Feeding Habits of Trichosurus Vulpecula

Possums in particular show very pronounced preferences for some plant species, and this intensifies their impact. Kean and Pracy (1949) listed 70 or so species eaten by the possum, and ascribed to them an order of preference shown by the animals. Subsequent work on the food of possums has largely substantiated this list and quantified some aspects of the diet (Mason 1958, Gilmore 1967, Fitzgerald 1976), but the habitats and areas studied by the more intensive method of stomach or faecal analysis have been more restricted than those covered by Kean and Pracy.

From all the work done on the food of possums in New Zealand, the same species occur regularly and reliably in the diet: fuchsia, rata (Metrosideros robusta or M. umbellata), kamahi, fivefinger Pseudopanax arboreus, kohekohe, page 46 makomako, titoki Alectryon excelsus, and toro Myrsine salicina. Where bush borders farmland the choice of food is widened to include grasses and clover.

However, the order of preference by possums is not always consistent. For each species mentioned as highly preferred there may be exceptions where these species are virtually ignored, and others where species normally considered unpalatable are preferred despite the presence of so-called highly preferred plants. The reasons for these exceptions, and for that matter the reasons for preference of any species, are not clear. Information on species eaten in Australia suggests that the leaves of these plants are highly sclerophyllous; this probably indicates that a large amount of food must be eaten in order to extract the nutrients required. Lipid content in eucalypt species is quite high, as it is in Metrosideros spp. - particularly M. fulgens during the months that it occurs most frequently in the diet of possums in the Orongorongo Valley (Fitzgerald 1978). Just how much of this lipid is or can be used by possums is not known.

Other preferred species in New Zealand seem to have amounts of protein and carbohydrates that are comparable with the amounts in those species which are not generally eaten. Plant toxicity (or lack of it) is another feature which may determine the possums' choice of food, but it is well known that possums can and do eat - with apparent immunity - leaves of Coriaria species which are actively toxic to cattle. Fluctuations in the presence and abundance of fruit may also alter the possums' dependence on leaves and change their selection patterns.

Our feeding trials with captive possums have suggested reasons for both preferential selection of some plant species and seasonal selection of others, but we do not yet know the habitats or nutritional plane necessary for possums to breed twice a year. More work which will throw light on the nutritional requirements of possums may help to sort out some of the management problems of possum control.

A comparative study of some aspects of the ecology of the possum in two different-aged stands of Pinus radiata has recently been completed (M. Clout, pers. comm.). One stand was planted in 1960, the other in 1971. Stomach analyses showed that, in the older stand, the possums ate pine foliage mainly in the autumn, when it formed up to 50% of the diet and was supplemented with female strobili. From June to September large quantities of male strobili were eaten and formed about 70% of the diet by August. Bibionid larvae page 47 (Insecta: Diptera) were also eaten, often in considerable numbers, from April to October. During late spring and summer, stomach contents consisted mostly of leaves of lower-storey shrub species with some ferns. The switch from this mixed diet in spring/summer to one comprising predominantly pine was quite abrupt.

In the 1971 stand the diet was more varied, less predictable, and with less seasonal pattern. Reproductive parts of the pines were not available, but pine foliage was still eaten in small amounts and bark was eaten mainly in winter and spring. Grass was an important food throughout the year; though dicotyledons formed a small proportion of the vegetation, they often accounted for about 40% of the stomach contents. Fern was also eaten in this habitat.

Possums in the two ages of pine stands thus had strongly contrasting diets, particularly in winter, largely as a result of the presence or absence of strobili. This difference in diet, with other variables possibly associated with the differences in the age of the two pine stands, produced quite marked differences in body weight and fecundity between the two groups of possums. Other such studies (e.g. Bell this symposium) together with laboratory experiments may eventually allow us to predict with confidence the response of the possum to its habitat and food supply.