Long-term changes and seasonal patterns in possum (Trichosurus vulpecula) leaf diet, Orongorongo Valley, Wellington, New Zealand

Possum leaf diet from 1976 to 1989 in the Orongorongo Valley was compared with the diet recorded from four previous years, using faecal analysis. There were large differences in the proportions of species eaten in different seasons and between different years. There was little overall change in the proportion of the main diet tree species, Metrosideros robusta and Weinmannia racemosa, and seasonal increases or declines in one were usually balanced by opposite changes in the other. However, the results of a study of forest composition in the same area showed that living stems of both species declined substantially in abundance, the former mostly before 1978. Melicytus ramiflorus maintained both an important contribution to the diet and its stem density in the forest. The lianes Metrosideros fulgens and Ripogonum scandens both increased their proportion of the diet, but no data were available on their abundance in the study area. At present browse levels, Weinmannia racemosa will become extinct locally, and Metrosideros robusta more rare, especially if drought and higher than average possum densities coincide. Given their large contribution to possum diet, the decline of these species will have a significant effect on possum feeding habits, including increased browsing pressure on Melicytus ramiflorus, Ripogonum scaldens and Metrosideros fulgens. The trend for formerly less palatable tree species to replace those previously important in the diet will continue, with consequent changes in forest species composition.