ABSTRACT The fungal pathogen of amphibians Batrachochytrium dendrobatidis ( Bd ) is the leading cause of disease‐related extinctions in vertebrates globally. The fungus has a biphasic lifecycle, and the free‐living infectious motile zoospore stage can remain viable in moist environments. This contributes to indirect and cross‐species transmission, as well as temporal persistence and spatial spread of the pathogen. Despite over two decades of surveillance of Bd in hosts, a better understanding of the ecology of this fungus also requires examining how environmental factors shape its temporal and spatial dynamics in water bodies. While environmental DNA (eDNA) methods have previously been used to detect Bd , few studies have applied this approach under extreme or shifting climatic conditions. We used eDNA to quantify Bd across 31 rainforest streams in Australia, providing rare insight into Bd dynamics during a period of unforeseen drought followed by record‐breaking rainfall. We collected water samples across four sampling periods during the breeding season of stream‐dwelling frogs. There was a high geographic variation in both detection and quantity of Bd . Bd eDNA was detected in 75 out of 222 water samples, with the greatest percentage of the 31 streams testing positive during spring (Sept. 2019). We detected Bd eDNA in most of the streams surveyed, particularly in areas where amphibians are known to have declined. Detection proportion and Bd eDNA quantity were both positively influenced by elevation and cumulative rainfall, with detection greatest at higher elevations and following increased rainfall. Bd eDNA quantity was positively related to water temperature, but declined at temperatures above 19.5°C. However, Bd eDNA detection also varied within samples, with low correlation between positive qPCR results. Through sampling Bd eDNA, we can expand our understanding of the ecology of a lethal pathogen, but our results highlight the need for multiple spatial, temporal and technical replicates to improve reliability of eDNA for Bd surveillance.
Sasso et al. (Sun,) studied this question.