The New Zealand archipelago has a total land area about the same as Britain (270,000 km2), but it has been isolated in the southwest Pacific for some 65 to 80 million years—that is, since before any modern placental mammals evolved. In the absence of terrestrial hunters (mammalian carnivores and snakes), many of the longest established birds of New Zealand found it safe to live, feed, and nest on the ground. In the course of time, many of them became large, flightless, slow breeding, and unique.
New Zealand was the last major land mass on earth to be colonized by people. A series of human invasions, beginning with the first permanent Polynesian settlements about 700 years ago, brought to the islands increasing numbers of colonists plus alien mammals and birds. The intruders arrived with stunning speed, numbers, and superiority, and they overwhelmed the previously undisturbed native fauna, with devastating consequences (Worthy & Holdaway 2002). Twenty-five species of introduced mammals are regarded as pests in New Zealand (Cowan & Tyndale-Biscoe 1997); 50% of New Zealand's breeding bird species have been lost, or 60% if the critically threatened survivors are included (Holdaway 1999).
Stoats and common weasels arrived in large numbers on a series of organized shipments from Britain over about 20 or 30 years after 1884 and liberated on pastures teeming with rabbits. They were part of a huge, unplanned experiment in biological control, intended to save New Zealand pastoral farmers from being ruined by plagues of European rabbits. The assumption was that, in Britain, predators normally kept rabbits under control. That was the only reason people could think of to explain why, in New Zealand, rabbits free of predators reached numbers unknown in Britain, competing with sheep for grass and undermining the pastures with burrows. Therefore, it stood to reason that all that was needed to reduce rabbits in New Zealand to the same nonnuisance level as back in Britain was to bring in predators that could eat rabbits. Foxes were banned, so the task fell to stoats, common weasels, and ferrets.
In the early 1880s, the New Zealand government began advertising for live stoats and common weasels to import. Many of the 17,000 or so gamekeepers then employed on the game estates of Britain (Chapter 12) were glad to take the opportunity to earn some extra pay. The £5 offered for a pair of common weasels was worth the equivalent of about £265 (US $410) today—close to a week's wages. The little animals were easily caught in box traps and shipped in small cages. The mortality rate among them, in the traps and on the long voyage under sail, must have been high. Nevertheless, the scheme ran for at least 20 years, and in the first 2 years, 1885 and 1886, 224 stoats and 592 common weasels reached the other side of the world alive and were released on the rabbitinfested pastures of the new colony (King 1984b).
Stoats certainly could and did kill rabbits, as in Britain, but the farmers' expectation that they would stay on the sheep runs and hunt rabbits indefinitely was asking too much of them. Right next to the pastures were the forests, where birds and wetas (large native insects) nesting in trees were certainly not out of reach of agile stoats. And despite a hundred years' attention from Norway rats and feral cats, semi-wild dogs and human hunters, some of the native birds and lizards that foraged and nested on the ground still survived.
Within a very short period, both species spread throughout the country. Common weasels have always remained scarce, probably because New Zealand has no voles (King & Moors 1979a), so the following account does not include them. By contrast, stoats can now be found almost anywhere, including in all the national parks.
Public attitudes toward stoats in New Zealand are universally hostile, especially where the destruction of the ancient, endemic birds was recent enough to have been witnessed by the parents or grandparents of people still living in the same areas today. Because stoats appear to be such efficient predators, they are sometimes blamed for all losses of native fauna, both historical and contemporary. But what stoats might have done in the past and what they can be shown to be doing today are quite different processes, and need to be assessed separately.
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