Assessing how multiple threats impact the green and golden bell frog for the purpose of improved conservation

The myriad of concurrent threats driving species declines make it difficult for managers to identify causal agents and preserve threatened populations. The decline of the green and golden bell frog (Litoria aurea) in Australia is an example of how multiple threatening agents such as the novel pathogen, Batrachochytrium dendrobatidis (Bd), invasive mosquitofish Gambusia holbrooki, and habitat degradation, can act independently or synergistically to rapidly decimate populations and prove difficult to manage. The broad focus of this study was to assess the impact of these threats on L. aurea, with the aim of improving current management techniques. Multi-state mark recapture analysis of three years of monitoring data from an extant and released population of L. aurea revealed that Bd significantly reduced survival probability, even within a habitat where the frogs persist. Analysis of data from the released population suggests the severity of Bd was affected by host age and condition, temperature, density, and food availability, and that both nutrition and Bd likely contributed to a lack of reproduction. Increased salinity may be tool for passively reducing Bd severity, as frogs used waterbodies of 3 parts per thousand (ppt), but care needs to be taken as salinities above 5 ppt elicited avoidance behaviour. Generalised linear models showed a marked separation between G. holbrooki and tadpole occupied ponds with a lack of avoidance by adult frogs. These patterns suggest that adults are naïve to G. holbrooki and the fish likely reduces recruitment through larval predation. A laboratory experiment identified few synergistic effects of Bd and G. holbrooki on tadpoles, although we argue that this may change in the wild. This thesis affirms the negative impact Bd and G. holbrooki have on L. aurea, and suggests why our study population persists in the face of these threats. We highlight factors that can improve in situ conservation projects of this nature, such improvements to habitat that may mitigate Bd, the importance of habitat that promotes a healthy abundance of prey species to supply nutrients for immune function and breeding, the importance of excluding G. holbrooki from protected sites, and the importance of habitat close enough to extant sites or large enough to support dispersal behaviours; all potentially achievable goals that may greatly benefit many threatened amphibians.