Rising temperatures putting koala populations at risk, scientists say

Rising temperatures are exacerbating threats to Australia's iconic koalas, an already vulnerable species, scientists warn.

Koalas are increasingly facing heat stress and even death when temperatures exceed a certain threshold, according to a paper published in Biology Letters on Tuesday.

An analysis of nearly 12,000 koala rescue records across New South Wales indicates koalas' risk of admission into care and death increased sharply when the average maximum temperature was 27 degrees Celsius -- or 80.6 degrees Fahrenheit -- or above.

Inland north-west koala populations faced the greatest risks, the researchers found.

Before this study, there had been many observational reports of koalas dying during heatwaves, particularly during extreme summer temperatures in Australia, Valentina Mella, a senior lecturer in animal behavior and conservation at the Sydney School of Veterinary Science, told ABC News. However, there was no empirical evidence directly demonstrating a clear link between temperature and koala mortality.

"We wanted to better understand how increasing temperatures and more frequent heatwaves may affect koala survival, especially as climate change intensifies," said Mella, lead author of the study.

The researchers explored the association between ambient temperature and koala risk of admission into care and subsequent death using a methodology "extensively" utilized in environmental epidemiology in humans when assessing associations between short-term exposures to heat and acute heat health effects.

They found that both the number of admissions into care and deaths started to increase when the seven-day average maximum temperature was 80.6 degrees Fahrenheit.

Admissions and deaths also continued to increase as the temperatures increased, Mella said.

Once the seven-day average maximum temperature reached 30 degrees Celsius -- or 86 degrees Fahrenheit -- the odds of koalas being admitted and dying after exposure were up to 3.5 times greater compared to an exposure of 25 degrees Celsius -- or 77 degrees Fahrenheit.

The trend was further compounded in koalas with chlamydiosis -- a common bacterial disease among the species -- and those found in unsuitable conditions, Mella said.

Koalas are classified as "vulnerable" on the International Union for Conservation of Nature's Red List of Threatened Species due to an estimated population decline of 28% over the last three generations.

In addition to drought, climate change and other temperature extremes, primary threats include residential and commercial development, logging, disease and fire, according to the IUCN.

A large population of koalas died in the massive bushfires that broke out in Australia at the end of 2019 and lasted through 2020. Nearly 47 million acres burned across southeast Australia and the eastern coasts, killing nearly 6,400 of the marsupial -- about 15% of the total population

Koala populations suffered enormous losses through direct mortality, injury, dehydration and long-term habitat destruction, Mella said.

"The intensity and scale of these fires were strongly linked to the extreme heat, prolonged drought, and dry conditions associated with climate change," Mella said.

But the latest research shows that even moderate heat can threaten the survival of the marsupial, the researchers said.

Combined with the increasing possibility of extreme weather events, these pressures create "compounding risks" for koalas, a species already vulnerable to habitat loss and fragmentation, Mella said.

The findings highlight that actions to improve heat-response strategies are necessary to protect vulnerable wildlife, the researchers said.