Emerging wildlife diseases are increasingly viewed as a major threat to biodiversity. Amphibian chytridiomycosis (caused by the pathogen Batrachochytrium dendrobatidis, “Bd”) is an emerging fungal disease associated with apparent extinctions of perhaps more than 200 amphibian species. However, many amphibian species long presumed extinct have been recently rediscovered, and these rediscoveries are difficult to reconcile with the prevailing paradigm that an amphibian mass extinction has already transpired. Here, we synthesize information on amphibian rediscoveries, and present a new conceptual framework to classify rediscoveries that will improve clarity concerning mechanisms for host-pathogen co-existence relevant for efforts to mitigate Bd.
We distinguish among four phenomena in rediscovery of amphibian populations. First, many rare amphibian species have been rediscovered after decades unseen by scientists, but are unrelated to chytridiomycosis and rather are associated with low detection probability. Second, some “relict populations” from chytridiomycosis-susceptible species exist in environmental refugia from chytridiomycosis (environments that are hostile for survival or growth of Bd). Environmental refugia should help identify the environmental limits of Bd and may help to illustrate how environmental management may promote pathogen eradication. Third, recolonization of upland sites from lowland refugia is a second form of recovery, and is widespread, but is not typical of the most severely threatened amphibian species or those presumed extinct. Finally, several species of amphibians highly sensitive to chytridiomycosis have populations surviving within ideal habitat for Bd yet persist with some unexplained resistance or tolerance to chytridiomycosis – the enigmatic survivors of amphibian extinctions. Enigmatic survival may result from evolutionary responses of hosts or pathogens, or shifts in the host community or community composition of the amphibian skin microbiome. Whichever mechanisms allow for host-pathogen coexistence, enigmatic survival illustrates that host-pathogen co-existence for the world’s most Bd-susceptible amphibians is possible. Research on these enigmatic survivors may inform strategies to mitigate wildlife emerging infectious diseases, including amphibian chytridiomycosis among others.