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.
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.
The first possum liberations in 1837 were unsuccessful, but that did not dissuade those who believed in a fur industry. The first possum population was established in 1858 in Southland. From then until 1922, 36 batches of possums were imported. By 1930, these animals and their descendants had been let loose in 450 locations around the country.
At first private individuals released possums, but from 1870 until 1921 acclimatisation societies took over the role. By 1921 the government prohibited further liberations, although trappers continued to release them illegally. Hunting and selling skins was regulated until 1946 when, after much debate, it was decided that possums were an environmental pest, and all protection was removed.
The first large-scale attempt to control possums was a bounty scheme which ran from 1951 to 1961. Eight million bounties of two shillings and sixpence were paid out for ‘possum tokens’ – the ears and a strip of fur. However, more than 75% of these animals were taken from near farms, picked off roads or caught in other easily accessible places. In the forests, possum numbers continued to grow.
In the early 2000s, there were possums in almost all of mainland New Zealand. It is estimated that they occupied about 54% of the country in 1950, 84% by 1963 and 91% by 1980. The last areas colonised were Fiordland and Northland. In the 1960s there were virtually no possums in Northland, but by the mid-1990s there were 10–15 million.
Possums have been eradicated from a number of islands, notably Rangitoto, Motutapu, Kapiti, Codfish, Whanganui and Tarakaipa, all of which have outstanding conservation importance.
Between 1980 and 1986, 19,612 possums were killed on Kapiti Island. Bird counts between 1982 and 1988 showed that the density of birds doubled, even though rats were still present. Obviously, possums had played a large part in keeping bird populations low.
Early botanist H. B. Kirk did not see the possum as a pest. He wrote in a 1920 government report that he believed the animals could ‘with advantage be liberated in all forest districts except where the forest is fringed by orchards or has plantations of imported trees in the neighbourhood’. 1
Once possum populations started to soar, trappers began to make a living from hunting them. Throughout the 1970s prices were good. They peaked in 1981, when 3.2 million skins were exported. In more accessible areas, hunting reduced possum numbers.
The possum fur business is only 1% of the worldwide fur market by value. Traditionally possum fur has been valued for trimming clothes, but since the 1990s prices have been low, forcing hunters out of the industry.
Since 1993 the possum-product industry has diversified to use pelts, leather, fibre and meat (sold to Asian countries). Fibre is used by itself to make hats and gloves, or spun with merino wool and made into a soft and luxurious fabric.
Possums are voracious eaters – in New Zealand they consume an estimated 21,000 tonnes of vegetation a night. Most noticeably, they destroy spectacular flowering trees such as pōhutukawa and rātā. They also change the overall structure and composition of native forests.
Although the amount they consume seems astonishing, it is just a small percentage of the amount of foliage produced each day in native forests – around 300,000 tonnes.
Favoured food species are tall canopy species such as tawa, northern and southern rātā, kohekohe, kāmahi and Hall’s tōtara.
Possums can cause catastrophic dieback, the complete collapse of a forest canopy – especially tree species that possums prefer, such as rātā and kāmahi. In the southern rātā–kāmahi forests of Westland, many valleys lost more than 50% of canopy trees within 15–20 years of possums arriving. The forest trees are then replaced by shrubs that are unpalatable to possums, and the area changes from tall forest to low open forest and shrublands.
New Zealand’s eight surviving species of native mistletoe are threatened by possums. These shrubs live on larger trees, and have fleshy leaves and juicy stems which are readily eaten. Possum control can lead to dramatic increases in mistletoe growth and flowering.
In more diverse (conifer–broadleaf) forests, possums selectively remove their preferred species over a number of decades, changing the composition of the forest. Possums often select one tree and systematically strip it. They feed on new shoots, making it harder for trees to recover from weather and insect damage, and slowing their growth.
By eating flowers, possums stop seeds forming. Many plants fail to regenerate under possum assault.
Once thought of as leaf eaters only, possums are now recognised as opportunistic consumers of any high-energy, high-protein foods. These include flowers, leaf buds, fruit, eggs, birds, insects and snails.
In 1993 evidence was first uncovered of the effect of possums on native wildlife. A time-lapse video camera captured possums eating the chicks and eggs of the endangered kōkako. Of 19 nests monitored over a four-year period, cameras recorded possums eating chicks and eggs in four. Possums had not been suspected in the past because they were considered vegetarian, and because it is hard to find traces of birds in possum faeces or intestines.
Since then, possums have been caught on film eating the eggs, chicks and even adults of other native bird species, including kererū, kiwi, harrier hawk, fantail, muttonbird, and tūī. This predatory behaviour has driven some species into decline.
In areas where possums are not controlled, few kōkako and kererū fledglings survive to adulthood. In a study in Pureora Forest in 1997, robins in an area treated with 1080 poison (used to kill possums) had 67% nesting success, compared with 30% in the area not treated. After a year, robin numbers had increased by 37% in the control area, compared with 16% outside.
