Trichosurus vulpecula have multiple impacts, as a browser of forest vegetation, frugivore, competitor for tree hollows, predator of invertebrates and bird nests, and disease vector. Long term changes in forest structure and composition (including canopy collapse in extreme cases) can result from sustained possum browsing pressure. Some highly palatable and chemically "unprotected" plant species are so preferred by brushtail possums that their selective browsing can result in local plant extinctions. Effects on native wildlife include depletion of fruit crops, competition for tree hollows, and predation by possums on invertebrates and the eggs and nestlings of birds (including threatened species). Possums are vectors of bovine tuberculosis, and consequently pose a significant threat to cattle, deer and dairy industries.
Location Specific Impacts:
Competition: Brushtail possums (Trichosurus vulpecula) and Rattus rattus also compete with Hemiphaga novaeseelandiae for fruit, reducing the number of breeding attempts, and possibly causing the starvation of adults (BirdLife International 2004).
Competition: The Kea (Nestor notabilis) is listed as 'Vulnerable (VU) in the IUCN Red List of Threatened Species. Introduced mammals may be competing for depleting winter food resources, they include brush-tailed possums (Trichosurus vulpecula), Himalayan thar (Hemitragus jemlahicus), hare (Lepus europaeus), Chamois (Rupicapra rupicapra) and Red deer (Cervus elaphus) (BirdLife International 2008).
Disease transmission: In 1967, it was realised that brushtail possums (Trichosurus vulpecula) were carriers of bovine tuberculosis (Tb), it was later found to be a significant vector in the transmission of Tb to cattle. This has the potential to severely threaten New Zealand's meat industry and subsequently, affect the country's economy (Atkinson, 1995).
Habitat alteration: Brushtail possums (Trichosurus vulpecula) prefer newly grown leaves over old leaves. In certain regions of New Zealand, possums are known to have consumed entire canopies of rata (Metrosideros umbellata), totara (Podocarpus totara), titoki (Alectryon excelsus), kowhai (Sophora microphylla) and kohekohe (Dysoxylum spectabile). A survey conducted in 1990 in South Westland’s mixed beech-broadleaved forests between the Mahitahi and Arawata Rivers concluded that, 16% of trees affected by conspicuous canopy dieback were caused by possum-browsing. This figure is estimated to climb as high as 44% if the population of possums are not kept under control.
Predation: The Kea (Nestor notabilis) is listed as 'Vulnerable (VU) in the IUCN Red List of Threatened Species. The Kea used to be hunted and killed in large numbers until its protected status. Farmers still kill birds. The main threat to this species is predation by introduced mammals- stoats (Mustela erminea), brush-tailed possums (Trichosurus vulpecula) and cats (Felis catus) (BirdLife International 2008).
Reduction in native biodiversity: Brushtail possums (Trichosurus vulpecula) feed on the berries and leaves of the tree fuchsia (Fuchsia excorticata). The flowers and berries of fuchsia are also considered as an important food source for tui, bellbirds and kereru. Furthermore, they indirectly support the growth of canopy trees by stabilising the soil around it. Two surveys conducted in 1975 and 1985 showed that the population of fuchsia trees in the Tararua ranges declined by at least 25% due to possums.
Threat to endangered species: Brushtail possums (Trichosurus vulpecula) directly affect a number of rare and endangered vertebrates. For example, the 'near threatened' New Zealand pigeon (see Hemiphaga novaeseelandiae in IUCN Red List of Threatened Species) is a forest pigeon endemic to New Zealand. It is in rapid decline in Northland - a 1993 survey indicated a 50% decline within 14 years. Studies indicate that declines are occurring elsewhere. Introduced predators are the primary cause of decline nationwide, in particular, brush-tailed possum, ship rat (Rattus rattus), stoat (Mustela erminea) and cats (Felis catus) (BirdLife International 2004). In addition, several studies have reported possums invading the nests of nationally endangered kokako (see Callaeas cinerea in New Zealand Threat Classification System lists) and nationally critical Okarito brown kiwis (see Apteryx rowi in New Zealand Threat Classification System lists (Atkinson, 1995).