Response of selected tree species to culling of introduced Australian brushtail possums Trichosurus vulpecula at Waipoua Forest, Northland, New Zealand Original Research Article

Population trends of colonising brushtail possums Trichosurus vulpecula were monitored between 1981 and 1993 at Waipoua Forest in Northland, New Zealand. Canopy defoliation and possum-related damage to eight plant indicator species were assessed annually for four years after an aerial poison operation in 1990, to determine whether the operation had reduced possum numbers and halted possum-related damage to the vegetation. Trap catch estimates of possum density increased by over 70% during the 1980s. The poison operation killed 87% of possums, and ongoing leg-hold trapping (1991–1993) maintained population levels at 7–9% trap catch. Reduction of the possum population did not produce a significant positive short-term vegetation response, but did halt the continuing downward trend in vegetation condition evident in nearby forested areas where possum densities were not reduced. Other measures of possum impact (browsed foliage, stem damage) improved significantly within a year of the poison operation, and may prove more sensitive short-term indicators of reduced possum-related damage. The apparent ability of residual possum populations to retard vegetation recovery has important implications for forest health strategies based on intermittent poison operations. While a large initial population reduction is required to halt vegetation decline, an ability to maintain residual possum populations at very low levels appears necessary for significant vegetation recovery. Targetting the increased resources now available for reducing possum populations to fewer areas of higher conservation value may well return greater ecological dividends than attempts to sustain long-term reduction of possum populations over large areas of more modified indigenous forest