Koalas, leaf chemistry and higher CO2
The koala eats almost nothing but eucalypt leaves, and the leaves are a poor meal. They are low in protein, high in fibre, and laced with tannins and oils that would make most mammals ill. The koala manages on this diet by being selective, by fermenting the leaves in an enormous caecum, and by doing very little. It sleeps most of the day because it cannot afford to do otherwise.
A diet with so little margin is sensitive to small changes in the food, and the food is changing.
What rising CO2 does to a leaf
Plants grown under elevated carbon dioxide tend to produce leaves with more carbohydrate and less nitrogen. Nitrogen is what an animal needs for protein, so the leaf becomes bulkier and less nutritious at the same time. Experiments growing eucalypt seedlings under CO2 levels expected later this century have found lower leaf nitrogen and, in some species, higher concentrations of the tannins that bind protein and make it harder to digest. The direction of the change is consistent even where the size of it varies.
For a koala, this means eating more to get the same nourishment, from a gut that is already working at capacity. Field studies have linked poor leaf nutrition to lower body condition and lower breeding success in some populations.
Heat and water
Koalas get most of their water from leaves and normally do not need to drink. During heatwaves and droughts, when leaves dry out and the trees shed them, that is no longer enough. Wildlife carers in inland New South Wales and Queensland have reported koalas coming down to the ground to drink from troughs and even approaching people with water bottles, which is behaviour that was once considered remarkable. On the ground they are exposed to dogs and cars. Trees that lose their canopy in a heatwave leave the animals with neither food nor shade.
Habitat
None of this would matter as much if koalas could move to where conditions are better. Clearing for agriculture and housing has fragmented their range, particularly along the eastern coast, and the 2019–2020 fires burned a large fraction of the remaining habitat in some regions. In 2022 the species was listed as endangered in Queensland, New South Wales and the Australian Capital Territory.
The koala is a specialist on a specialist tree, and the tree is under stress. There is no single fix. Protecting and reconnecting habitat gives the animals room to cope with the rest.
Further reading
- Western Sydney University, Hawkesbury Institute, eucalypt leaf chemistry under elevated CO2
- Australian Koala Foundation, habitat mapping
- IUCN Red List, koala assessment