Cool-season environmental water delivery increases extinction risk for chytrid-infected amphibians

Chytridiomycosis, the disease caused by the aquatic fungus Batrachochytrium dendrobatidis (Bd), has caused large declines in amphibian populations globally. The pathogen often interacts with other threatening processes to exacerbate declines, including predation by introduced species, habitat loss and fragmentation, and regulation of river systems. In the highly regulated floodplains of south-western New South Wales, Australia, Bd has been detected in populations of the southern bell frog Litoria raniformis (vulnerable; IUCN Red List of Threatened Species, version 2020–3), despite the region's hot, semi-arid climate. However, the effect of chytridiomycosis on the long-term persistence of L. raniformis populations in this region is unknown. To ameliorate the effects of flow regulation, water is diverted into certain wetlands by water managers, a process called environmental water delivery. Given the higher prevalence of Bd infection occurring in winter and spring, the timing of environmental water delivery could influence disease prevalence. We developed a stochastic population model to test the impact of the timing of environmental water delivery on the extinction risk of L. raniformis. Our simulations suggested populations are less likely to go extinct when environmental water is delivered in spring compared to late winter. Modeled inflow of cold (~ 10 °C) river water in August produced a 0.13 probability of local extinction within the 50-year forecast window. When inflows were ~ 15 °C (September) or ~ 20 °C (October), the probability of local extinction was more than halved to 0.05 and 0.04, respectively. This outcome indicates that in addition to the frequency and scale of environmental water delivery, timing flows to coincide with warmer conditions should be a component of environmental-water planning to reduce the negative impacts of chytridiomycosis on threatened amphibians in floodplain systems.