It’s a conservationist’s dream. Two endangered Australian mammals are bouncing back from the brink of extinction after being relocated to a remote, predator-free island. The once-abundant brush-tailed bettong and black-footed rock-wallaby have been under threat since cats and foxes were introduced to Australia by European settlers.
Brush-tailed bettongs, or woylies, have now almost completely disappeared from the mainland, while black-footed rock-wallabies are listed as endangered.
However, on Wedge Island in South Australia some 200 kilometres off Adelaide, it’s a different story. Here, 11 bettongs and 11 rock-wallabies were released between 1975 and 1983 as part of a conservation effort. Now, the 10 square kilometres of land are teeming with more than 1500 bettongs and 200 rock-wallabies.
About 300 southern hairy-nosed wombats are now also living on the island, after six of the non-threatened mammals were introduced in 1971 to boost tourist appeal.
“It’s pretty spectacular if you walk over the island – the three species are everywhere,” says Bertram Ostendorf of the University of Adelaide.
Although Wedge Island is the first place in which three new mammals have been introduced around the same time, it has been subject to surprisingly little monitoring. “It’s expensive to get to, so not much information is available on the success of these introductions or their impact on the island’s endemic fauna,” says Ostendorf.
To address this, Ostendorf and colleagues set up 34 motion-activated cameras around the island. Over two separate four-week periods, the cameras snapped more than 9000 photos of the island’s fauna, including the three introduced mammals, 15 different native bird species and five native lizard species.
The diversity of native species hinted that the island’s ecosystem had weathered the mammal introduction, but there were no detailed historical records to compare with, says Ostendorf, who presented the work at the Natural Resource Management Science Conference in Adelaide earlier this month.
An encouraging finding was that native birds and reptiles appeared to be taking advantage of the new shelter provided by the wombats, with 67 photographs capturing them leaving or entering wombat burrows.
“The wombat burrows are like hotels, with penguins and lizards checking in and checking out,” says Ostendorf.
The wombats acted as soil engineers, building burrows that could be used for shelter, while rock-wallabies acted as gardeners, helping to create open grasslands suitable for wombat grazing.
Wedge Island was chosen for the mammal relocation because the ecosystem had already been degraded by extensive sheep grazing after its settlement in 1858. “When grazing was abandoned, there was limited flora or fauna conservation value,” says Ostendorf.
Australia has more than 1000 islands larger than 50 hectares that could potentially serve as refuges, but the risks and benefits need to be weighed up, he says.
Mike Letnic of the University of New South Wales agrees. “Increasingly, we are realising that threatened mammals can be translocated to islands to escape threats on the mainland,” he says. “However, they will change the islands, often in unexpected ways.”
Reintroduction of mammals to the mainland may also be difficult, because they will not have developed the skills to avoid predators, Letnic says.
Nevertheless, with 10 per cent of Australia’s mammals already extinct and another 40 per cent listed as vulnerable, conservationists are increasingly resorting to island refuges for species including Gilbert’s potoroo, quolls and Tasmanian devils.
In the last few decades, for example, the bilby has been introduced to Thistle Island, the tammar wallaby to Greenly Island and the banded hare-wallaby to Bernier and Dorre Islands.
“If it wasn’t for these islands, a lot more species would have gone extinct,” says Adrian Manning of Australian National University. “Foxes and cats are so pervasive on the mainland and we still have a lot to learn about how to control them.”
“The key thing is that islands buy you time,” Manning says.