Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd), is a major threat to amphibian species worldwide. Bd is widespread across a range of freshwater habitats, with highest prevalence in cool, wet environments. However, the range of the pathogen extends into semi-arid regions with perennial river systems, including the Murray-Darling Basin in Australia. Susceptible frog species in these environments are dually threatened by floodplain alteration and heavy water extraction, and reliant on wetland watering for persistence.
This study sought to assess how the timing and duration of managed water delivery may influence Bd prevalence, virulence and ultimately the likelihood of persistence of the endangered Southern Bell Frog (Litoria raniformis).
We monitored populations of L. raniformis in a large wetland complex in semi-arid south-western NSW. Bd prevalence was driven by wetland pH, salinity and temperature and displayed seasonal patterns of prevalence.
Demography was established using skeletochronology and used to model growth and survival rates of L. raniformis. These models informed a population viability analysis to model survival probability under different environmental watering strategies. In our selected wetland on the Lower Murrumbidgee floodplain, extinction probability was lower 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 maintaining biodiversity in a heavily managed river system. Water delivery timing and frequency can reduce the impact of Bd in amphibian populations in situ. This work demonstrates how stochastic population models can be used to assist water management decisions by calculating the extinction risk for endangered species under different water management scenarios.