Tasmanian Devil Population Research Articles

Evidence that disease-induced population decline changes genetic structure and alters dispersal patterns in the Tasmanian devil.

Infectious disease has been shown to be a major cause of population declines in wild animals. However, there remains little empirical evidence on the genetic consequences of disease-mediated population declines, or how such perturbations might affect demographic processes such as dispersal. Devil facial tumour disease (DFTD) has resulted in the rapid decline of the Tasmanian devil, Sarcophilus harrisii, and threatens to cause extinction. Using 10 microsatellite DNA markers, we compared genetic diversity and structure before and after DFTD outbreaks in three Tasmanian devil populations to assess the genetic consequences of disease-induced population decline. We also used both genetic and demographic data to investigate dispersal patterns in Tasmanian devils along the east coast of Tasmania. We observed a significant increase in inbreeding (F(IS) pre/post-disease -0.030/0.012, P<0.05; relatedness pre/post-disease 0.011/0.038, P=0.06) in devil populations after just 2-3 generations of disease arrival, but no detectable change in genetic diversity. Furthermore, although there was no subdivision apparent among pre-disease populations (θ=0.005, 95% confidence interval (CI) -0.003 to 0.017), we found significant genetic differentiation among populations post-disease (θ=0.020, 0.010-0.027), apparently driven by a combination of selection and altered dispersal patterns of females in disease-affected populations. We also show that dispersal is male-biased in devils and that dispersal distances follow a typical leptokurtic distribution. Our results show that disease can result in genetic and demographic changes in host populations over few generations and short time scales. Ongoing management of Tasmanian devils must now attempt to maintain genetic variability in this species through actions designed to reverse the detrimental effects of inbreeding and subdivision in disease-affected populations.

How to Build an Efficient Conservation Fence

Barriers are used to achieve diverse objectives in conservation and biosecurity. In conservation management, fences are often erected to exclude introduced predators and to contain diseased animals or invasive species. Planning an efficient conservation fence involves a number of decisions, including the size and design of the enclosure. We formulated the first general framework for building a fence that minimizes long-term management costs by balancing the expense of constructing a more secure barrier against the costs of coping with more frequent failures. The approach systematically considers the range of potential solutions to a well-defined fencing problem and results in a solution that maximizes conservation return on investment. We illustrated this method by designing efficient fences to address two different conservation goals: exclusion of invasive predators from populations of threatened eastern barred bandicoots (Perameles gunnii) and maintenance of isolated populations of healthy Tasmanian devils (Sarcophilus harrisii). A systematic approach to conservation fencing allows the best fence design to be chosen quantitatively and defensibly. It also facilitates conservation decisions at a strategic level by allowing fencing to be compared transparently with alternative conservation management actions.

Contact networks in a wild Tasmanian devil (Sarcophilus harrisii) population: using social network analysis to reveal seasonal variability in social behaviour and its implications for transmission of devil facial tumour disease

The structure of the contact network between individuals has a profound effect on the transmission of infectious disease. Using a novel technology--proximity sensing radio collars--we described the contact network in a population of Tasmanian devils. This largest surviving marsupial carnivore is threatened by a novel infectious cancer. All devils were connected in a single giant component, which would permit disease to spread throughout the network from any single infected individual. Unlike the contact networks for many human diseases, the degree distribution was not highly aggregated. Nevertheless, the empirically derived networks differed from random networks. Contact networks differed between the mating and non-mating seasons, with more extended male-female associations in the mating season and a greater frequency of female-female associations outside the mating season. Our results suggest that there is limited potential to control the disease by targeting highly connected age or sex classes.

