Heterogeneous occupancy and density estimates of the pathogenic fungus Batrachochytrium dendrobatidis in waters of North America

Abstract/Summary

Biodiversity losses are occurring worldwide due to a combination of stressors. For example, one estimate is that 40% of amphibian species are vulnerable to extinction and a major contributing factor to losses in amphibian biodiversity in disease. One pathogen, the emerging infectious fungus Batrachochytrium dendrobatidis (Bd) is a contributor to amphibian population declines worldwide. Bd research has focused on the dynamicas of the pathogen within amphibian hosts with little emphasis on investigating the potential dynamics of free-living Bd. Therefore, we investigated patterns of Bd occupancy and density in amphibian habitats using occupancy models, powerful tools for estimating site occupancy and detection probability. Occupancy models have been used to investigate diseases 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 Bd seasonality, relationships between Bd site occupancy and habitat attributes, and probability of Bd detection from water samples as a function of the number of samples, sample volume, and water quality. We also report on the temporal patterns of Bd density from a 4-year case study of a Bd-positive wetland. 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. We detected Bd in 47% of sites sampled but estimated Bd occupied 61% of sites, highlighting the importance of accounting for imperfect detection. When Bd was present, there was a 95% chance of detecting it with four samples of 600 ml of water or five samples of 60 ml. Our findings provide important baseline information to advance the study of Bd disease ecology in temperate-zone systems, and advance our understanding of amphibian exposure to free-living Bd in aquatic habitats over time.

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