The Ecology of the possum (Trichosurus Vulpecula Kerr) in Pinus radiata plantations

Abstract

Damage caused by possums (Trichosurus vulpecula Kerr) in New Zealand's extensive Pinus radiata plantations has caused concern among foresters. This study set out to examine the nature of tire problem and investigate the ecology of possums in radiata plantations in relation to damage and control. The work was carried out in the Kinleith Forest near Tokoroa in the central North Island. A review of existing literature and survey results showed that damage is mainly a problem of young plantations, consisting of bark-stripping on young trees and browsing of newly-planted seedlings. A simulated browsing experiment indicated that the latter damage was unlikely to result in discernible production losses. Bark-stripping was found to occur during the winter and spring. It had a patchy distribution and was only thought likely to prove serious where stocking was already low for other reasons. Analysis of stomach contents from animals poisoned in 1971 plantings and 1960 age class stands revealed that in both habitats possums relied heavily on foods other than pine, although in the older (1960) stands radiata pollen cones were an important winter and spring food. Insect larvae (Philia nigrostigma) were also eaten at this time of year, particularly in older stands. Little bark was consumed in either habitat, but it was a more common food in the younger stands, were it was eaten during the winter and spring. It is suggested that bark-stripping in young stands may be a response to relatively poor nutrition during the winter and early spring. Data on fat reserves, reproductive performance and growth support the view that the overall plane of nutrition was better among possums in the older stands. Possum populations were studied by livetrapping and radiotelemetry in two areas, one within a 1960 age class radiata stand (FRSA), the other extending across the boundary between a 1971 planting and cutover native forest (MSA). In the latter area there was a seasonal pattern of habitat use, with animals tending to use the native forest to a greater extent than the plantation during the winter. Population densities were similar in the two areas (2-3/ ha), with the MSA population having an apparent trend of increase. The two populations differed in composition, the FRSA one having an excess of females and the MSA an initial excess of males but an increasing proportion of females. It is suggested that disturbance of the MSA population, by burning in 1971 and subsequent poisoning, resulted in partial depopulation with initial recolonisation predominantly by dispersing males. Results from both study areas showed that young males have a greater tendency to disperse from their native area than do young females. Radiotelemetry revealed that possums in the MSA used more dens than those in the FRSA and good dens appeared to be few in the latter area. It is suggested that dens may be a limiting resource for some possum populations. Since females are known to control the use of dens the survival of males should be poorer where dens are limited, resulting ultimately in a biased sex ratio. Experimental depopulation of the FRSA and monitoring of its recolonisation over one year suggested that population re-establishment on cleared areas does not depend on local reproduction by survivors, but is primarily by dispersal in the initial stages, with an early influx of young males. The MSA population was also poisoned, in a standard 1080 airdrop, and the effectiveness of this operation was monitored. Current control practices are critically appraised in the light of the results obtained in this study and the value of widespread aerial poisoning operations in pine plantation habitats is questioned.