The Australian brushtail possum (Trichosurus vulpecula), a cat-sized marsupial, was introduced to New Zealand in 1837 for the fur trade. In Australia, the possum is protected as a native species. But in New Zealand, it has become the country’s most damaging animal pest, wreaking havoc on native forests.
Possums are found virtually everywhere on mainland New Zealand and Stewart Island, although they have been eradicated from major offshore islands since the early 1990s.
Possum numbers are difficult to accurately assess, but in the early 2000s, estimates ranged from 50 to 70 million.
Not only do possums destroy forests, they also infect cattle with bovine tuberculosis, threatening the country’s valuable dairy industry.
Their cost to the economy is considerable: in 2006 government agencies spent $111 million on possum control, a level of funding that will continue for another decade. In addition, possums eat pasture and cause a fall in farm production (estimated at $35 million annually). They also eat planted pine seedlings and horticultural produce.
From 1934 Australian scientists decided on the name of ‘possum’ for members of the Phalangeridae family, including the brushtail possum, to differentiate them from the American opossum, a marsupial from a completely different family.
The brushtail possum is a furry animal weighing between 2 and 5 kilograms, and has a thick, bushy tail, pointed face, protruding eyes and large ears. It is a nocturnal tree-dweller with long claws to help it grip branches. In New Zealand its coat is either black or grey – most stock came from Tasmania, where black possums predominate. Their soft, luxurious fur was preferred by furriers, and the copper-coloured variety that lives in Queensland was never introduced.
Two-thirds of possums are thought to live in the North Island, which has more of their preferred habitat (scrub gullies bordering farmland) and a wider variety of forest types. The South Island’s beech forests and alpine grasslands do not support dense possum populations.
The average density in the North Island is equivalent to four possums per hectare, but can reach 25 animals per hectare on pasture bordering areas of native forest and shrubland.
Black possums predominate in wetter, forested areas, including the Kaimai Range, Te Urewera, Westland, Fiordland and western Southland, and on Stewart Island and Chatham Island. Grey possums are more common in drier open country.
Largely solitary, possums do not form strong pair bonds. Males in captivity have been observed mating with more than one female. Females usually give birth to one young a year, although they sometimes produce two. The pregnancy lasts for just 17–18 days, when a tiny (0.2 gram), furless, blind baby is born. The baby possum immediately crawls into its mother’s pouch and attaches itself to a nipple for about 70 days.
Young possums are weaned at 5–8 months, but they remain close to their mother until they are almost a year old.
Home ranges and dispersal
Females have a home range of about 1.3 hectares, and males of 1.9 hectares. On average a juvenile possum moves 5 kilometres from the maternal den – females further than males. In the King Country two females were recorded travelling 32 and 41 kilometres from where they were born.