Ecology of the Possum (Trichosurus Vulpecula Kerr) in the Karori Water Supply Reserve, Wellington, Sounds Ecological District

Abstract

The abundance and condition of possums in various areas of the Karori Reserve, Wellington - Sounds Ecological District was assessed. Possums were most abundant in the upper part of the reserve in an area dominated by bishop and radiata pines with an undergrowth of native species and in an area dominated by gorse and flax shrublands. In the lower part of the reserve, possums were less abundant and there a number of characteristics were observed: possums had a higher body weight for age, their reproductive output was higher and the population was marked by a higher proportion of females and juveniles. This was interpreted as a "compensatory response" characteristic of possum populations maintained at low densities. Possum stomachs were sampled in spring and their contents analysed in order to investigate the importance of Fuchsia excorticata and other plant species to possum diet. In the lower part of the reserve the leaves of Fuchsia excorticata and the flowers of Cytisus scoparius made up more than 50 percent of the possums' forage. In the upper part of the reserve exotic species such as the flowers of Cytisus scoparius and Ulex europeans as well as the leaves of native species such as Aristotelia serrata, Pseudopanax arboreus and Coprosma robusta made up the bulk of possum forage. Exotic plants accounted for a significant proportion of the spring possum diet in the reserve. Whereas most leaves originated from native plants, the exotic species contributed mainly carbohydrate-rich flowers and pollen. The canopy condition of two possum-susceptible tree species Fuchsia excorticata and Pseudopanax arboreus was assessed early in autumn over the years 1993, 1995 and 1996. Over these years, 47 percent of marked fuchsia trees were completely defoliated and most trees are unlikely to recover even if possums are eradicated from the reserve. All defoliated fuchsia trees were located in the upper part of the reserve. On the other hand, 37 percent of marked fuchsia trees exhibited only a little possum damage. Most of these trees were located in the lower part of the reserve where possum density was lower. Increasing possum densities could therefore be correlated with an increasing level of possum damage to fuchsia canopies. Even though Pseudopanax arboreus leaves and petioles were encountered in 10 percent of possum stomachs, the canopies leaves and petioles were encountered in 10 percent of possum stomachs, the canopies of assessed trees did not seem to be greatly affected. It appeared that due to the high abundance of this plant species in the reserve the effects of possum browse were spread over the whole reserve to levels which could be tolerated by an individual Pseudopanax arboreus tree.