Australia’s koala paradox: why is the beloved marsupial endangered in parts but overabundant in others? | Wildlife

Koalas are dropping from trees on French Island in Victoria’s Western Port Bay, as the marsupials overpopulate the area and deplete their food source. The island’s koala population has thrived since being introduced from the mainland in the 1880s, but is now facing a paradoxical situation alongside other parts of southern Australia – eating themselves out of house and home while populations decline elsewhere.

Koala Overabundance in the Mount Lofty Ranges

The koala population in South Australia’s Mount Lofty Ranges – about 10% of Australia’s total – is booming. According to the latest estimate from the CSIRO’s National Koala Monitoring Program, there are between 729,000 to 918,000 koalas nationally, figures that are up from a 2023 estimate of 287,830 to 628,010, though experts attribute the increase to more accurate survey methods.

A new study co-authored by Dr. Frédérik Saltré of the University of Technology Sydney, published in the scientific journal Ecology and Evolution, projects the Mount Lofty Ranges koala population will grow by between 17% and 25% over the next 25 years. Saltré warns that a large population causes ecological problems and issues with feeding.

Koala overpopulation in the Mount Lofty Ranges was identified as a potential problem as early as 1996. Saltré, also a research scientist at the Australian Museum, is concerned about the risk of overbrowsing, which damages the trees koalas rely on for food.

Many of the overabundant koala populations in southern Australia are in locations where the animal was not originally found, according to Professor Mathew Crowther of the University of Sydney. “It means that the eucalyptus trees are not necessarily that resistant to them, so the koalas overbrowse,” he says.

Koalas were introduced to the Mount Lofty Ranges from Kangaroo Island, which in turn originated from French Island – specifically, from 18 animals sent interstate in the 1920s.

Koalas are notoriously picky eaters, consuming fewer than 50 of the 800-odd species of Australian eucalypts. However, the three locations all have abundant manna gum, one of the koala’s preferred food trees, according to Assoc Prof Desley Whisson, a terrestrial wildlife ecologist at Deakin University. It also tends to grow in a monoculture, supporting higher koala populations.

Commercial plantations of blue gum have also contributed to the increase in koala populations. In Victoria’s south-west, about 42,500 koalas live in such plantations, and when the trees are harvested, the displaced koalas move on, contributing to the decline of native vegetation nearby.

Habitat Loss Overshadowed

Koalas in Queensland, New South Wales and the Australian Capital Territory have been listed as endangered by the Australian government since 2022. In these areas, land-clearing, habitat fragmentation, disease, vehicle strikes, and dog attacks have led to population declines, according to Crowther.

“By far the biggest issue is habitat loss,” Whisson agrees, adding that climate change is also a threat. “We’re seeing a lot of drought; we’re also seeing an increase in fire in some locations.” Analysis has shown that nearly 2m hectares of forests suitable for koalas have been destroyed since the marsupial was declared a threatened species in 2011, with 81% of the clearing occurring in Queensland.

While southern states have a reputation for overabundant koala populations, Whisson says populations are generally at low densities aside from a few locations. “I’m actually a bit concerned that we might be losing some of our populations,” she says, adding that the issue is overshadowed by the overabundance problem.

Move or Sterilise?

Koala overabundance has no easy fixes. “The only way you can get any kind of rapid knockdown of a population is to remove some and put them somewhere else,” Whisson says, noting that this is expensive and doesn’t always work. Koala translocations have been successful in the past, but there have also been failures, most recently earlier this year in NSW.

Culling is banned and unlikely to be adopted. Fertility control – involving sterilisation or long-term contraception – is another avenue, which has been used since the 1990s. New modelling by Saltré suggests that sterilising approximately 22% of adult females annually in the highest-density areas of the Mount Lofty Ranges would be enough to stabilise populations, at an estimated cost of $34m over 25 years.

Saltré says it is more cost-effective than translocation, but concedes “we’re playing the long game here.” Because koalas can live for up to 15 years, fertility control takes a long time to have an impact, Whisson says. A successful intervention would require a holistic approach from the government.

Both declining koala populations and overabundance can be addressed through landscape-scale restoration of habitat. The creation of the Great Koala national park in NSW, with a moratorium on logging, was welcomed by forest advocates. Bigger expanses of mixed forest are needed in the southern states, Whisson says.

“With climates changing, the predictions are that the southern states of Australia will be the stronghold for koalas,” she says. As Saltré puts it, “we need to find a sweet spot where the species can be sustainable over a long time.”