By eating fruits and flowers, possums deprive nectar-feeding birds like tūī, kākā and bellbirds of the high-energy food they need in key periods such as the breeding season. Possums also take over dens that kiwi use – sometimes their mere presence disturbs native birds in their activities.
Both of New Zealand’s endangered native bat species have been recorded as attacked by possums, including in South Canterbury where the bat population is declining by 5–9% a year. In the central North Island possums eat flowers of the threatened wood rose, a nectar source for short-tailed bats.
A study in the Orongorongo River valley near Wellington has shown that native invertebrates form an important part of the possum diet. Over half of the possum faecal pellets contained invertebrates, the most likely targets being large, slow-moving species such as giant wētā, stag beetles and weevils.
Possums also eat endangered native snails. One possum can eat 60 giant Powelliphanta snails in a single night.
Bovine tuberculosis (TB), a contagious disease caused by the bacterium Mycobacterium bovis, is a serious threat to the cattle and deer farming industries. It poses an economic risk to New Zealand, as dairying is one of the country’s most valuable industries, with exports worth $6.5 billion in 2007.
Bovine TB arrived in New Zealand with the cattle of the first European settlers. Despite attempts to eradicate it, small pockets have always remained. In 2007 around 0.4% of national cattle herds and 1.4% of deer herds were infected.
The mystery of why some herds continued to be infected was solved in the 1960s with the discovery that possums carried TB. Cattle were sniffing or licking dead or dying possums that came out onto pasture, and so contracting the disease. Ferrets also spread TB.
Although on average only about 2% of possums actually have TB, there are pockets where the numbers infected may be up to 40%. Infected possums were present in about 38% of the country (10 million hectares) by 2003. Most die six months after contracting the disease. They become lethargic and stay out in the open during the day, where curious cattle lick the dead or dying animals and become infected. In their native Australia possums do not have TB, possibly because they have less chance of contact with cattle.
The Animal Health Board is responsible for managing the possum TB problem. In 2006 it spent $60 million on controlling TB, mostly by poisoning possums, and an additional $27 million on research, herd testing and compensation.
There are two phases in bovine TB control: first, possums are heavily reduced over a large area, usually by aerial poisoning. Then, in the maintenance phase, possum numbers are kept low. If this is not done, TB returns to its previous level within five to eight years. The disease has been eradicated so far in only a few small areas.
Possum control responsibility is divided between the Animal Health Board, the Department of Conservation, regional councils and private individuals or companies.
Possums are killed by either poison or traps. Researchers are also working on biological control solutions.
A biological-control vaccine to reduce possum breeding is being developed. Trials have shown that possums treated with the vaccine have reduced breeding by 70–80%. If permission is given for the vaccine to be used in the wild, it will be made into baits and fed to possums from bait stations.
Six main poisons are used against possums: 1080 (sodium monofluoroacetate), phosphorus, cholecalciferol, pindone, cyanide and brodifacoum. Each is used in different situations.
The active ingredient in 1080, fluoroacetate, occurs naturally in about 40 plants in Brazil, south and west Africa, and western Australia, which can be dangerous if eaten. There are low concentrations of fluoroacetate in tea, but not enough to be harmful.
The most common and most controversial poison for possum control is 1080, which has been used in New Zealand since the 1950s. It is especially lethal to mammals, although it also kills birds, amphibians and insects. In the US it is used to protect sheep and goats from coyotes, and in Australia to kill pests such as rabbits, foxes, wild dogs and pigs. In 2002–3 New Zealand used 80% of the world’s total 1080 production: 2.3 tonnes of powder.
1080 is usually added to cereal, carrot or gel baits and dropped from helicopters in areas where there is a high level of possum infestation, or where the terrain is too difficult for trapping or laying poison by hand. Using GPS technology, pest control experts are able to accurately drop the poison to within a few metres of target sites.
People oppose the use of 1080 for three main reasons: they consider it harmful to the environment, they believe it is inhumane, and it also kills other ‘non-target’ animals.
Supporters point out that 1080 degrades in the environment, especially in water. Being soluble, it is rapidly diluted. In soil it does not persist for more than a few weeks, especially at warm temperatures.
Dogs sometimes die when they scavenge on possum carcasses after 1080 drops, although owners are warned to keep their dogs away from poisoned areas. Pests such as deer, stoats and rats are also killed, either because they eat 1080 directly or they feed on possums.
Green, Wren. The use of 1080 for pest control: a discussion document. Wellington: Animal Health Board and Department of Conservation, 2003.
Montague, T. L., ed. The brushtail possum: biology, impact and management of an introduced marsupial. Lincoln: Manaaki Whenua, 2000.
Saxton, Frank. Possums. Auckland: Reed, 2005.