Billie Lazenby (Save the Tasmanian Devil Program) discusses new research into devil facial tumour disease and the recent article, Density trends and demographic signals uncover the long-term impact of transmissible cancer in Tasmanian devils.
Tracking population trends, particularly in response to a threat, is an essential component of conservation management. Moreover understanding what is driving these trends gives insight into whether they are likely to be sustained. Such consideration of the processes driving patterns is an essential component for guiding the extent and form of recovery action necessary for threatened species.
Tasmanian devils are the world’s largest marsupial carnivore and are endemic to the 65 000 km2 island of Tasmania. Their population viability has been threatened by a transmissible cancer called Devil Facial Tumour Disease (DFTD). The disease was first reported in 1996, and manifests as soft tissue masses around the head and inside the oral cavity. DFTD is invariably fatal, and has a high infection rate. Early reports showed devil numbers had declined dramatically in affected areas and there was a shift towards an age structure dominated by young devils. These impacts, combined with a lack of observed density thresholds for transmission, led to international, national, and state recognition of the devil as a threatened species.
Within five years of its emergence, the impact of DFTD was well-documented from a small number of study sites, but there was no information on the longer-term impact of the disease. Furthermore, molecular investigations revealed that the disease had the potential to evolve, therefore it is feasible that there is a dynamic interaction between devils and DFTD. More than 20 years after the first confirmed case of DFTD, we analysed the trends in spotlight counts of devils, and density, across the area impacted by DFTD, and investigated the demographic processes that might be driving these trends within nine capture-release sites across the devil’s range.
Spotlight surveys have been conducted systematically across most of the island of Tasmania since 1985, and were designed to monitor mammalian species subjected to harvesting. However all animal species observed during the surveys, including devils, are recorded and the resulting dataset was invaluable because it covered the area impacted by DFTD before and after the emergence of the disease. The nine capture-release sites used for our investigation were established as early as 2004, and monitoring commenced at some of them prior to the emergence of DFTD. From these sites we were able to estimate density, and population parameters relevant to the demographic signals we were testing.
Collaboration with an analytical specialist from San Diego Global was pivotal to our analysis. Post-disease sites typically supported very low densities of devils, and we needed a robust approach for analyzing the densities resulting from these small sample sizes. In addition, we needed to take into account the potential for any differences in trappability associated with age, sex or disease status, that might affect the accuracy of our estimates of density or demographic signals such as age structure, sex ratio, and disease prevalence. This was particularly important given the known shift in age structure towards young devils following disease emergence.
We found ongoing small declines in spotlight counts and pooled density estimates across all sites with years since DFTD emergence. Despite this, devils persisted within all individual monitoring sites. There was some variability in individual site responses, with one site showing small, as yet unexplained, increases in density 8-10 years after the emergence of DFTD. Post-disease sites continue to be dominated by young devils, and the prevalence of DFTD has not abated regardless of declines in density of around 80%. Devils have partially offset the impact of DFTD by increasing fecundity: female devils are regularly commencing breeding at the age of one whereas they rarely breed until they are two in non-diseased sites, and there are more pouch young per female in diseased sites. Earlier breeding however has come at the cost of an increase in prevalence of DFTD in one-year old devils. Essentially this means that females in diseased areas tend to breed at the age of one, and then presumably die as a consequence of an earlier sexual interaction which increases their chances of contracting DFTD.
Clearly the reduced densities and single early breeding induced by DFTD renders diseased populations susceptible to stochastic events such as road mortality, bushfire, and drought. Moreover the shift towards low densities of predominantly young devils in areas where DFTD is present means the influence of devils on the ecosystem through predation, scavenging, and life in general, has likely changed dramatically. DFTD has spread across over 80% of the devil’s range, so these ecosystem changes are widespread. In the current absence of a method to manage DFTD in wild devils, we suggest managing the small densities resulting from the disease. Management options include mitigation of threats such as road mortality and bushfire, and translocation of healthy devils to augment genetic diversity, age structure, and density.
Read the full article, Density trends and demographic signals uncover the long-term impact of transmissible cancer in Tasmanian devils in Journal of Applied Ecology.