Tassie tiger extinction: humans solely to blame
HUMANS WERE EXCLUSIVELY RESPONSIBLE for the disappearance of the Tasmanian tiger, new research has revealed.
Using a new method, scientists at the University of Adelaide, SA, have confirmed that human-induced pressures, and not an epidemic, wiped out the Tasmanian tiger or thylacine, which became extinct in the 1930s.
The finding, which was published in the Journal of Animal Ecology this week, contradicts the widespread belief that disease played a major role in the species’ disappearance.
“The thylacine’s extinction has been debated many times and often researchers have concluded that some unknown cause, such as a disease epidemic, must have contributed to its extinction,” lead researcher Dr Thomas Prowse told Australian Geographic.
“We have weakened this argument by showing how the thylacine extinction can be explained by the well-documented impacts of Europeans.”
Why the thylacine became extinct
The thylacine, which was abundant in Tasmania before European settlement in 1803, was a unique marsupial carnivore that fell into rapid decline between 1886 and 1909, when the Tasmanian government encouraged people to hunt the iconic predator by paying a bounty for carcasses.
Few animals were located once the bounty was lifted and the last known wild thylacine was captured in 1933. Although the species was granted full protection in July 1936, by September that year the last surviving thylacine died in Hobart Zoo.
Thomas and his team developed a mathematical model to recreate the pre-European food web in Tasmania, incorporating thylacines, their prey (kangaroos and wallabies), and the growth of vegetation, which supported these prey species.
“We then simulated the diverse impacts of Europeans, including habitat modification, bounty hunting, prey harvesting, and the rapid growth of the sheep population that competed with kangaroos and wallabies for food,” says Thomas.
“We were able to simulate the thylacine extinction without needing to invoke disease and also recreated the rapid early 1900s population decline that is evident from the government bounty records.”
Programming Tasmanian tiger extinction
The research technique they developed extends from a process known as population viability analysis, or PVA, which is commonly used by conservation biologists to evaluate the extinction risk for threatened species. “PVA takes information on rates of birth, death, immigration, and emigration for a single species, and uses this to predict the fate of animal populations into the future, often using a dedicated computer program,” says Thomas.
“Together with our collaborators overseas, we developed and tested new software that can link multiple PVA models to create what we call a ‘metamodel’,” he says. Metamodels can simulate complex ecological systems involving different types of species interactions, such as predation, competition and even disease transmission.
“In this case, we constructed a metamodel by linking separate models for thylacines, kangaroos and wallabies,” he says.
In developing the first multi-species PVA model, the team was able to examine the interactions of several species and impacts at once. “Single-species PVA ignores important species interactions so it may fail to identify threatening processes or misrepresent the effectiveness of different management alternatives,” says Thomas.
Bounty wiped out Tasmanian tigers
The technique will allow conservation managers to generate more realistic predictions of extinction risk in the future. “Since the fate of all species is inextricably linked to those around it, be they predators, prey or pathogens, it’s not surprising that there’s a host of conservation problems to which our multi-species approach could be applied,” Thomas says.
Thomas and his team are currently using metamodels to consider the economic impacts of dingoes in Australia’s cattle rangelands and to aid in the conservation of a number of threatened species including native rodents in Tasmania, long-necked turtles in the Northern Territory, and black-footed ferrets in North America.
Dr Brandon Menzies, a zoologist at the University of Melbourne who has conducted extensive research into the genetic diversity of thylacines, says the metamodelling approach has provided new insights into the species’ disappearance.
“I think this is a really nice study that quantitates the contribution of a range of factors that would have contributed to thylacine population health prior to their extinction,” says Brandon.
“Importantly, it suggests that the impact of the government bounty and competition from introduced herbivores had the largest influence on thylacine numbers and would have been sufficient to eliminate the population in this simulated metamodel,” he says. “There is now good evidence… to explain their rapid decline in the early 1900s, which contradicts the idea of a wild epidemic.”