Disease Risk in Temperate Amphibian Populations Is Higher at Closed-Canopy Sites

Abstract

Habitat loss and chytridiomycosis (a disease caused by the chytrid fungus Batrachochytrium dendrobatidis - Bd) are major drivers of amphibian declines worldwide. Habitat loss regulates host-pathogen interactions by altering biotic and abiotic factors directly linked to both host and pathogen fitness. Therefore, studies investigating the links between natural vegetation and chytridiomycosis require integrative approaches to control for the multitude of possible interactions of biological and environmental variables in spatial epidemiology. In this study, we quantified Bd infection dynamics across a gradient of natural vegetation and microclimates, looking for causal associations between vegetation cover, multiple microclimatic variables, and pathogen prevalence and infection intensity. To minimize the effects of host diversity in our analyses, we sampled amphibian populations in the Adirondack Mountains of New York State, a region with relatively high single-host dominance. We sampled permanent ponds for anurans, focusing on populations of the habitat generalist frog Lithobates clamitans, and recorded various biotic and abiotic factors that potentially affect host-pathogen interactions: natural vegetation, canopy density, water temperature, and host population and community attributes. We screened for important explanatory variables of Bd infections and used path analyses to statistically test for the strength of cascading effects linking vegetation cover, microclimate, and Bd parameters. We found that canopy density, natural vegetation, and daily average water temperature were the best predictors of Bd. High canopy density resulted in lower water temperature, which in turn predicted higher Bd prevalence and infection intensity. Our results confirm that microclimatic shifts arising from changes in natural vegetation play an important role in Bd spatial epidemiology, with areas of closed canopy favoring Bd. Given increasing rates of anthropogenic habitat modification and the resulting declines in temperate and tropical frogs, understanding how vegetation cover and disease interact is critical for predicting Bd spread and developing appropriate management tools for wild populations.

Environmental fluctuations and host skin bacteria shift survival advantage between frogs and their fungal pathogen

“…Our findings showed that just a slight increase in temperatures experienced during the warm-wet season provides frogs with a significant advantage that helps minimize the impact of infections ( Figure 5). Our results corroborate many others that show that higher temperatures decrease the severity of infection (for example, Woodhams et al, 2008;Murphy et al, 2011;Becker et al, 2012;Rowley and Alford, 2013). In many of these cases, behavioral fever (that is, the process by which ectotherms seek areas of warmer temperatures to increase body temperatures) was the mechanism providing resistance to hosts.…”

Chytridiomycosis risk among Central European amphibians based on surveillance data

“…Susceptibility to Bd infection varies substantially, as does its incidence among different taxa (Searle et al 2011, Baláž et al 2014). These differences appear to be influenced by geography, life history and habitat (Becker et al 2012, Murray & Skerratt 2012. The pathogen's original host and biogeography context remains an unresolved issue that has received a great deal of attention (Weldon et al 2004, Fisher 2009, Schloegel et al 2012.…”

Spatial distribution of Batrachochytrium dendrobatidis in South American caecilians

“…Specifically, we detected a positive effect of precipitation and a negative effect of temperature variables on the likelihood of Bd infection in caecilian hosts. Vegetation density, which is often positively associated with Bd infection in anurans (Raffel et al 2010, Becker & Zamudio 2011, Becker et al 2012, showed a weak negative effect on Bd in caecilians. This finding might be due to the high degree of fossoriality of terrestrial caecilian species, which spares them from the direct or indirect effects of habitat quality, with downstream shifts in both macro-and microclimates.…”