Potential causes of geographic variation in the severity of chytridiomycosis

Abstract Amphibians are among the most endangered vertebrate groups. One of the main reasons for their biodiversity loss is the relatively recent global spread of chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis ( Bd ), which has contributed to the decline or extinction of at least 200 amphibian species. With relevant knowledge accumulating, it has become evident that the effects of chytridiomycosis can differ among geographic regions not only on the interspecific level but also within species. Ultimately, temporal and spatial variation in the outcome of chytridiomycosis is driven by the evolutionary arms race between the parasite and its hosts. Proximately, this variation may arise due to variation in host susceptibility, differences in the pathogenicity of different lineages of Bd , variation in prevailing environmental conditions, or interactions between these factors. Here, we summarize the evidence for each one of these possibilities and draw attention to the fact that studies considering several of these factors simultaneously are scarce, and those taking all into account are lacking entirely. Consequently, the overall interpretation of the differences in the outcomes of existent studies and experiments has remained difficult. We conclude that future research should focus on the interactive effects of co-evolutionary history, host susceptibility, Bd virulence, and relevant environmental factors on disease outcomes. These studies will help in deepening our understanding of disease dynamics, in predicting future hotspots of chytridiomycosis outbreaks, and in focusing conservation efforts to where they are most needed.