Spatial dynamics and population impacts of disease in threatened amphibians

Abstract

Globally, amphibians are the most endangered vertebrate taxon, with over one third of species at risk of extinction. A prominent driver of this extinction crisis is the disease chytridiomycosis, which is caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). Bd has been implicated in major amphibian declines in Australia, Europe and the Americas. In this thesis, I investigated; (1) the spatial distribution of Bd and its impacts on amphibian species in Australia and Europe, and (2) the impact of Bd on amphibian demography in Australia. In a sub-alpine amphibian community in south-eastern Australia, I demonstrated that the presence of an amphibian reservoir host was strongly associated with increased Bd prevalence in a highly susceptible, sympatric amphibian species. At the landscape level, high reservoir host abundance was associated with increased severity of decline in the sympatric species, indicating that variation in disease susceptibility caused pathogen-mediated apparent competition, leading to extirpation of the susceptible species. In a heterogeneous landscape in central Romania, I examined the spatial distribution of Bd infection in an ephemeral pond breeding amphibian. I found that Bd presence was strongly associated with proximity to perennial water sources and forest cover. These results indicated that perennial water sources may act as source habitat for Bd, with amphibian movements resulting in Bd spill-over into ephemeral ponds. In south-eastern Australia, I investigated long-term changes in the distribution of a chytridiomycosis-impacted amphibian species. Using a 37 year dataset, I documented a period of decline between the mid-1970s and 1990s, consistent with the spatiotemporal emergence of Bd in eastern Australia. A sustained period of population re-expansion has occurred since the 1990s, despite high Bd prevalence in recovering populations. In a sub-alpine environment in south-eastern Australia, I investigated how remnant populations of a highly susceptible amphibian species are able to persist with Bd. I showed that Bd was associated with high adult mortality between years. Importantly, I found that adults breed before succumbing to chytridiomycosis and Bd prevalence is low in early life-history stages. This allows recruitment to remain sufficiently high to facilitate population persistence. I then compared population age structure of long-exposed populations, Bd-naive populations and historical specimens. I found that diseased populations exhibited severe age structure truncation. Individuals from diseased populations matured earlier in life and at smaller sizes, indicating that chytridiomycosis can drive phenotypic shifts in host life-history. I used population simulations to illustrate that this shift is likely to increase population vulnerability to stochastic events. Many of the insights gained from this research are highly relevant to amphibian conservation. I have combined these insights with a synthesis of the broader literature to develop a management framework for amphibians threatened by Bd. This framework is designed to help guide the implementation of intervention strategies to reduce extinction risk in chytridiomycosis-threatened species. Collectively, this thesis constitutes an important contribution to global amphibian-Bd research and has important implications both for understanding chytridiomycosis impacts and dynamics, and for developing effective conservation responses.