Amphibian Declines and Chytridiomycosis: Pathogen Biology, Impacts and Conservation Responses

Amphibians are declining faster than any other class of vertebrates, and a substantial part of that loss is attributable to a single group of fungal pathogens. This review covers the biology of the amphibian chytrids, the scale of the damage they have caused, and what conservation practice can and cannot do about it. Batrachochytrium dendrobatidis was described in 1999 after mass mortality events in Australia and Central America, and a second species specific to salamanders was described in 2013. Both infect keratinised skin, and the fungus kills by disrupting the transport of electrolytes across an organ that amphibians use for respiration and osmoregulation, leading to cardiac arrest. A global assessment associated the pathogen with declines in more than five hundred species, of which roughly ninety are presumed extinct, the largest documented loss of biodiversity attributable to a disease. Genomic evidence traces the hypervirulent lineage to east Asia and its global spread to the trade in live amphibians during the twentieth century. Outcomes vary with temperature, host skin peptides and skin bacterial communities, and a few populations have recovered without any decline in pathogen virulence. Effective measures remain limited to biosecurity, trade regulation and captive assurance colonies.