Climate change is predicted to cause an increase in drought frequency and fires in many parts of Australia as a result of reduced rainfall levels, increased evaporation rates, and an overall temperature increase of about 1ºC by 2030, according to a CSIRO report (IUCN 2009). Increasing frequency and intensity of droughts or periods of extreme heat would force Koalas to descend from trees more frequently in search of water or new habitats. This would make them more vulnerable to wild and domestic predators, as well as to road traffic. Dispersing Koalas often have to cross main roads and come into contact with domestic animals. It is estimated that around 4,000 Koalas are killed each year by dogs and cars alone (IUCN 2009).
Koalas’ warm fur and thick skin enables them to endure cold conditions in southern Australia, but they do not cope well with extreme heat. Unlike most arboreal marsupials, Koalas do not use nest hollows, which contributes to their greater susceptibility to extreme temperatures and drought. Bushfires, which have already wiped out numerous populations of Koalas, are likely to increase in both frequency and severity with climate change. Koalas are particularly vulnerable to bushfires as their slow movement and tree-dwelling lifestyle makes it difficult for them to escape and their food supply can be destroyed (IUCN 2009).
Climate change is likely to have some less obvious negative impacts on Koalas as well. Atmospheric carbon dioxide concentrations globally have increased from 280 ppm to 387 ppm since the Industrial Revolution. Projections for 2050 suggest that carbon dioxide concentrations are likely to increase markedly to between 500 and 600 ppm, depending on future emissions scenarios (IUCN 2009). Increased carbon dioxide levels tend to result in faster plant growth, but also to reduce protein levels and increase tannin levels in plants’ leaves, making them less nutritious and more difficult to digest (Lawler et al. 1997). As carbon dioxide levels continue to rise, Koalas and other browsers will need to cope with increasingly nutrient-poor and tannin-rich leaves.
Given the altered chemical composition of their food plants, Koalas could meet their nutritional needs by spending more time feeding and thus eating more. However, there is a limit to how much Koalas can increase the size of their guts. Furthermore, eating more leaves causes food to pass more quickly through the Koala’s digestive system, resulting in less thorough digestion and decreased nutrient uptake. Another possibility would be to exercise greater selectivity in tree and leaf choice (see Moore and Foley 2000, 2005; DeGabriel et al. 2009). Although Koalas could be more selective in their food selection, however, this would require more travel time to find the best trees and Koalas travelling in search of food are at an increased risk of predation and road accidents. Furthermore, the nutritional demands of breeding female koalas are higher than those of non-breeding individuals, raising the possibility that even if nonbreeding individuals were able to sustain themselves when faced with less nutritious food options than they have today, failure of breeding females to meet their nutrition needs could nevertheless lead to widespread reproductive failure and population declines (IUCN 2009).
Reports of large population declines in the first years of this century have prompted reassessments of the Koala’s threat status by the Australian government (IUCN 2009). In addition to factors related to climate change, other major factors contributing to Koala declines include disease and habitat destruction. The primary disease threat is from chlamydia, a widespread sexually-transmitted disease that causes blindness, pneumonia, and urinary and reproductive tract infections and death in Koalas. Habitat loss is also a major problem. Destruction and degradation of Koala habitat is particularly prevalent in the coastal regions of Australia, where urban development is rapidly encroaching on eucalyptus forests. In addition, habitat fragmentation limits Koalas’ ability to disperse to suitable areas and can intensify inbreeding problems (IUCN 2009).