EFFECTS OF INTENSIVE TRAPPING ON BREEDING AND AGE STRUCTURE OF BRUSHTAIL POSSUMS, TRICHOSURUS-VULPECULA, ON KAPITI-ISLAND, NEW-ZEALAND

Between 1980 and 1986, introduced Australian brushtail possums (Trichosurus vulpecula) were eradicated from Kapiti Island, a 1965 ha nature reserve. The possum population was monitored by live- and kill-trapping, before and at various times during the eradication programme, to assess the effects of population reduction on the reproduction and age structure of the survivors. About 70-75% of the island's possums were killed in the first three years, but the massive and rapid population reduction had little effect on the percentage of females breeding, or the timing of the main breeding season in autumn, although the percentage of females breeding in spring increased from 0.3 to 5%. There was no evidence that trapping produced local shortages of males and disrupted normal breeding. The age structure of the population changed as numbers were reduced, to contain higher proportions of young (0-3 years old) and old (greater-than-or-equal-to 10 years old) animals. The last 80 animals killed were mostly either recent recruits (26%) or older possums which had survived throughout the trapping programme (56%). The minimal effect of population reduction on reproduction may have reflected the short time available for habitat recovery relative to the period of eradication and to possum longevity. The observed changes in age structure suggested density-dependent changes in survival, particularly of juveniles.