Using site-occupancy models to prepare for the spread of chytridiomyosis and identify factors affecting detectability of a cryptic susceptible species, the Tasmanian tree frog

Abstract

Context The global reduction of amphibian biodiversity as a result of the disease chytridiomycosis (caused by the fungus Batrachochytrium dendrobatidis; Bd) has highlighted the need to accurately detect local population declines in association with Bd presence. Although Bd has spread globally, some remote regions such as the Tasmanian Wilderness World Heritage Area (1.40 million ha; TWWHA) in Australia, remain largely, but not entirely, disease free. The Tasmanian tree frog (Litoria burrowsae) resides primarily within TWWHA boundaries, and is believed to be susceptible to chytridiomycosis. Aims In the absence of historical abundance data, we used a single-season multi-state site-occupancy model to investigate the impact of Bd on L. burrowsae populations, on factors affecting species detection and to inform ongoing surveillance and conservation. Methods We recorded frog detection and ranked call intensity (estimation of population size) from repeated independent surveys within a season to estimate the role of covariates, such as presence of Bd and environmental variables, on species occupancy and detection probability. Key results Modelling revealed large frog populations are more likely to be present at naturally formed than human-formed ponds, strong winds negatively affect detection of populations, and time after sunset affects detection of large populations. Large frog populations were more likely to be Bd-negative; however, models including Bd presence were not well supported, in part a result of the small number of Bd-positive sites recorded. Conclusions and Implications The utility of site-occupancy modelling in understanding the impact of disease on populations is little known, but has the potential to improve the accuracy and efficiency of many conservation programs.

Current trends and future directions in koala chlamydial disease research

“…Mark-recapture data should also be collected in association with information on individual covariates (particularly infection and disease states, age and gender; Bonner et al 2010). Diseasestate occupancy surveys and other spatial techniques may also assist in better understanding the role of source-sink dynamics on the transmission of infection and manifestation of disease across populations in a range of environments and under a range of threatening processes (Adams et al 2010;Cashins et al 2015;Mackenzie et al 2009). Given the generally poor long-term efficacy of chlamydial vaccines in multiple species (Entrican et al 2012;Rey-Ladino et al 2014), research into alternative field-based management options may also be beneficial in addition to the current koala vaccine studies (Khan et al 2016;Waugh et al 2016).…”