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T h e f u n g a l s k i n d i s e a s e "Chytridiomycosis" is an important cause for amphibian diversity loss. This parDcular disease is emblemaDc for human mediated spread of pathogens from their naDve into new ranges, with the potenDal to exDrpate the newly invaded host populaDons. The Asian chytrid fungus Batrachochytrium salamandrivorans (Bsal) is held responsible for this disease and it has been recently vectored into Europe. There it is currently causing populaDon crashes in fire salamanders. These crashes result in a loss of over 90% of a populaDon in just sixth months' Dme, which makes a populaDon vulnerable to exDrpaDon. Understanding the mechanisms that underpin these processes is key to develop miDgaDon strategies. With our research, we unravelled the complex interacDon between host, pathogen and environment that creates the perfect storm that is currently threatening the survival of salamanders in Europe. Bsal is an aggressive fungus that quickly kills fire salamanders by destroying their skins. As soon as the disease makes its appariDon in a salamander populaDon, thanks to the high densiDes of individuals, the infecDon likely spreads mainly through direct contacts between animals. This happens for example during courtship and through severe contaminaDon of the environment. More intense interacDons between adult salamanders (maDng, territorial issues) may explain why adult salamanders are infected at a higher proporDon compared to younger individuals. This leads to a reduced reproducDve capacity of the populaDon, compromising compensaDon for the losses suffered. However, one would expect the outbreak to stop once the salamander density has dropped below the level that is necessary for Bsal to find and colonize a new host. Unfortunately, this does not happen and the decline conDnues unDl exDrpaDon of the populaDon becomes likely. There might be two different explanaDons for that: The first one is the presence of an infecDve reservoir (a sort of protected hideout) where Bsal persists and from which it can spill over to the few uninfected fire salamanders that are sDll around. Other salamander, frog and toad species that are less suscepDble to the disease but may be infected persistently consDtute one part of this reservoir. Furthermore, Bsal produces spores that endure in the environment for long periods of Dme and thus create a highly infecDous environment. The presence of these reservoirs allows BsaI to persist within the infected ecosystem. A second explanaDon can be found in that Bsal remains highly virulent for fire salamanders even a\er conDnuous presence for at least two years in a suscepDble populaDon. This may seem strange, since exDrpaDon of its host would cause Bsal to disappear as well, yet can be explained by maintenance of Bsal in its reservoir host populaDon as menDoned above, from which it spills over to the fire salamanders. What remains a mystery is why selecDon for increased resistance in the remaining fire salamanders appears not to take place. Indeed, these animals did not show any ability to mount a protecDve response against Bsal, even a\er repeated vaccinaDon. In conclusion, this study depicts a grim image of how the encounter of a pathogen (Bsal) and a host (fire salamander) that are new to each other may end up in a destrucDve confrontaDon 1:Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium.

How a fungus causes the collapse of salamander populations