THE DIET OF INTRODUCED BRUSHTAIL POSSUMS TRICHOSURUS-VULPECULA IN A LOW-DIVERSITY NEW-ZEALAND NOTHOFAGUS FOREST AND POSSIBLE IMPLICATIONS FOR CONSERVATION MANAGEMENT

We evaluate the diet of introduced brushtail possums Trichosurus vulpecula in a relatively low-diversity New Zealand Nothofagus forest. Although possums only invaded this area 30-40 years ago, densities (0.5-1.0 ha-1) are similar to those recorded for other Nothofagus forests. Thirty-three food types are eaten by possums, but the four most important contribute 68.4% of annual diet. Preference indices show that foliage consumption is non-random, with some species being highly favored, particularly short-lived 'seral' tree species. Although possums rely heavily on foliage, the type of food consumed is strongly influenced by seasonal shifts in availability of key food types. Invertebrates and fruit are seasonally important in the diet and are probably taken opportunistically to supplement the staple foliage diet. Possum damage is less obvious in Nothofagus forests than in floristically more diverse forests as palatable species form only a small proportion of the biomass. However, these species make a major contribution to overall forest diversity and, through selective browsing, possums reduce diversity and accentuate the strong bias towards unpalatable biomass. These changes have significant implications for forest birds, as there is considerable overlap between possum and bird diet. If possum browsing continues, it is likely to restrict food supply for some birds. Most concern in possum control has gone into more diverse mixed-species forests, but even in low-diversity Nothofagus forests with few possums, these animals can have a disproportionately large impact on forest composition. Possum control in these forests is therefore likely to have considerable long-term benefits for ecosystem health.