The chytrid fungus threatens the last epidemiologically naive Neotropical montane amphibian biodiversity hotspot.

The amphibian chytrid fungus Batrachochytrium dendrobatidis (B. dendrobatidis) has caused catastrophic biodiversity loss worldwide.1,2,3 Although B. dendrobatidis is now widespread across environmentally suitable regions,4 the absence of amphibian declines in some high-suitability areas suggests the existence of refuges supporting epidemiologically naive communities. Epidemiologically naive refuges have not experienced past B. dendrobatidis epizootics despite suitable environmental conditions and may be B. dendrobatidis-free, harbor less virulent B. dendrobatidis lineages, or contain resistant host communities. As eradication of virulent B. dendrobatidis lineages is rarely successful after establishment, identifying epidemiologically naive refuges is imperative.5,6 Here, following extensive multi-year disease surveillance, we report a major epidemiologically naive refuge in the Neotropical highlands, the Sierra Nevada de Santa Marta (SNSM) in northern Colombia. We found no evidence for contemporary B. dendrobatidis presence, despite detecting the hypervirulent Global Panzootic Lineage in a neighboring Andean mountain range. Population genomic analyses in SNSM-endemic harlequin toads found no demographic signatures indicative of cryptic epizootic decline. Niche modeling predicted high suitability for B. dendrobatidis occurrence and B. dendrobatidis-induced declines throughout the SNSM, with climatic conditions in lowland regions surrounding the SNSM likely acting as a dispersal barrier and preventing B. dendrobatidis invasion, creating a "sky island." Infection trials using three SNSM-endemic amphibian species revealed tolerant and susceptible species. Together, our results indicate that the SNSM is an epidemiologically naive refuge at risk of invasion by hypervirulent B. dendrobatidis, which could result in the loss of 25 endemic amphibian species. We highlight the urgent need for proactive conservation action and the strict implementation of biosecurity to safeguard the unique and vast amphibian diversity of the SNSM.