at the broader population level because of spatial decoupling of infection among subpopulations. Our findings provide evidence of epidemic dynamics in a terrestrial Bd host and underscore a broader principle: observational scale fundamentally shapes our ability to detect and interpret infection dynamics in spatially structured populations.