Why Are Stoats a Problem in New Zealand? Invasive Predators

How stoats got to New Zealand

Before people arrived, New Zealand had no land mammals except bats. Polynesian and European settlers brought rats, dogs, cats, pigs, possums and many other animals. Among them were European rabbits, released for food and sport, which multiplied so quickly that by the 1870s they were stripping sheep pasture.

Farmers pushed for the import of mustelids, the family that includes stoats, ferrets and weasels, as a natural control for rabbits. Naturalists in New Zealand and Britain, including the ornithologist Walter Buller, warned that these predators would turn on native birds. The warnings were ignored, and stoats from Britain were released from the 1880s onward. Within a few years observers were already reporting declines in native birds.

Introduced rabbits caused a similar crisis across the Tasman Sea, a story told in how did rabbits get to Australia. Australia, however, never brought in stoats.

Why New Zealand birds are so vulnerable

New Zealand’s birds evolved for millions of years without mammal predators on the ground. Many of them lost the ability to fly, nest on the ground or in tree holes, and freeze or stay on the nest when threatened instead of fleeing. These are good strategies against birds of prey, but they are fatal against a small mammal that hunts by smell.

The stoat is especially dangerous because it:

• climbs trees well and enters tree holes and nest cavities;
• hunts by day and by night;
• swims well enough to reach many offshore islands;
• kills more than it needs when prey is easy, and caches the surplus;
• breeds fast, with litters of about 4–9 kits a year.

Which native animals do stoats harm?

Stoats eat a wide range of native birds and their eggs and chicks. In New Zealand, birds make up a much larger part of their diet than they do in Europe.

Kiwi are among the best-known victims. In places without predator control, only about 5% of kiwi chicks reach adulthood, and stoats are responsible for roughly half of those chick deaths. Once a kiwi weighs more than about 1 kg (2.2 lb) it is usually big enough to defend itself against a stoat. Read more in are kiwis endangered and the North Island brown kiwi profile.

Beech masts and stoat plagues

Some of the worst damage happens after a “mast” year, when southern beech trees produce a huge crop of seed. The seed feeds an explosion of mice and rats, and the stoats that eat them breed well and raise large litters. When the seed and rodents run out the next year, the many young stoats turn to birds.

After the 2005–2006 beech mast, for example, a stoat outbreak in Fiordland killed more than half of the takahe in areas without trapping, and the wild takahe population dropped by about a third in a single year.

Stoats as swimmers and island invaders

Small islands kept free of predators are the last safe homes for many rare birds, including the kakapo. Stoats are strong swimmers, and genetic studies have shown that they regularly reach islands in Fiordland after being removed, especially in mast years.

Even a single stoat on a sanctuary island is treated as an emergency. In 2022 one male stoat reached Chalky Island, an important kakapo refuge, and it took conservation staff about eight months, with dogs, cameras and helicopters, to remove it.

What New Zealand is doing about stoats

The Department of Conservation and many community groups run predator control across large areas of forest and around important bird sites. Methods include networks of traps, predator-proof fences around “mainland islands” and sanctuaries, and removal of stoats from offshore islands. Researchers are also working on new tools, such as better lures and self-resetting traps for remote areas.

In 2016 the New Zealand government announced Predator Free 2050, a national goal to remove introduced rats, possums and mustelids, including stoats, from the whole country by 2050. It is one of the most ambitious conservation projects in the world, and many scientists think it will need new technology to succeed on the mainland.

Stoats among the world’s worst invaders

Stoats are listed among the “100 of the world’s worst invasive alien species” by the IUCN Invasive Species Specialist Group. In their native range in Europe and Asia they are a normal and valued part of the ecosystem, listed as Least Concern. The problem in New Zealand is not the stoat itself but its arrival in a place whose wildlife never evolved to cope with it. A similar story is playing out on the Orkney Islands off Scotland, where stoats arrived around 2010 and threaten the local Orkney vole and ground-nesting birds.