Timing of environmental water delivery influences extinction risk for chytrid infected amphibian populations

Chytridiomycosis infection by the aquatic fungus Batrachochytrium dendrobatidis (Bd) is a major threat to frog species worldwide. Bd is widespread across a range of freshwater habitats, its highest prevalence in cool persistently wet environments. While the timing and duration of managed water delivery to amphibian habitats may influence Bd occurrence, there is limited knowledge about the role of water temperature and timing of water delivery on Bd prevalence. Such knowledge is crucial to making informed water management decisions to limit chytridiomycosis.We monitored populations of the endangered southern bell frog (Litoria raniformis) including prevalence and intensity of Bd in a large wetland complex, the Lower Murrumbidgee, in semi-arid south-western NSW. Bd prevalence was driven by wetland pH, salinity and temperature and displayed seasonal patterns of prevalence with higher detection rates in spring and autumn when conditions were cooler and more favourable for Bd.

Population age structure was used to model growth and survival rates of L. raniformis. These models informed a population viability analysis to predict survival rates under different environmental watering strategies. In our selected wetland on the Lower Murrumbidgee floodplain, extinction risk was smaller with later, and therefore warmer, inflows due to the impact of temperature on Bd prevalence. Furthermore, predicted extinction risk was significantly reduced when watering scenarios ensured sites were not dry for more than two years, with further reduction in extinction risk when sites were not dry for more than one year.

Water delivery to wetlands for environmental benefits is vital for maintain biodiversity in a heavily managed river system. Decisions on timing, duration and frequency of water delivery is complex. This work demonstrates how stochastic population models can be successfully used to assist water management decisions by calculating the extinction risk for endangered species under different water management scenarios.

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