Efficiently locating conservation boundaries: Searching for the Tasmanian devil facial tumour disease front

Conservation management actions and decisions are often defined by the location of ecological boundaries, for example, the present range of invasive or threatened species. The position of these boundaries can be cryptic, and managers must therefore directly sample sites, an expensive and time-consuming process. While accurate boundary location techniques have been considered by ecological theorists, the issue of cost-effective, or optimal boundary location has not. We propose a general framework for boundary location which incorporates both cost-efficiency and uncertainty. To illustrate its application, we use it to help locate an infectious disease front in the endangered Tasmanian devil population. The method ensures optimal spatial sampling by maximizing the expected information gained from each sample. When resources are limited, our method provides more accurate estimates of the boundary location than traditional sampling protocols. Using a formal decision theory sampling design encourages economically efficient actions, and provides defensible and transparent rationale for management actions.

Demography, disease and the devil: life‐history changes in a disease‐affected population of Tasmanian devils (Sarcophilus harrisii)

1. Examining the demographic responses of populations to disease epidemics and the nature of compensatory responses to perturbation from epidemics is critical to our understanding of the processes affecting population dynamics and our ability to conserve threatened species. Such knowledge is currently available for few systems. 2. We examined changes to the demography and life-history traits of a population of Tasmanian devils (Sarcophilus harrisii) following the arrival of a debilitating infectious disease, devil facial tumour disease (DFTD), and investigated the population's ability to compensate for the severe population perturbation caused by this epizootic. 3. There was a significant change to the age structure following the arrival of DFTD to the Freycinet Peninsula. This shift to a younger population was caused by the loss of older individuals from the population as a direct consequence of DFTD-driven declines in adult survival rates. 4. Offspring sex ratios of disease mothers were more female biased than those of healthy mothers, indicating that devils may facultatively adjust offspring sex ratios in response to disease-induced changes in maternal condition. 5. We detected evidence of reproductive compensation in response to disease impacts via a reduction in the age of sexual maturity of females (an increase in precocial breeding) over time. 6. The strength of this compensatory response appeared to be limited by factors that constrain the ability of individuals to reach a critical size for sexual maturity in their first year, because of the time limit dictated by the annual breeding season. 7. The ongoing devastating impacts of this disease for adult survival and the apparent reliance of precocial breeding on rapid early growth provide the opportunity for evolution to favour of this new life-history pattern, highlighting the potential for novel infectious diseases to be strong selective forces on life-history evolution.

Life-history change in disease-ravaged Tasmanian devil populations.

Changes in life history are expected when new sources of extrinsic mortality impact on natural populations. We report a new disease, devil facial tumor disease, causing an abrupt transition from iteroparity toward single breeding in the largest extant carnivorous marsupial, the Tasmanian devil (Sarcophilus harrisii), in which males can weigh as much as 14 kg and females 9 kg. This change in life history is associated with almost complete mortality of individuals from this infectious cancer past their first year of adult life. Devils have shown their capacity to respond to this disease-induced increased adult mortality with a 16-fold increase in the proportion of individuals exhibiting precocious sexual maturity. These patterns are documented in five populations where there are data from before and after disease arrival and subsequent population impacts. To our knowledge, this is the first known case of infectious disease leading to increased early reproduction in a mammal. The persistence of both this disease and the associated life-history changes pose questions about longer-term evolutionary responses and conservation prospects for this iconic species.

Lessons from the Looming Extinction of the Tasmanian Devil

Extinction in the wild is now regarded as likely for the Tasmanian Devil Sarcophilus harrissi. In 1996, a disease, Devil Facial Tumour Disease (DFTD), was seen in a Devil in the wild. By mid-2008, the population had declined by about 70%, and the disease was spreading rapidly from east to west across Tasmania. DFTD is an infectious cancer, passed from one Devil to another principally, or entirely, by biting. The bite implants the cancer cells; the low genetic diversity in the Tasmanian Devil population from previous population bottlenecks or selective sweeps means that those cancer cells are not recognized by the immune system and the cancer grows in the infected Devil. Estimates of the time to extinction in the wild range from about 20 to 35 years. If, however, there are resistant genotypes within the Devil population then extinction in the wild may be averted, with numbers augmented through captive breeding programmes and active management to spread these genotypes in the wild.

