Impact of Invading Brushtail Possum Populations on Mixed Beech- Broadleaved Forests, South Westland, New Zealand

The impact of browsing by introduced brushtail possums on mixed beech—broadleaved forests in South Westland was estimated from the amount of conspicuous canopy dieback present in 1989- 1990. Aerial and ground-based reconnaissance in all catchments indicated most canopies (84%) were intact. The remaining 16% of canopies were affected by conspicuous dieback, principally of southern rata (Metrosideros umbellata) and/or fuchsia (Fuchsia excorticata). Major dieback nuclei were located in the three areas with the longest history of browsing by possums, which had spread from three known liberation centres. At each dieback nucleus, the amount of dieback reflected the abundance of possum-preferred canopy species. Because South Westland forests contain lower proportions of such species, they are less susceptible to dieback than the conifer-broadleaved forests of central Westland. However, the present low amounts of dieback in South Westland mainly reflect low overall possum densities and a short period of occupation. The occurrence of key possum- preferred species indicates that about one-third of the forests could develop conspicuous canopy dieback if possum numbers increase and 44-94% are susceptible to canopy and/or understorey depletion. By 1992, the few areas selected for sustained possum control effort in Westland under-represented the range of forest composition. However, recently boosted funding for possum control has provided the opportunity to protect representative forest tracts before the onset of widespread ecosystem depletion in South Westland.