This chapter discusses the nutritional factors that determine the densities of arboreal folivores - principally the koala, greater glider, common ringtail possum and common brushtail possum - in eucalypt forests. We examine the subject on two scales. The first is a broad approach that relies on the pioneering work of Braithwaite and colleagues, who examined attributes of forests, particularly the nutrient status, in relation to animal abundance. Our second approach is a fine-scale one that relies on studies of captive animals to decipher the role of anti-nutritional factors or plant secondary metabolites in feeding decisions by arboreal folivores. The knowledge accruing from these studies enables us to better explore the feeding ecology of wild animals. If we understand why animals feed on particular trees we can then explore why they use trees that they do not feed from. Similarly, by knowing the secondary metabolites that influence feeding it is then possible to measure their concentrations along environmental gradients to expand our understanding of plant-animal interactions. By reviewing information on the food preferences of the species and combining this with our laboratory feeding studies, we argue that the marsupial folivores select fundamentally different diets. What separates the species most are their preferences to feed from trees within a particular eucalypt subgenus, something we trace to chemical differences.

In reviewing the state of our knowledge, we highlight many questions that remain concerning how the nutritional quality of foliage varies across Australian landscapes. Several technological advances offer great hope for investigating these questions in both the laboratory and in the field. These same techniques will prove invaluable in monitoring changes in the nutritional quality of eucalypt foliage resulting from human activities and as a result of climate change.