Thursday 3 May 2012

What killed the Australian Thylacine.

The Thylacine, also known as the Tasmanian Tiger or Marsupial Wolf, was a large, predatory, marsupial mammal which roamed Australia from about four million years ago until about 3500 years ago, when it became extinct on the Australia mainland, though it persisted on the island of Tasmania till the twentieth century. The period when the Thylacine went extinct on the Australian mainland co-insides with the adoption of a number of new technologies by human populations across mainland Australia (but not Tasmania) leading to a rise in human populations across the continent, and also the introduction of Dingos (wild dogs) to mainland Australia (but again, not Tasmania). These two events are both likely to have contributed to the downfall of the mainland Thylacines, though the contribution of each to the eventual extinction has been hard to assess.

A Thylacine, front, and a Dingo, behind. Carl Buell.

Invasive predators can cause problems for their indigenous rivals in a number of ways. Most obviously they often tend to outcompete them. Prey animals that evolved alongside the indigenous predator will have developed methods of avoiding predation by it, but are often unable to cope with the invasive predator, which may use novel, unfamiliar hunting strategies (such as hunting in packs, which Dingos do, but which Thylacines are not thought to have done). This will cause the incoming predator to suppress prey numbers till they have time to adapt to the novel predation methods. This is more damaging still if individual members of the new species have a larger individual prey requirement; again this is likely to have been the case with the placental Dingos being presumed to have had a faster metabolism than the marsupial Thylacines.

Introduced predators can be even more of a threat if they directly attack members of the indigenous species. Predators will often attack and kill members of smaller predatory species with which they compete, even if they do not actually eat the rival species. Like prey items, predators may be particularly vulnerable if they encounter a novel rival species which behaves in ways they have not evolved to counter.

Until now Thylacines have been thought to have been reasonably safe against direct attacks by Dingos, as members of the surviving twentieth century population in Tasmania were too large, with the females matching the largest Dingos in size, and the males being considerably larger.

In a paper published in the journal PLoS One on 2 May 2012, Mike Letnic of the School of Biological, Earth and Environmental Sciences at the University of New South Wales, Melanie Fillios of the Department of Archaeology at the University of Sydney and Mathew Crowther of the School of Biological Sciences at the University of Sydney, discuss the results of a study of subfossil Dingo and Thylacine specimens from the Western Australian Museum, gathered in temperate southwest Australia and the semi-arid Nullarbor region.

This revealed that he Thylacines of southwest Australia and the Nullarbor region were considerably smaller than those of Tasmania, with only the largest males being comparable in size to a Dingo, and the females typically being around half the size. This suggests that any hostile encounters between the solitary Thylacines and the pack-hunting Dingos would have gone very badly for the Thylacines. Even if the Dingos only targeted female Thylacines the effect on the Thylacine population level would have been devastating; most animals can cope with a sharp drop in the number of male animals, but females are essential to maintaining the population.

Skulls from the Western Australian Museum. Left, female Thylacine. Center, male Thylacine. Right, Dingo. All from the Nullarbor region. Scale bar is 5 cm. Letnic, Fillios & Crowther (2012).

Sufficient material was not available from other regions to assess the sizes of Thylacines across continental Australia, but individual teeth from southeast Australia and a partial skull from the Cape Range in the northwest of the continent suggest that the samples from the Western Australian Museum are a better model than those from Tasmania.

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