Between them, possums and stoats have wreaked havoc on New Zealand's biodiversity. Their appetites for native species, both plant and animal, have had devastating impacts.
Possum
The kiwi may be New Zealand's national icon but today all six species are threatened and one no longer survives on the mainland. The biggest single culprit is most likely the stoat, Mustela erminea. In parts of New Zealand forest canopies are under threat of collapse. The main culprit here is the brushtail possum, Trichosurus vulpecula. Not only that, but possums are the major reservoir and vector of a cattle and deer disease, bovine tuberculosis (Tb), which threatens New Zealand's dairy and beef exports.
Both species were deliberately introduced, and both species absorb millions of dollars in efforts to control their impacts.
The Weaselly Stoat
Stoat
The stoat is undoubtedly one of New Zealand's worst immigrant pests. Together with weasels and ferrets, they were brought here in the 1880s in a desperate attempt to halt the rocketing increase in rabbit numbers, ironically enough, themselves introduced for fur and hunting sport.
With hindsight, the introduction of stoats is commonly regarded as one of the worst mistakes ever made by European colonists in New Zealand. Their arrival did knock rabbit numbers, for a while, but native birds, insects and bats were easier to catch. New Zealand's flora and fauna had evolved in the absence of mammalian predators and so were poorly equipped to survive their presence and it was not long before stoats' impact became obvious and the forests ever-more silent. Forty-four birds have become extinct since human colonisation brought pests, such as stoats, to New Zealand.
Kiwi numbers have declined rapdily
Predation of young kiwi, chiefly by stoats, is the most important factor contributing to the continuing decline of mainland kiwi populations. Stoats kill an average of 40 North Island brown kiwi chicks per day - this adds up to 15,000 per annum and accounts for 60 per cent of North Island brown kiwi born. Stoat numbers can be extremely low and yet can still make a substantial difference to kiwi survival. Part of the problem is that stoats are 'surplus killers' - they kill everything they find and stash the surplus as insurance for lean times. They do not just kill enough to eat at that time.
Future Control Options
In 1999 the New Zealand Government set aside an extra NZ$6.6 million for a five-year research project to develop an integrated stoat control research programme. At present, stoat control relies largely on labour-intensive trapping and the use of poisoned hen eggs. Because control will have to be ongoing to enable some endemic New Zealand species to survive on the mainland, the aims are to:
- Make stoat control more cost effective where it is already successful;
- Develop new techniques so that control can realistically be undertaken in more and larger areas;
- Expand the arsenal of methods to ensure sustainability of control; and
- Seed new, longer-term projects for their potential to dramatically increase control effectiveness.
The Pesky Possum
Possum invasion
Since the common brushtail possum was introduced in 1837 it has invaded nearly every corner of New Zealand's mainland - only two per cent remains possum-free.
The possum is a relatively small solitary Australian marsupial, the size of a cat, but its impacts on New Zealand's ecology have been huge. An opportunistic eater, it feeds on foliage, flowers and fruits, insects, bird eggs and fungi. It has no natural competitors for food and shelter, and no natural predators or parasites that significantly affect its numbers are in New Zealand.
For many years possums were protected to all but licensed hunters and a traded skins provided a good source of revenue. In the 1940s, when the first scientific evidence of their impacts on native forest was collected, the government's policy changed dramatically. Ever since New Zealand has been fighting to control the animal. Despite this, illegal liberations into possum-free areas were reported up until the 1970s.
The possum is considered the primary wildlife reservoir of Tb in farmed cattle and deer, and the single greatest barrier to eradication of the disease. Possum populations infected with Tb occupy about 24 per cent of New Zealand and these areas correlate to where 75 per cent of the reactor herds and Tb-infected cattle are found.
Control Measures
A great deal of research has been done, and is being done, on possum biology, social behaviour and diseases, impacts, types of control, monitoring and the benefits of control. Methodologies are continually up-dated as new or improved information becomes available.
The Costs of Control
Costly eradication
Indicative costs of possum management range from NZ$5 - $10 per hectare for annual culling operations at low densities, to more than NZ$61 per hectare for eradication of possums from islands.
At present New Zealand spends a total of about NZ$50 million on control. In 1997/98, the control of Tb vectors cost NZ$24.7 million, three times more than in 1990. Research costs for possum/Tb control research reached NZ$15.5 million in 1998/99. However, if New Zealand decided to aim to eradicate possums from the highest priority areas for conservation protection and Tb management, this cost has been estimated at NZ$1 billion.
Agencies Involved
Many agencies are involved in control with the main funding coming from the Animal Health Board (AHB), Regional Councils and the Department of Conservation (DOC).
The AHB's control strategy aims to reduce bovine Tb with the long-term goal of official freedom from the disease by 2011. So far it has achieved a 60% reduction. DOC has a ten-year National Possum Control Plan (1993 - 2002) which aims to achieve sustained control in critical areas, and to protect high-ranking environmental values most at risk from the marsupial. DOC uses both staff and contractors to carry out its animal pest control operations.
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