University of Adelaide modelling finds koala numbers on island have more than tripled over past five years

This article is more than 3 years old

This article is more than 3 years old

Koala numbers on Kangaroo Island have more than tripled over five years but the South Australian government has rebuffed calls to cull the animals, saying they can be managed through existing sterilisation programs.

Koalas were introduced to Kangaroo Island in the 1920s because of concerns they were facing extinction from hunting and habitat destruction on the mainland.

Eighteen animals grew to tens of thousands by 1997, when the SA government instituted a koala management plan to curb koala numbers in an effort to preserve the shrinking manna gum habitat.

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The management plan involved sterilising adult female koalas and in some cases moving the sterilised females to manna gum forests south-east of Adelaide, providing some relief for the Kangaroo Island trees.

Between 1997 and 2016, about 12,500 koalas were sterilised and about 3,800 were relocated.

The population dropped from 27,000 in 2001 to 13,000 in 2010 but modelling done by the University of Adelaide based on 2015 survey data found the number had since grown to about 50,000.

The final results of that survey have not been released but the Department of Environment, Water and Natural Resources regional manager for Adelaide and the Mount Lofty ranges, Brenton Grear, said the estimates appeared to be accurate.

It has prompted calls by the state MP for the island to hold a koala cull, a proposal that the ABC reports has been backed by the Conservation Council of SA.

Grear said the massive increase in numbers would prompt the department to review its plans but that a cull was not on the cards. He said one option could be moving from surgical sterilisation to a hormonal treatment, which has been successfully trialled in south-western Victoria.

“You might have to do more sterilisation, you might have to look at techniques around sterilisation that are more effective,” he told Guardian Australia.

Under-skin hormonal implants, similar to contraceptive implants available to women, can be delivered “at the bottom of the tree” and are much quicker and cheaper than surgery.

Grear said culling koalas would be contrary to both state and federal policy and would take a massive public information campaign to convince locals and tourists that it was an effective idea.

On top of that, he said, you would have to be assured that the animals could be culled humanely.

SA allows the commercial harvesting and culling of kangaroos and some other native species but Grear said that, despite not being native to Kangaroo Island, koalas were still placed in “a different category to other native animals”.

A spokeswoman for the SA environment minister, Ian Hunt, said it was not government policy to cull koalas and that the sterilisation program would be extended.

“Culling will never happen,” Deakin University wildlife and conservation lecturer Dr Desley Whisson said. “It becomes a political issue. We do cull other populations of other species but I don’t think the public will ever support a cull of koalas.”

Whisson managed Kangaroo Island’s koala program for a number of years and said it had a finely tuned sterilisation program that involved sending out a team of koala wranglers every year to find and catch female koalas and send them to a vet, who would perform the keyhole surgery to cut the oviduct. The whole procedure, from capture to release, took about five hours.

In 2015-16 Natural Resources Kangaroo Island caught 517 female koalas, or 6.9 a day. Of those, 503 were sterilised, 10 had already been sterilised and four were euthanised due to poor health.

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Whisson said it was “plausible” that the koala population had grown by 37,000 in five years but it was also possible that previous surveys had underestimated the population by discounting the bluegum plantations, which the most recent survey found were prime koala habitat.

Booming koala numbers in south-west Victoria are linked to the growth of bluegum plantations, which are a favourite koala food tree along with manna gums.

Grear said problems on Kangaroo Island were caused not by the sheer number of koalas but by their density in specific locations, particularly the vulnerable manna gum forests. Preliminary results from the 2015 survey showed the densities were at “acceptable” levels in the manna gum forests but unacceptable levels in the bluegum plantation, which is ready to be harvested.

Whisson said that targeting the sterilisation program on the manna gum forests would leave those areas open to “constant invasion” by koalas from other areas of the island moving toward their preferred food.

“You might sterilise or translocate koalas out of those areas but there will be others that want to come in,” she said.