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 vulpe cula in a relatively low-diversity New Zealand Nothofagus forest. Alth ough 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. Th irty-three food types are eaten by possums, but the four most importan t contribute 68.4% of annual diet. Preference indices show that foliag e consumption is non-random, with some species being highly favored, p articularly short-lived 'seral' tree species. Although possums rely he avily on foliage, the type of food consumed is strongly influenced by seasonal shifts in availability of key food types. Invertebrates and f ruit are seasonally important in the diet and are probably taken oppor tunistically to supplement the staple foliage diet. Possum damage is l ess obvious in Nothofagus forests than in floristically more diverse f orests as palatable species form only a small proportion of the biomas s. However, these species make a major contribution to overall forest diversity and, through selective browsing, possums reduce diversity an d accentuate the strong bias towards unpalatable biomass. These change s have significant implications for forest birds, as there is consider able overlap between possum and bird diet. If possum browsing continue s, it is likely to restrict food supply for some birds. Most concern i n possum control has gone into more diverse mixed-species forests, but even in low-diversity Nothofagus forests with few possums, these anim als can have a disproportionately large impact on forest composition. Possum control in these forests is therefore likely to have considerab le long-term benefits for ecosystem health.