On French Island in Victoria\u2019s Western Port Bay, koalas are dropping from trees. Eucalypts have been eaten bare by the marsupials, with local reports of some found starving and dead. Multiple koalas \u2013 usually solitary animals \u2013 can often be seen on a single gum.<\/p>\n
Koalas were first introduced to French Island from the mainland in the 1880s, a move that protected the species from extinction in the decades they were extensively hunted for their pelts<\/a>. In the absence of predators and diseases such as chlamydia, the population thrived.<\/p>\n But the island is now struggling with an overabundance, part of a paradoxical threat facing the marsupial across the country: in the north-eastern states, koala numbers are declining, but in parts of southern Australia, the animals are eating themselves out of house and home. What makes saving the celebrated species so difficult to get right?<\/p>\n Too much of a good thing<\/p>\n There are between 729,000 to 918,000 koalas nationally, according to the latest estimate from the CSIRO\u2019s National Koala Monitoring Program. Those figures are up from a 2023 estimate of 287,830 to 628,010 koalas nationally, but experts have pointed out<\/a> the updated figures reflect more accurate technology and extensive survey work, rather than a true increase in the koala population.<\/p>\n The koala population in the Mount Lofty Ranges in South Australia has grown so large that it is a threat to its own long-term survival. Photograph: Desley Whissen<\/p>\n But in South Australia\u2019s Mount Lofty Ranges, the koala population \u2013 about 10% of Australia\u2019s total \u2013 is booming. The ranges \u201ctick all the right boxes\u201d, says Dr Fr\u00e9d\u00e9rik Saltr\u00e9, a senior lecturer in ecology at the University of Technology Sydney. \u201cRainfall, temperature, soil acidity really boosted the habitat suitability \u2026 so they could actually thrive in this ecosystem.\u201d<\/p>\n It\u2019s now a victim of its own success. A new study co-authored by Saltr\u00e9 has found that the Mount Lofty Ranges koala population has grown so large that it is a threat to its own long-term survival. Published in the scientific journal Ecology and Evolution<\/a>, it projects the population will grow by between 17% and 25% over the next 25 years.<\/p>\n \u201cWe might think it\u2019s a good thing having a lot of individuals, but it causes a lot of ecological problems down the track,\u201d he says. \u201cThey\u2019re going to have massive issues feeding themselves.\u201d<\/p>\n Researchers are concerned about the risk of overbrowsing, which damages the trees koalas rely on for food. Photograph: Desley Whissen<\/p>\n Koala overpopulation in the Mount Lofty Ranges was identified as a potential problem as early as 1996<\/a>.<\/p>\n Saltr\u00e9, also a research scientist at the Australian Museum, is concerned about the risk of overbrowsing, which damages the trees koalas rely on for food and can eventually result in mass starvation.<\/p>\n Mathew Crowther, a professor in quantitative conservation biology at the University of Sydney, says many of the overabundant koala populations in southern Australia are in locations where the animal was not originally found.<\/p>\n \u201cIt means that the eucalyptus trees are not necessarily that resistant to them, so the koalas overbrowse,\u201d he says.<\/p>\n Koalas are notoriously picky eaters \u2013 of 800-odd species of Australian eucalypts, they eat fewer than 50. Photograph: Desley Whissen<\/p>\n Koalas were introduced to the Mount Lofty Ranges from Kangaroo Island, home to another overabundant population. The Kangaroo Island koalas, in turn, originate from French Island \u2013 specifically, from 18 animals sent interstate in the 1920s.<\/p>\n Koalas are notoriously picky eaters \u2013 of 800-odd species of Australian eucalypts, they eat fewer than 50. But these three locations are all abundant in manna gum, which Assoc Prof Desley Whisson, a terrestrial wildlife ecologist at Deakin University, says is one of the koala\u2019s preferred food trees.<\/p>\n \u201cIt\u2019s high in available nitrogen, low in toxins, and also high in moisture, so the best thing for a koala,\u201d she says. It also tends to grow in a monoculture, supporting higher koala populations than mixed forests with lower densities of the animals\u2019 favourite trees to feast on.<\/p>\n Exacerbating the problem are commercial plantations of blue gum \u2013 another preferred food tree \u2013 which Whisson says \u201chave facilitated an increase in koala populations\u201d. In Victoria\u2019s south-west, about 42,500 koalas<\/a> live in such plantations. When the trees are harvested, the displaced koalas move on, contributing to the decline of native vegetation nearby.<\/p>\n Habitat loss overshadowed<\/p>\n Further north, the picture is also dire. Koalas in Queensland, New South Wales and the Australian Capital Territory have been listed as endangered by the Australian government since 2022.<\/p>\n In these places, Crowther says, native populations have declined due to land-clearing, habitat fragmentation caused by urban development, disease, and death from vehicle strikes and dog attacks.<\/p>\n \u201cBy far the biggest issue is habitat loss,\u201d Whisson agrees, adding that climate change is also a threat. \u201cWe\u2019re seeing a lot of drought; we\u2019re also seeing an increase in fire in some locations.\u201d<\/p>\n Analysis has shown that nearly 2m hectares of forests<\/a> suitable for koalas have been destroyed since the marsupial was declared a threatened species in 2011. Most of the clearing \u2013 81% \u2013 occurred in Queensland.<\/p>\n While the southern states have a reputation for overabundant koala populations, Whisson says that aside from a few locations \u2013 including islands, the Mount Lofty Ranges and Budj Bim national park in Victoria\u2019s south-west \u2013 for the most part koala populations are at low densities.<\/p>\n \u201cI\u2019m actually a bit concerned that we might be losing some of our populations,\u201d she says. The issue is overshadowed by the overabundance problem, because \u201cit\u2019s a very visible, very difficult situation to deal with\u201d.<\/p>\n Move or sterilise?<\/p>\n Koala overabundance has no easy fixes. \u201cThe only way you can get any kind of rapid knockdown of a population is to remove some and put them somewhere else,\u201d Whisson says. \u201cThat is really expensive, and it doesn\u2019t always work.\u201d<\/p>\n Koala translocations were clearly once successful in establishing the now overabundant populations in Victoria and South Australia, but there have also been notable bungles, most recently earlier this year in NSW<\/a>.<\/p>\n