Koalas die at sharply higher rates once the seven-day average maximum temperature tops 27 degrees Celsius, a threshold well short of the heat extremes long assumed to put the species in danger. That is according to a study published in the journal Biology Letters, which matched 11,862 koala rescue admissions across New South Wales against weather records spanning 2000 to 2022.
The work, led by University of Sydney behavioural ecologist Valentina Mella, found the odds of a koala being admitted to care or dying increased by 1.5 to 3.5 times in weeks when average maximum temperatures exceeded 30 °C, relative to a 25 °C baseline. Populations across inland north-west NSW carried the heaviest exposure, because those ranges run hotter than the coastal forests to the east.
Mella, who has run several koala studies in the state’s north-west, told ABC Science that prolonged drought and more frequent heat were now compounding habitat fragmentation and disease, with some local extinctions already beyond reversal. “These are the first populations that are going to go,” she said.
The koala population in Gunnedah, once promoted as the koala capital of the world, is now functionally extinct, with the remaining animals infertile and no replacements to take their place once they die.
The study mapped every admission against local weather records, producing a statewide gradient that runs from the heat-prone inland districts of the Pilliga and Liverpool Plains to the cooler, koala-dense forests of the Mid North Coast. That coastal belt takes in the 176,000 hectares of state forest earmarked for the proposed Great Koala National Park, which the NSW government has committed to legislating late in 2026, as Wood Central reported.
The finding carries direct weight for land managers and private native forestry, because koala survival rests not only on the eucalypts the animals feed on, but also on the shade trees they shelter in during the hottest hours. University of Queensland ecologist Bill Ellis, who took no part in the study, said non-food trees mattered as much to koala survival as the eucalypts the animals browse.
Ellis has urged landowners to keep patches of bush on private property, particularly along creek lines, arguing those cooler pockets are exactly where koalas retreat when heat builds. “They can’t survive without the shady trees that they sit in during the day,” he said.
Separate research at the Australian National University is testing whether eucalypts can be bred to have higher protein and lower toxin levels, an effort aimed at building what its scientists describe as nutritional refugia in a warming climate. ANU landscape and nutritional ecologist Kara Youngentob, who was not involved in the Biology Letters study, said koalas lost their appetite in the heat and held almost no fat reserves to draw on.
Youngentob said the margin was far thinner for koalas than for most mammals, because the animals must eat every night and keep nothing in reserve. “Koalas can only last a few days; they basically have no fat reserves,” she said.
Koalas draw most of their water from the eucalyptus leaves they eat, yet trees pull moisture back from their foliage during dry spells, leaving the animals chewing what Mella likened to dry cardboard. The loss matters because panting is the main cooling mechanism koalas have, and every pant spends water that a drought-stressed animal cannot easily recover.
University of Melbourne ecologist Natalie Briscoe, who had no role in the research, said using care admissions in this way was a clever way to capture an association that is otherwise hard to measure in wild animals. The result, she said, lined up with what is already known about koala physiology and behaviour under sustained heat.
Briscoe said the outlook was bleak, particularly for koalas in the northern and inland parts of the range where heat is building fastest. “Things are likely only going to get worse for koalas under climate change,” she said.
Mella has called for water stations and built shade structures across the hottest regions, the same provision long made for livestock. The koala is listed as endangered in NSW, Queensland and the ACT, and the study sets the point at which deaths begin to climb at 27C — a mark inland ranges now cross for weeks at a stretch each summer.