The impact of disease on the survival and population growth rate of the Tasmanian devil

1. We investigated the impact of a recently emerged disease, Devil Facial Tumour Disease (DFTD), on the survival and population growth rate of a population of Tasmanian devils, Sarcophilus harrisii, on the Freycinet Peninsula in eastern Tasmania. 2. Cormack-Jolly-Seber and multistate mark-recapture models were employed to investigate the impact of DFTD on age- and sex-specific apparent survival and transition rates. Disease impact on population growth rate was investigated using reverse-time mark-recapture models. 3. The arrival of DFTD triggered an immediate and steady decline in apparent survival rates of adults and subadults, the rate of which was predicted well by the increase in disease prevalence in the population over time. 4. Transitions from healthy to diseased state increased with disease prevalence suggesting that the force of infection in the population is increasing and that the epidemic is not subsiding. 5. The arrival of DFTD coincided with a marked, ongoing decline in the population growth rate of the previously stable population, which to date has not been offset by population compensatory responses.

The Immunohistochemical Characterization of Devil Facial Tumor Disease (DFTD) in the Tasmanian Devil (Sarcophilus harrisii)

Immunohistochemical techniques were used to characterize the disfiguring and debilitating fatal neoplastic disease, devil facial tumor disease (DFTD), which has recently affected a significant proportion of the wild population of Tasmanian Devils (Sarcophilus harrisii). The diagnostic values of a number of immunohistochemical stains were employed to further characterize 50 representative cases. The neoplasms were negative for cytokeratin (0/48), epithelial membrane antigen (0/42), von Willebrand factor (vWF) (0/11), smooth muscle actin (SMA) (0/26), desmin (0/47), glial fibrillary acid protein (0/13), CD16 (0/13), CD57 (0/43), CD3 (0/18), and LSP1 (0/16). DFTD cells were positive for vimentin (50/50), S-100 (41/48), melan A (11/39), neuron specific enolase (35/35), chromogranin A (12/12) and synaptophysin (29/30). The cells were negative for amyloid (0/30) and stained negatively with Singh's silver (0/34) but were weakly argyrophilic (3/40) using Grimelius histochemical stain. These staining characteristics are consistent with cells of neuroectodermal origin.

Time to raise the devil

A horrible facial cancer is decimating the Tasmanian devil population. But researchers in Australia think they have found a way to save the species. Carina Dennis reports. The Tasmanian devil, the largest surviving carnivorous marsupial, was hunted to extinction on the Australian mainland. Now it could be wiped out in Tasmania too. The big threat is the fatal devil facial-tumour disease. Remarkably, and unlike other malignancies, the tumours in every affected animal carry identical chromosome rearrangements and the tumour seems to be passed on by contact through biting, consistent with transmission by an allografting process. Meanwhile, conservation efforts focus on removing sick animals from wild populations.

Road upgrade, road mortality and remedial measures: impacts on a population of eastern quolls and Tasmanian devils

The impact of road mortality on local populations of wildlife has rarely been quantified. In June 1991, the access road into the northern end of the Cradle Mountain – Lake St Clair National Park in Tasmania was widened and sealed. This occurred part-way through an ecological study of the dasyurid carnivore guild, during which populations were being monitored. In 17 months, the resident population of 19 eastern quolls became extinct and the devil population, of 39 individuals, halved. Concurrently, there was a dramatic increase in the number of road-kills. The main causal factor was probably an increase in modal speed of about 20 km h–1 and a greater increase in maximum speed. Measures were implemented to reduce the incidence of vehicle/wildlife collisions. Measures directed at people included physically slowing traffic speed (using ‘slow points’) and increasing driver awareness (signs and pamphlets). Those directed at wildlife included deterring wildlife from crossing the road in the path of approaching vehicles (wildlife reflectors), and encouraging escape off the road (ramps across gutters and banks, and pipes for shelter). The ‘slow points’ were effective in reducing vehicle speeds by 20 km h–1. Wildlife used the ramps and pipes. The eastern quoll population was re-establishing within six months, and after two years, had recovered to 50% of its former level. There was some indication that devil populations were recovering.