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Tasmanian tiger may not be extinct - scientists

Computer modelling suggests that Australia's thylacine, or Tasmanian tiger, was not wiped out by bounty hunters and may still survive today in isolated areas.

Dutch and U.S. environmental economists have combined biology and economics into a single mathematical model to re-evaluate whether demand for dead 'tigers' 100 years ago drove the species to extinction in 1936, as scientists have long believed.

The economists report in the latest issue of the journal Ecological Economics that "our results indicate that economic forces could not have led to the species' demise."

Moreover, using an open access hunting model that simulates harvest levels and abundance of thylacines and human hunters over time, they claim their results "support the view that tigers could still exist in the wilds of Tasmania today."

"Such models exist for many species, including herring, whales and elephants," lead author Dr Erwin Bulte of Tilburg University in the Netherlands told ABC Science Online in an interview. "But to our knowledge, this type of model has never been used to evaluate whether a species has gone extinct."

Known mostly from the north and east coasts and central plains of Tasmania, the thylacine, or Thylacinus cynocephalus, is a dog-like carnivorous marsupial with short legs, a kangaroo-like tail, striped short fur, and powerful jaws with a wide gape. Females have a backward-facing pouch.

From the 1830s to 1909, pastoral companies and governments in Tasmania paid bounties on the scalps of thylacines as part of systematic programs to exterminate the predator, suspected of taking grazing livestock brought to the island by European settlers.

Despite many claimed sightings and sporadic search parties since the last member of the species died in a Tasmanian zoo in 1936, the thylacine has never been found. It is officially considered, both in Australia and internationally, as an extinct mammal.

The economists offer hope to those who dream the thylacine may still be found. "We estimate a significant wild population existed until the 1920s, and abundance might have been sufficient to ensure enduring survival," they write.

Their model uses biological elements such as the minimum viable population of thylacines, and assumes that the island's carrying capacity decreases over time as agriculture replaces habitat.

The economic parameters estimate harvest rate and the speed with which people switch jobs when they can increase their income. It assumes hunting effort increased when the returns of snaring became more attractive than other occupations, such as agriculture.

"People adjust imperfectly - they don't go hunting right away when hunting appears profitable, and they don't give up hunting right away when returns drop," Bulte said. This gradual adjustment causes cycles in hunting behaviour and cycles in tiger abundance.

"This pattern is seen in many resource harvesting industries, such as fisheries. Our model predicts that during these cycles, the population was never hunted down to such low levels that recovery was impossible," he said.

The authors estimate a 'steady state' level of a remnant 779 animals after hunting ceased in 1909. They used a Monte Carlo simulation - running the numerical model 5,000 times while randomly varying the parameter values - to check the robustness of this survival result.

"We get 5,000 different predictions of harvesting and tiger abundance over time. There were no combinations of parameters that killed the tigers off," said Bulte. "If thylacines are extinct, it was probably not due to the bounty system alone."

They calculated that a much greater level of bounty (more than five times higher than the ₤1 paid per scalp), or a 78% reduction in the labour wage rate, would have been required to entice enough hunting effort to drive the population below its minimum viable level.

Tasmania's entire carrying capacity for thylacines is estimated as between 2,000 and 4,000 animals, or a maximum of one pair in 50 to 60 square kms. Records show that just one tannery exported 3,482 skins to London between 1878 and 1896.

One Australian scientist disagreed with the findings, saying the model oversimplifies the thylacine's world and the complex forces of extinction. Dr Stephen Wroe, a palaeontologist at University of Sydney, is researching the thylacine's extinction from mainland Australia around 3,000 years ago, and the impacts of competition with both dingoes and humans.

"The thylacine went extinct [in Tasmania] for a whole raft of reasons, and the bounty system was one of them - others include loss of habitat, loss of preferred prey species, competition with feral dogs, disease, changing fire regimes ... the list goes on," he told ABC Science Online.

"But one thing is absolutely indisputable - European Australians drove it [to extinction]. The way in which this paper is crafted appears to suggest that this is not the case," Wroe said. "This is not only misleading, but damned irresponsible."

The authors conclude their paper with an important caveat: in order to better model the dynamics and recovery of Tasmania's thylacine population, they write, more spatial population data and a greater knowledge of migratory movements was needed.

"In theory, such a model is feasible, but in practice the data requirements (on the biological side, but also on the economics side) are such that this is virtually impossible," said Bulte.