Stoats in New Zealand

Stoats (Mustela erminea) were introduced into New Zealand to control introduced rabbits and hares, but are now a major threat to the native bird population. The natural range of the stoat is limited to parts of the Northern Hemisphere. Immediately before human settlement, New Zealand did not have any land-based mammals apart from bats, but Polynesian and European settlers introduced a wide variety of animals.

The rabbit was introduced by European settlers as a food and game animal, and by the 1870s it was becoming a serious threat to the newly developed farming economy. Farmers began demanding the introduction of mustelids (including stoats) to control the rabbit plague. Warnings about the dangers to bird life from stoats were given by scientists in New Zealand and Britain, including the New Zealand ornithologist Walter Buller. The warnings were ignored and stoats began to be introduced from Britain in the 1880s. Within six years, drastic declines in bird populations were noticed.

In December 2010, a stoat was seen on what was thought to be the stoat-free Kapiti Island, and by August the next year the New Zealand Department of Conservation had managed to kill three. It is thought that they could not have swum the five-kilometre stretch of open sea from the Kapiti Coast.

New Zealand has a high proportion of ground-nesting and flightless birds, due to its long geographical isolation and a lack of mammal predators. Native birds have evolved to fill niches that are occupied by mammals in most other places. Stoats are the greatest threat to these ground-nesting and hole-nesting birds, which have very limited means of escaping stoat predation. In addition to birds, stoats eat insects, mice and rats. During "beech masts", when southern beech trees (Nothofagus species) produce a far greater amount of seed than normal, the stoat population undergoes changes in predation behaviour. With high beech-seed numbers, rats and mice become more plentiful, and the increase in prey encourages stoat breeding. The higher stoat numbers reduce the rodent population and the stoats then prey on birds. For instance, the wild population of the endangered takahe dropped by a third between 2006 and 2007, after a stoat plague triggered by the 2005–2006 mast wiped out more than half the takahe in areas where stoat numbers were not limited by trapping.

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