The koala is a specialized Australian marsupial with a diet consisting entirely of eucalyptus leaves, making it uniquely vulnerable to environmental change. Koalas rely entirely on their forest habitat for sustenance and water. Climate change is introducing a cascade of threats, pushing this species to its physiological and ecological limits. The inability of koalas to quickly adapt to rapidly changing conditions, such as rising temperatures and habitat degradation, poses an existential threat to their survival.
Direct Impacts of Heat and Drought
Rising temperatures and prolonged drought directly threaten koala thermoregulation. Koalas have a low-energy lifestyle, and their thick fur slows heat dissipation, making them susceptible to hyperthermia above 37.7 degrees Celsius. To cope, koalas practice behavioral thermoregulation by hugging the lower, cooler trunks of certain trees. This conductive heat loss reduces the need for the evaporative cooling, such as panting, which prevents dangerous water loss.
As heatwaves become more frequent, this passive cooling is often insufficient to prevent dehydration and mortality. Koalas historically derive almost all moisture from leaves, but they are now forced to descend to the ground to seek water sources. This behavioral shift exposes them to new risks, including predation by dogs and vehicle collisions. Extreme events have caused high mortality rates, such as the estimated 25% loss in a Gunnedah population during a 2009 heatwave.
Alteration of Eucalyptus Habitat and Nutrition
Climate-driven changes undermine the koala’s sole food source, the eucalyptus leaf. Prolonged drought drastically reduces foliar moisture content, forcing koalas to reject leaves below a necessary threshold. This lack of hydration from their diet exacerbates the dehydration and heat stress koalas already face.
Atmospheric changes also alter the leaf’s chemical composition, making it less nutritious and more difficult to digest. Increased carbon dioxide (CO2) levels favor the production of carbon-based defensive compounds, such as tannins. These tannins bind to the protein in the leaves, making them indigestible and reducing the available nutritional content. Koalas must consume greater quantities of these lower-quality leaves, which increases their intake of the toxins and leads to chronic nutritional stress, malnutrition, and kidney failure.
Increased Frequency and Intensity of Bushfires
Climate change contributes to longer, hotter fire seasons, resulting in intense bushfires. Koalas are vulnerable due to their slow, arboreal nature, which prevents effective escape from fast-moving crown fires. The 2019–2020 “Black Summer” bushfires resulted in catastrophic losses, with estimates suggesting over 61,000 koalas were killed, injured, or affected. Mortality rates in severely impacted areas, such as Kangaroo Island, reached as high as 80% of the local population.
Mega-fires destroy extensive habitat, leading to severe fragmentation of koala populations. By 2070, nearly 45% of koala habitat is projected to have a high susceptibility to bushfire. This destruction isolates remaining groups, limiting genetic exchange and making recolonization of burnt areas significantly difficult. Survivors are often malnourished and face increased secondary threats, such as being struck by cars or attacked by dogs while searching for food in the burnt landscape.
Climate-Related Stress and Disease Vulnerability
Heat, drought, and habitat loss create chronic stress. Environmental stress elevates levels of glucocorticoid hormones, such as cortisol, which are measurable in their feces. These stress hormones suppress the koala’s immune system, making them highly susceptible to endemic diseases.
The most devastating consequence of immune suppression is the activation of latent Chlamydia pecorum infection. While many koalas carry the bacterium without symptoms, chronic stress facilitates the disease’s progression, especially alongside a high viral load of Koala Retrovirus (KoRV). Active chlamydiosis causes severe clinical signs, including painful urinary tract infections (visible as “wet bottom”), severe conjunctivitis leading to blindness, and reproductive tract damage. This damage causes infertility in female koalas, severely curtailing the species’ ability to recover after climate-driven disasters.