Introduction and establishment
Possums (Trichosurus vulpecula) were first brought from Australia in 1837, to Riverton in Southland, to form the basis of a fur industry. This initial attempt failed, and it was not until 1858 that they were successfully introduced at the same place. They are now found in about 95% of all farmland, scrubland and bush.
Most possums – estimated at over 50 million – live in farmland or scrubland. With at least five possums per hectare, another 16 million are reckoned to live in the bush. Their population density in New Zealand is much greater than in most parts of Australia.
Possums eat and damage the forest at every point, taking mainly leaves, but also buds, flowers, fruit and seeds from the tallest tree tops. On the ground they eat seedlings, saplings and sometimes bark. Possums also eat parts of perching plants, scramblers, climbers, some tree ferns and fungi. Favoured foods include mistletoes, tree fuchsias, kāmahi and rātā trees, which is why these are among the first species to be eaten out of the forest.
Damage to plants and animals
When possums eat a plant’s flowers, the nectar and fruit are reduced. The few berries that grow are also eaten. In one study, an entire crop of kaikōmako berries was eaten in a few nights. Another study revealed that possums ate some 60% of hīnau berries. They rob native birds, bats, lizards and insects of food.
With a repertoire of at least 20 different sounds, a possum’s calls can be startling:
‘There are screeches, grunts, growls, hisses, and chatters (mostly used in aggressive interactions); zook-zooks and squeaks (dependant juveniles and pouch young); and shook-shooks and clicks (males during courtship). The shook-shook sound of the male, resembling the call of juveniles, may serve to reduce female aggression.’ 1
By feeding selectively, possums have radically altered the makeup of forests. In the 1960s, possums opened up large tracts of forest in the Ruahine Range to invading scrub. They have eaten nearly all of the rātā trees from the Aorangi Range, and have almost wiped out mistletoes in the North Island. Large stretches of Westland forest are dotted with possum-killed rātā and kāmahi. When possums are eradicated, as they have been on Kāpiti Island, the bush regenerates.
Possums also eat insects, bats, birds and their eggs and nestlings. They drive native animals out of their dens and nesting sites.
Traps, poisons and guns can control possum numbers. However, these are expensive methods, and only practical in easily accessible bush or in bush that is valuable to conservation or farming. This leaves possums uncontrolled over large areas of hinterland. Scientists are searching for new methods of control – the most promising are ways of making possums less fertile.