Reconstructing historical and contemporary disease dynamics: A case study using the California slender salamander

Abstract

The fungal disease chytridiomycosis, caused by the pathogen Batrachochytrium dendrobatidis (Bd), has been implicated in the extirpation and extinction of amphibian species throughout the world. Recent Bd epizootics (i.e. epidemics in wildlife) have driven hundreds of species to near-extinction. Several species in California have been severely affected by Bd epizootics, but most work has focused on aquatic species. Our study focused on the most abundant and widespread terrestrial amphibian species in California, the California slender salamander, Batrachoseps attenuatus. This species is known to be infected by Bd, but little is known of its disease dynamics. We examined the effect of disease history on contemporary disease dynamics by combining retrospective tests for Bd emergence in museum specimens with contemporary field-collected infection data from the same locations. We found that Bd rapidly emerged in B. attenuatus and exhibited a non-linear pattern of spread throughout Northern California, and that modern-day persistence was negatively correlated with time since first detection of infection. To understand what factors are associated with Bd emergence, we correlated standard environmental variables (e.g. temperature, precipitation) from Bd-positive sites with Bd prevalence. We also compared and contrasted the degree of sociality between B. attenuatus populations that were recently Bd-infected with those that had a longer history of infection. We found that Bd infection in B. attenuatus was positively associated with distance to nearest lentic aquatic habitat, suggesting that aquatic carriers of Bd may be important in prevalence of pathogen within terrestrial B. attenuatus populations. Among our 14 field sites, we also found that recently infected populations had larger group sizes, after standardizing for population density, than populations that had been infected over multiple decades. This result suggests that sociality may facilitate disease spread in terrestrial hosts and that populations with longer exposure to this pathogen may evolve away from the ancestral condition of sociality.