Top down or bottom up? Comparing the impacts of introduced arboreal possums and ‘terrestrial’ ruminants on native forests in New Zealand

We review and contrast the impacts on New Zealand's native forests of the two main types of introduced mammalian herbivore; ‘arboreal’ browsers, represented solely by the brushtail possum Trichosurus vulpecula, and ‘terrestrial’ ruminants, represented by deer (Cervidae) and goats Capra hircus. Because of their large size and sophisticated digestive anatomy, the ruminants are able to consume virtually all of the foliage of preferred native plants that is available to them and thereby exert a strong ‘top down’ regulatory effect on forest composition, particularly where they can supplement their diet with fallen leaves from the forest canopy. Possums appear to be less efficient as folivores than the ruminants as a consequence of their smaller size and simpler digestive anatomy. This may sometimes severely limit their top-down influence on plant abundance. However, possums appear able to make much greater use of foliage where they are able to combine an abundant but nutritionally inadequate leaf resource with high-quality foods such as fruit. For both possums and deer, the size and nature of supplemental food sources (i.e. other than growing foliage) appears crucial in determining both herbivore carrying capacity and the severity of their impacts on forest composition. Particularly important is whether or not the availability of the key supplemental foods is directly and negatively affected by herbivore abundance. We explore the implications of this hypothesis for herbivore management.