Effects of canopy cover on the landscape epidemiology of an amphibian chytrid fungus

Batrachochytrium dendrobatidis (Bd) is a globally distributed fungal pathogen of amphibians that causes the disease chytridiomycosis and is one of the major contributors to recent population declines of amphibians. The prevalence of Bd should vary spatially across landscapes because survival of the pathogen is influenced by environmental variables. In particular, wetland temperatures should vary with forest canopy cover in heterogeneous landscapes. However, few studies have examined the relationship between Bd prevalence and canopy cover. I sampled anurans for Bd at 33 wetlands in east-central Illinois representing a gradient in canopy cover. I hypothesized that amphibians from closed canopy wetlands would have higher Bd prevalence than amphibians from open canopy wetlands because high canopy cover would prevent temperatures from exceeding Bd’s upper critical maximum as often. My sampling was conducted in spring during the severe drought of 2012, and during more typical weather conditions in 2013. I used occupancy modeling to examine how Bd prevalence varied spatially in relation to canopy cover and other environmental covariates, and to determine if weather conditions altered such relationships. I analyzed all common species combined and Spring Peepers (Pseudacris crucifer) separately, which was the most abundant species. Bd was widespread throughout the study wetlands. During the drought year of 2012, however, prevalence of Bd was substantially reduced compared to 2013 for common species combined and for Spring Peepers. As expected, air and water temperatures at closed canopy wetlands were lower than temperatures at open canopy wetlands. The influence of canopy cover on Bd was complex, however, and I found only limited support for my hypothesis. Bd prevalence for Spring Peepers during 2012 was related positively to canopy cover. In contrast, Bd prevalence of Spring Peepers was not affected by canopy cover in 2013. Likewise, Bd prevalence for common species combined was not related positively to canopy cover in either year. Effects of other environmental covariates, such as water depth, differed between years and were probably influenced by the variable weather conditions. Lastly, connectivity to other wetlands was generally not an important predictor of spatial heterogeneity in Bd prevalence. Weather conditions had a large impact on Bd prevalence patterns, which makes predicting pathogen dynamics difficult. In the Midwest region, climate change models predict more frequent droughts, which may generally inhibit Bd while also altering the effect of canopy cover on Bd prevalence for amphibians