18 koalas moved to Kangaroo Island in the 1920s; a century on, 27,000 descendants are stripping eucalyptus bare and risk mass starvation

Most Americans picture a fluffy marsupial clinging to a tree when they think of koalas and a species slowly disappearing from the wild. That image isn't wrong. But in one corner of Australia, this situation is reversed. The koalas are not disappearing. They're multiplying so fast that scientists are now warning of an impending ecological disaster.

According to a new study published in Ecology and Evolution led by Dr. Frédérik Saltré of the University of Technology Sydney and the Australian Museum, alongside researchers from Flinders University and the University of Wollongong, South Australia’s Mount Lofty Ranges has a koala population that has grown far beyond what the land can cope with. According to this research, which delivers the first detailed population estimate ever conducted for the area, this one area now holds roughly 10% of all koalas in Australia. And without action, that number could increase by another 17 to 25 percent over the next 25 years, putting enormous strain on the forests that these animals rely on for food.

This isn't the first time South Australia has been here
The story of how koalas can overrun a habitat has played out before, just a quick boat ride away. A peer-reviewed study in Wildlife Research by Masters, Duka, Berris and Moss, "Koalas on Kangaroo Island: from introduction to pest status in less than a century", states that only 18 koalas were released on Kangaroo Island between 1923 and 1925. The population grew rapidly according to this research, reaching an estimated 27,000 animals by 2001. The ecological profile of the koala on Kangaroo Island changed within a century from being a species introduced for conservation reasons to one of pest status, with overbrowsing of the eucalyptus trees threatening the forests and the koalas themselves. The 2019-20 Black Summer bushfires temporarily reduced the population, but numbers have since recovered to at least 15,000. Researchers warn that the pattern could happen again.


Too many koalas, and that's the problem
The twist is that koalas are in steep decline across much of eastern Australia but are thriving in South Australia. According to the Australian Government's Department of Climate Change, Energy, the Environment and Water, the koala, specifically its combined populations across Queensland, New South Wales, and the Australian Capital Territory, was federally listed as endangered under the Environment Protection and Biodiversity Conservation Act in February 2022, largely because of bushfires, drought, deforestation and disease. This makes the South Australian population a conservation bright spot, but also a serious warning.

The problem with too many koalas in one place is deceptively simple: they eat eucalyptus leaves, and when there are too many of them, the trees can’t regenerate. Many parts of the Mount Lofty Ranges already have koala numbers well above what the ecosystem can support, creating what Dr. Saltré describes as an increasing risk of “severe overbrowsing,” which could quickly wipe out the very forests that koalas rely on for food, according to the Ecology and Evolution study. Without intervention, the researchers say mass starvation and widespread habitat collapse are not hypothetical; they are a serious risk.

Why the typical fixes don’t work here
Conservation managers face a real dilemma. According to the study, traditional methods such as culling and relocation pose serious ethical issues with the public and are just not appropriate for such an iconic native species. As co-author Dr. Katharina Peters of the University of Wollongong put it: "How do we manage a species that is now threatened by its own abundance, and do so in a way that protects both animal welfare and long-term ecosystem health?" Koalas are part of Australian culture, as Americans might feel about bald eagles or wolves, and controlling their population is a hot-button issue.

The science-backed, humane fix
In this same research, the team combined thousands of observations from citizen science projects with advanced spatial modeling before running computer simulations to test multiple management scenarios. The best approach was to sterilize approximately 22% of adult female koalas annually, but only in the highest-density areas, not the entire region. According to the study, this targeted approach would stabilize the population at a cost of $34 million over 25 years, a small price to pay for the scale of crisis that would be averted.

A 2021 study in Wildlife Research by Watters, Ramsey, Molsher and Cassey found that at the population level, surgical sterilization and hormonal implants directly affect breeding outcomes, with body condition and population density identified as key drivers of breeding success. Thus, this research points to targeted intervention in dense areas as having a disproportionate stabilizing effect. That is supported by a 2020 paper in Biological Conservation, which used real field data to demonstrate that sustained fertility control programs can significantly reduce koala numbers and slow damage to eucalyptus forests. The study found that the productivity losses from fertility control could not be fully offset by compensatory demographic responses, precisely the outcome needed.

The Ecology and Evolution research builds on previous Australian Museum work that successfully sequenced around 20,000 koala genes, work that has opened new opportunities for medical treatments, deepened understanding of koala evolution, and provided insights into how best to conserve the species going forward.

What this means beyond Australia
This story is relevant and hits home for Americans who care about wildlife conservation, and research consistently shows that millennials and Gen Z do. The US has a paradox of its own: suburban ecosystems stripped by overpopulation of deer, grazing lands in the West overwhelmed by wild horse populations, and invasive species spreading unchecked by natural predators. Managing animal populations in ways that are both scientifically sound and publicly acceptable is a challenge without borders.

What is particularly instructive about the South Australian case is the predictive modeling prior to committing to an expensive strategy. According to Dr. Saltré: "Instead of spending money on a conservation plan without knowing whether it will succeed, we use computer simulations to identify in advance which strategies are most likely to work, optimizing both costs and taxpayer investment."

The takeaway is both hopeful and pressing. There is a humane, evidence-based solution, but time is running out. Damage occurs when the population grows beyond the carrying capacity of the land. For a generation raised on nature documentaries and climate anxiety, the koala crisis in South Australia is a real reminder: conservation is never easy. Sometimes the hardest thing isn't saving an endangered animal. It's about figuring out how to protect one from itself.