Spatial and temporal patterns in occupancy and density of the pathogenic amphibian fungus, Batrachochytrium dendrobatidis, in waters of North America

Background/Question/Methods

Biodiversity losses are occurring worldwide due to a combination of natural and human-induced stressors including habitat loss and fragmentation, urbanization, invasive species, contaminants, global climate change, and emerging infectious diseases. For example, one estimate is that 40% of amphibian species are vulnerable to extinction. One pathogen, the fungus Batrachochytrium dendrobatidis (Bd) is a major contributor to many amphibian population declines worldwide. Most research on Bd has focused on how Bd interacts with its amphibian hosts with little emphasis on the ecology of free-living Bd outside of the host. We investigated spatial and temporal patterns of Bd occupancy and density in amphibian habitats of the United States using occupancy models, which are powerful tools for estimating site occupancy and detection probability. Occupancy models have been used to investigate diseases in amphibians, fishes, and birds, where the focus was on pathogen occurrence in the host. We applied occupancy models to investigate free-living Bd in North American surface waters to determine: 1) the seasonality of Bd, 2) the relationships between Bd site occupancy and habitat attributes, and 3) the probability of Bd detection from water samples as a function of the number of samples taken, sample volume, and water quality. We also report on the temporal patterns of Bd density from a four-year case study of a Bd-positive wetland, and the relationship between water quality and Bd density.

Results/Conclusions

We provide evidence that Bd occurs in the environment year round. Bd exhibited temporal and spatial heterogeneity in density, but did not exhibit seasonality in occupancy. Bd was detected in all months, typically less than 100 zoospores L^-1. The highest density observed was ~3 million zoospores ^L-1. Bd was found in 47% of sites sampled and was estimated to occur in 61% of sites. Bd occupancy was highest at low elevation sites and decreased as elevation increased. The probability of detecting Bd increased as pH increased. When Bd was present, there was a 95% chance of detecting it with 4 samples of 600 ml of water or 5 samples of 60 ml. Our findings address important baseline information to advance the study of Bd disease ecology in temperate-zone systems, and advance our understanding of the likelihood of amphibian exposure to free-living Bd in aquatic habitats over time.