University of Tasmania
Comte_whole_thesis_ex_pub_mat.pdf (5.69 MB)

The spatial epidemiology of the devil facial tumour disease in long-diseased populations of its unique host, the Tasmanian devil

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posted on 2023-05-28, 09:51 authored by Comte, S
Emerging infectious diseases in wildlife are recognized as a major conservation threat, globally. Their impact is rarely as acute as the novel transmissible cancer, devil facial tumour disease (DFTD), on its unique host species, the Tasmanian devil (Sarcophilus harrisii). Since discovery in 1996, the disease has caused overall population declines of 77%, with local declines in excess of 90%. While localized extinction has not been documented, much of Tasmanian landscapes are functionally depleted of the largest carnivore. There are encouraging signs of tolerance and resistance emerging in wild populations, but these have not yet resulted in measurable, sustained recovery. This thesis proposes a multiscale framework to understand the effect of the cancer on the spatial ecology and movement ecology of Tasmanian devils with potential repercussion on transmission of DFTD in long-diseased areas of the north-east Tasmania. The research presented here is the result of intensive fieldwork from 2015 to 2018, building on long-term datasets since 1999, bringing deeper knowledge on demography and spatial ecology of Tasmanian devils, from daily movements to large-scale distribution. In 2015 and 2016, GPS tracking devices were fitted to adult devils in the Freycinet Peninsula, including animals presenting clinical signs of DFTD. Previous tracking data (using VHF radiotracking technology) gathered in the Freycinet peninsula in 2001, allowed the comparison of spatial organization before and after the outbreak. In 2017, a large-scale (2900 km2) camera trap survey was implemented in the north-east Tasmania to predict the abundance distribution of devils in the long-diseased areas. Additionally, as part of a long-term monitoring program, live trap surveys were conducted every year in two coastal populations, wukalina (30 km\\(^2\\)) and Freycinet (300 km\\(^2\\)). These surveys were used to challenge the large-scale spatial distribution when down-scaling and up-scaling the empirical survey. In Freycinet, DFTD infection was associated with a decrease in daily movement activity with males showing a stronger response than females. At the population level, after controlling for differences in tracking technologies (VHF vs GPS), the reduction in devil density in Freycinet resulted in smaller home range sizes for females and a more clustered spatial organization, a trend not observed for males. At a larger spatial scale in the north-east of Tasmania, abundance distribution was strongly influenced by the design of the empirical survey and the spatial scale at which the effect of the habitat was measured. Distribution and abundance of devils was predominantly driven by the presence of low cover (heathland and scrub) and forest cover, while rugged landscape was negatively selected. The predicted distribution in the north-east was heterogenous with hot spots in coastal areas, interconnected with low altitude dry sclerophyll forests. High altitude wet forest and large open areas were the least suitable habitat in these long diseased populations. Collectively, this research indicates that the DFTD outbreak influenced the spatial and movement ecology of Tasmanian devils at different scales. This likely resulted in lower contact rates and less competition for resources. Less aggressive behaviours in the population could in turn reduce disease transmission. The heterogenous abundance distribution of devils suggests that meta-population mechanisms could underlie the current epidemiology of DFTD where local extinction and colonization may increase the resilience of the host species at landscape scales. Additionally, the results suggest that females are more important for the local persistence of the disease. This research provides new insight on how pathogens directly and indirectly influence the spatial ecology and movements of their hosts. The study of these relationships at different spatial scales opens new perspectives to understand host persistence to infectious diseases. This research supports and encourages the future development of spatial epidemiology for wildlife diseases to better inform conservation strategies.


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Copyright 2019 the author

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