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European settlers aiming to establish a wild source for food and fibre and fur pelts for clothing introduced the Common Brushtail Possum from Australia to New Zealand. In 2009, the New Zealand population was estimated to be 30 million.
Possums are vectors of bovine tuberculosis, which is a major threat to the dairy, beef and deer farming industries. The disease is endemic in possums across approximately 38 per cent of New Zealand (known as ‘vector risk areas’). In these areas, nearly 70 per cent of new herd infections can be traced back to possums or ferrets. The Biosecurity Act 1993, which established a National Pest Management Strategy, is the legislation behind control of the disease in New Zealand. The Animal Health Board (AHB) operates a nationwide programme of cattle testing and possum control with the goal of eradicating M. bovis from wild vector species across 2.5 million hectares – or one quarter – of New Zealand’s at-risk areas by 2026 and, eventually, eradicating the disease entirely.
The TBfree New Zealand programme is regarded as "world-leading". It has successfully reduced cattle and deer herd infection rates from more than 1700 in 1994 to fewer than 100 herds in July 2011. Much of this success can be attributed to sustained possum control reducing cross-infection and breaking the disease cycle. For example, at Hohotaka, in New Zealand's central North Island, control work from 1988 to 1994 achieved a sustained mean reduction of 87.5% in the density of Tb‐infected possums. As expected, annual Tb incidence in local cattle herds consequently declined by a similar amount (83.4%).
From 1979–1984, possum control was stopped due to lack of funding. In spite of regular and frequent TB testing of cattle herds, the number of infected herds snowballed and continued to increase until 1994. The area of New Zealand where there were TB wild animals expanded from about 10 to 40 per cent.
That possums are such effective transmitters of TB appears to be facilitated by their behaviour once they succumb to the disease. Terminally ill TB possums will show increasingly erratic behaviour, such as venturing out during the daytime to get enough food to eat, and seeking out buildings in which to keep warm. As a consequence they may wander on to paddocks, where they naturally attract the attention of inquisitive cattle and deer. This behaviour has been captured on video.
The introduction of possums has been ecologically damaging because the native vegetation has evolved in the absence of mammalian omnivores. Possums selectively browse native vegetation causing particular damage to broadleaved trees, notably Metrosideros species including rata. This leads to competition for food with native forest birds, changes in forest composition and eventually canopy collapse. Possums are opportunists and will eat the eggs of native birds. They do not have so much impact on Southern Beech (Nothofagus), but their presence tends to reduce the species diversity of Nothofagus forest since they eat many of the other species that would naturally be present. The predation of bird eggs and chicks has led them to be referred to as "reluctant folivores" in that they eat foliage to survive but prefer other foods.
Attempts to reduce numbers by trapping and poisoning have had some success. Trapping and cyanide are generally used by individual hunters as pest control or fur harvesting, while the Department of Conservation, Animal Health Board and Regional Councils use biodegradable 1080 poison to target larger areas. Studies by DoC have found that the risk of the few losses of native birds; (34 individuals in 70 aerial drop operations) and native invertebrates and mammals are outweighed by the recovery of the native species once the competition for food and predation has been eased. There is the added benefit of killing other invasive species. There have been isolated reports of pets, in particular dogs, succumbing to poisoning from 1080, probably through eating possum carcasses. The Animal Health Board and the Department of Conservation jointly operate poison drops. The effects of these drops on the environment have been considered by ERMA in allowing the use of 1080. and in consultation with Māori. There is opposition against the use of 1080 from some quarters since New Zealand is now the world's largest consumer of 1080.
To prevent damage to young trees it seems to be necessary to keep numbers very low, perhaps 5% of the levels that would be reached without interference. DoC is the largest single agency involved in possum control and much possum control is also carried by the AHB, councils and regional authorities, especially to combat the threat of bovine tuberculosis.
In 2009, it was announced that conservation measures had achieved some significant success, especially in certain regions, and had reduced possum numbers to around 30 million animals from a carrying capacity of 48 million. (Estimates produced in the 1980s of 70 million possums were flawed.) Almost half (13.3 million hectares) of New Zealand's vegetated land is under some form of possum control, either for conservation reasons, or to reduce the spread of bovine tuberculosis.
A small cottage industry in fur pelts and wool mixed with possum fur fibre has developed, and trappers and hunters provide the raw material. The fur is often sold as 'eco-fur' by a number of small manufacturing and retailing businesses. The numbers of animals taken for fur is rising and has almost equalled the numbers killed in poisoning. A number of New Zealand companies are exporting possum carcasses to Taiwan, Hong Kong and Malaysia for human consumption, where possum is regarded as a delicacy and known as "Kiwi bear". There is also a small industry processing possum meat as 'Possyum' dog food, also for export. In an example of "coals to Newcastle", NZ Biosecurity has a regulation governing the import of frozen possum meat from Australia for human consumption.