Effect of one-hit control on the density of possums (Trichosurus vulpecula) and their impacts on native forest

Introduced possums (Trichosurus vulpecula) have unwanted impacts on New Zealand’s indigenous forest, but how forests respond to possum control is not well understood. Here, we document how possum populations, and the tree species they feed on, responded over 6–8 years following aerial 1080 poisoning. In each of three areas, possum density and tree condition were monitored at 2–7 poisoned sites and 1–3 unpoisoned sites (each 150–250 ha in area). Trap Catch Indices (TCIs) of possum density were much reduced after poisoning, but usually recovered quickly to near pre-control levels within 6 years. The overall mortality of possum-preferred tree species was about 25% lower at poisoned sites than at unpoisoned sites, but this varied between species. Browse levels initially fell sharply at most sites, but with greater declines at the poisoned sites. Browse pressure then increased again as possum numbers increased to previous levels. Canopy condition (as indexed by % Foliar Cover; FCI) increased at both poisoned and unpoisoned sites, but more so at the poisoned sites. Foliar cover of some the most common and widespread species such as kämahi (Weinmannia racemosa), mähoe (Melicytus ramiflorus) and tawa (Beilschmedia tawa) continued to increase (i.e. recover from presumed previous defoliation) even when the possum numbers had substantially recovered. Although the interactions between possums and their food supply are complex, possum control does reduce possum browse, and therefore tree defoliation and, ultimately, tree mortality. Importantly, we infer that reducing possum density by 60% will usually be sufficient to protect most of the possum-preferred tree species we studied.