The koala (Phascolarctos cinereus) is an iconic Australian marsupial whose survival is linked to the health of its eucalyptus forest habitat. Koalas are specialized tree-dwellers, relying exclusively on the leaves of a few dozen eucalyptus species for both food and moisture. Because of this narrow ecological niche, the koala is sensitive to environmental changes, making it uniquely vulnerable to the impacts of a warming climate. Koala populations across Queensland, New South Wales, and the Australian Capital Territory were officially listed as endangered in 2022. This status reflects how environmental shifts intensify existing threats, accelerating the decline through acute, chronic, and physiological stressors.
Acute Threats from Extreme Weather
Increased frequency and intensity of extreme weather events pose immediate and devastating threats to koala survival. The mega-fires driven by hotter, drier conditions directly cause massive mortality and destroy habitat on a scale that prohibits natural recovery. Koalas are ill-equipped to flee approaching fire because their primary defense mechanism is to climb higher into the canopy, a strategy that becomes fatal when flames and intense heat rise through the tree crowns.
The 2019–2020 “Black Summer” bushfires, for example, affected an estimated 61,000 koalas, killing many and leaving survivors with severe burns and respiratory damage. These intense fires also fragment remaining habitat, isolating small populations and reducing their long-term resilience.
Heatwaves present an equally serious threat. Koalas are sedentary animals with a narrow thermal tolerance, making them highly susceptible to hyperthermia during heat events. When ambient temperatures exceed approximately 37 degrees Celsius (98.6 degrees Fahrenheit), koalas struggle to thermoregulate and maintain a stable core body temperature.
This heat stress quickly leads to dehydration and death, forcing koalas to descend from their trees in search of water. Seeking ground-level water sources exposes them to dangers such as predators and vehicle collisions.
Nutritional Stress and Eucalyptus Quality
The long-term effects of climate change are subtly undermining the koala’s primary food source, the eucalyptus leaf, leading to chronic nutritional stress. One mechanism is nutrient dilution, where rising atmospheric carbon dioxide (CO₂) levels alter the chemical composition of the leaves. As CO₂ increases, eucalyptus trees grow faster and increase carbohydrate content, but this growth comes at the expense of protein and nitrogen content.
Koalas have a low-energy lifestyle adapted to a diet that is already nutritionally poor, requiring them to eat large quantities of leaves to meet their protein needs. When the protein-to-fiber ratio drops due to nutrient dilution, koalas must consume significantly more leaf volume to gain the same amount of nutrition, which they are often unable to do. This results in chronic malnutrition, weakening their immune systems and reducing the energy available for reproduction and movement.
Drought stress further complicates the koala’s diet by affecting toxin concentration. When trees are water-stressed, they increase the production of defensive chemical compounds, known as anti-nutrients, such as tannins and phenolics. These compounds bind to the scarce protein in the leaves, preventing the koala’s digestive system from absorbing it. This dual challenge of reduced protein and increased anti-nutrients forces koalas to expend more energy on detoxification and digestion for a diminishing nutritional return.
Increased Physiological Vulnerability
The cumulative burden of heat stress, drought, and poor nutrition significantly compromises the koala’s overall health, resulting in heightened physiological vulnerability to disease. Chronic water stress and dehydration, driven by hotter conditions and drier leaves, severely impact the koala’s energy budget and mobility. A dehydrated koala has less energy to forage for quality leaves or evade threats, contributing to a cycle of decline that makes recovery difficult.
This state of chronic stress also acts as a profound immune system suppressant, making koalas far more susceptible to endemic diseases, most notably Chlamydia. The bacterium Chlamydia pecorum causes severe health issues, including painful urinary tract infections, blindness, and reproductive tract disease leading to infertility. In some populations, infection rates have climbed to over 85%, rendering the population functionally non-viable due to widespread infertility.
Environmental stressors like malnutrition and habitat loss compound the effects of underlying health issues, such as the Koala Retrovirus (KoRV), which further weakens the immune response. The combination of a compromised immune system from environmental stress and the prevalence of a serious pathogen like Chlamydia creates a lethal synergy. This link between climate change, nutritional decline, and disease susceptibility is a primary driver of the rapid population decline observed across eastern Australia.