Seasonal prevalence of the chytrid fungus in populations of the motorbike frog Litoria moorei

Date: 01, Jun, 2020
Author(s):   Matt West, Sarah Anderson, Robert A. Davis, Rebecca Will, Michael Scroggie and Kirsten M. Parris
Publisher: Clean Air and Urban Landscapes Hub

Amphibian chytrid fungus Batrachochytrium dendrobatidis (hereafter Bd), has caused at least 500 amphibian species to decline globally (Scheele et al. 2019). While some amphibians are highly susceptible to developing the disease chytridiomycosis, when infected by Bd, other species appear to cope despite an association with the pathogen (West 2015). Understanding the environmental factors that influence Bd prevalence and disease manifestation is crucial for developing strategies to reduce the impact of the pathogen on natural populations (Blaustein & Kiesecker 2002). In some cases, environmental conditions may influence the ability of species to cope with Bd. Bd infection prevalence – the proportion of individual frogs in a population infected with the pathogen – can be influenced by environmental conditions (Becker et al. 2012; Becker & Zamudio 2011; Clemann et al. 2013; Heard et al. 2014). Temperature, humidity, salinity, and water pH can influence Bd prevalence by limiting the pathogen’s growth and survival rates (Bramwell 2011; Stockwell et al. 2012; Stockwell et al. 2015). For example, optimal pathogen growth occurs at 17–25 °C, growth slows outside of this range, and the pathogen dies at ≥28°C (Piotrowski et al. 2004). Wetland salinity may also act to protect frogs against chytridiomycosis, by reducing the transmission rates within populations (Clulow et al. 2018). The prevalence and pathogenicity of Bd increase in mild and humid climates (Becker & Zamudio 2011; Berger et al. 2004). Consequently, Bd prevalence can be higher during the cool winter months than during the warm-hot summer months (Woodhams & Alford 2005). Environmental conditions are known or predicted to be highly suitable for Bd in multiple regions throughout Australia including the Wet Tropics in Queensland, coastal regions of Queensland and NSW, a vast expanse of Victoria and Tasmania and south-west Western Australia (Murray et al. 2010; Murray et al. 2011). Amphibians in these areas of Australia are therefore considered at a higher risk of infection than other regions. For example, two related threatened species in south-eastern Australia, the growling grass frog Litoria raniformis and green and golden bell frog Litoria aurea have both suffered population declines that are associated with Bd (Heard et al. 2013; Heard et al. 2015; Klop-Toker et al. 2018a; Stockwell et al. 2010). Logically, other related species that occur in highly Bd suitable regions in Australia might also be assumed to be susceptible to Bd. Another species of related bell frog, the motorbike frog Litoria moorei, is known to occur in the highly Bd suitable south-west region of Western Australia. Nevertheless, L. moorei is not known to have declined despite exposure to this pathogen. Bd is currently not perceived as a critical management issue (i.e., it is not known to have caused significant population declines) for any amphibians in south-west Western Australia even though this is a highly suitable region, and the pathogen has been present since at least 1985 (Riley et al. 2013). However, few studies have evaluated Bd prevalence or impacts on Western Australian amphibians. If L. moorei can persist despite the association with Bd, then understanding the factors that influence infection prevalence and pathogenicity could help inform management of threatened eastern Australian bell frogs. Conversely, if L. moorei is at risk of Bd-mediated declines, then pathogen management in south-western Australia will need to become a higher priority issue. As a first step to address these uncertainties, we conducted a study to determine if the environmental conditions in south-west Western Australia could influence infection risk in L. moorei populations. To achieve this, we examined environmental conditions and evaluated the prevalence of Bd infection at 45 wetlands throughout the Swan Coastal Plain region of south-western Australia from December 2016 to March 2018.