An enquiry into the economic significance of possum damage in an exotic forest near Taupo

Abstract

The significance of possum damage in a small Pinus radiata forest near Taupo was assessed using a formal evaluation process. 1. The initial identification of bark stripping damage and the resulting tree malformation had been made by the forest managers. 2. Identification of the brush-tailed possum as the culprit was largely indirect, although captive possums caused similar damage to small trees planted in their pens. 3. The dues at risk from possum damage were found to be timber production, and perhaps employment. The values at risk from traditional control measures related to the impact of poisons on non-target species. 4. Only leader damage was found to have significant effects on tree growth and form, and then only if the leader was killed or broken down. Malformation resulted from about 55% of the dead leaders and 67% of the broken leaders. The incidence of possum induced malformation was found to be similar to malformation of genetic origin. 5. When the young stands of the forest were surveyed for damage, prevalence and incidence were generally found to be low, although a few stands were heavily damaged. The incidence of damage could not be easily related to stand characteristics, but slash disposal and stand age appeared to be important in determining incidence. 6. Possum abundance was studied with cage trapping techniques. The populations in young stands were found to be in an active colonisation phase, with a large rate of increase, some turnover of residents and large numbers of transients passing through the cage trapping grids. 7. The relationship between possum dispersion and the incidence of damage in a stand was not clear. Some low density areas had a high incidence of damage, and vice versa, but the patterns of damage could be related to slash disposal practices. Possum damage was found to be aggregated, and the degree of aggregation increased with the incidence of damage. Slash disposal practices also influenced the incidence of damage, but the relationship to silvicultural operations was thought to be coincidental. 8. The possum populations in the forest were characterised by density-dependent reproduction, a high dispersal rate, low pouch-young mortality and low turnover. The progeny of single-breeding females were heavier and older at weaning than the progeny of double-breeders. 9. Traditional control techniques involving the use of poisons were considered to be of limited effectiveness because of the high rate of dispersal and density-dependent reproduction. The costs of these operations were found to exceed the losses due to damage when they were compared at rotation. 10. A more effective means of dealing with possum populations was considered to be the use of alternative slash disposal techniques with the removal of malformed trees by routine silvicultural operations. The least environmentally disruptive population control method was harvesting the animals for their pelts. It was felt that the possum populations could be effectively managed as a fur resource.