Introduced pest or predatory species threaten the health of our native ecosystems. They can cause catastrophic effects on the environment, including preying on and competing with native birds, reptiles and invertebrates.
Stoat © Charlie Marshall


Mustelids include Ferrets, Stoats and Weasels.

Ferrets were introduced to New Zealand from Europe in the 1880s, along with stoats and weasels, to control rabbits that were breeding out of control. Scientists and bird lovers warned at the time that they would be a danger to our native birds, but their warnings were ignored.

Their main prey are rats, mice, birds, rabbits, hares, possums and insects (particularly weta). Mustelids will also eat lizards, freshwater crayfish, roadkill, hedgehogs and fish. If they get the chance, they’ll kill more than they need for food and hide the rest in their den to eat later. They often take on animals much bigger than themselves.

All mustelids have very good eyesight, good hearing and a strong sense of smell. They move quickly and are good at climbing trees so they can eat baby birds and eggs in the nest.

Mustelids can live just about anywhere in New Zealand, as long as they can find prey. They can travel long distances very quickly; one young stoat travelled 70 km in just two weeks. They are also strong swimmers and can swim 1 km or more to reach islands.

A big problem with mustelids is the size of their families; they can have up to 12 babies at a time, but usually have four-to-six. A female stoat can get pregnant when she is still a blind, deaf, toothless and naked baby – at only 2-3 weeks old. Even though she is pregnant, her babies won’t grow inside her until she is an adult. They will be born the following spring.


There are three kinds of rats in New Zealand, the kiore (Rattus exulans) which is the smallest, the Norway rat (Rattus norvegicus, also called the brown rat) which is the biggest, and the ship rat (Rattus rattus, also known as the common rat or black rat) which is the most common. All species eat weta and other insects, snails, frogs, lizards, tuatara, birds and bats, as well as the flowers, fruits and seeds of plants.

Kiore came to New Zealand with early Māori voyagers, while Norway rats and ship rats travelled here on whaling ships and with early European explorers and settlers. Kiore have been outcompeted by the more recent arrivals and are now only found on several offshore islands and in remote parts of Fiordland.

The Norway rat has a short, thick tail which is shorter than its body and it has small ears. Norway rats are particularly good swimmers and are able to swim up to 1 kilometre. Norway rats also prey on ground-nesting birds, their eggs and their chicks, such as the New Zealand dotterels and shore plover. They are able to climb trees but spend most of their time on the ground.

The ship rat is the biggest threat to wildlife, as it is a good climber and can reach nests in trees. It is thought predation by ship rats was instrumental in the extinction of many of New Zealand’s native birds including the bush wren, huia, laughing owl, New Zealand little bittern, and South Island snipe.

Brown rat
© Zeynel Cebeci / Wikimedia
Brushtail possum
Brushtail possum © Brisbane City Council


Possums are a major threat to our environment and are widespread throughout New Zealand. Their only predators (feral cats) do not have much effect on controlling possum numbers.

The Australian possum – the common brushtail possum, Trichosurus vulpecula – was first introduced to New Zealand in 1837 to establish a fur trade. This release was unsuccessful, but a second release 20 years later at the same Southland location did succeed.

They are opportunistic omnivores. They eat buds, flowers, fruit, berries and nectar, which means they compete with native birds and reptiles for food sources. The growth and life cycle of a tree or plant can be significantly affected when all parts of it are eaten. Possums also have ‘favourites’ such as rātā or kamahi trees, leading to an even greater impact on these species.

In 1993, possums were filmed eating the eggs and chicks of kōkako, and this evidence changed many people’s views of their threat to wildlife. They also eat invertebrates, including weta, and are significant predators of New Zealand land snails such as Powelliphanta. They often occupy holes in tree trunks for their nests, holes which would otherwise be used by nesting birds such as kākāriki and saddlebacks.

Dairy and deer farmers have the added worry of possums spreading bovine tuberculosis. The economic loss in primary production due to damage and control of possums is valued in the tens of millions of dollars.


Hedgehogs were first brought to New Zealand by acclimatisation societies to remind settlers of their homeland, but were later introduced in greater numbers to control garden pests such as slugs, snails and grass grubs. They are abundant in New Zealand and are a significant threat to many native species.

Their preferred habitat is lowland pastoral areas, although recent studies show they are regularly trapped in large forest tracts. Hedgehogs were thought to be less common at altitude, but they can be found high above the bush line at altitudes up to 1800m. 

They are a major predator on eggs of riverbed breeding birds such as banded dotterel and black-fronted tern, and kill and eat chicks of a variety of ground-nesting birds. They have a voracious appetite for invertebrates and take many local endemic species. They eat the rare giant native centipede, wētā, and other rare insects. They also eat the native snail Wainuia urnula. Lowland populations of Powelliphanta snails may also be severely affected. They also eat lizards and skinks.

Many ground-set trapping programmes in Central Otago, braided river systems in South Canterbury or forest ecosystems in the North Island catch hedgehogs.

Don’t encourage them into your backyard by creating burrows for them or leaving milk out. Set a suitable trap for the situation you are in (suburban or rural). Prints are five-toed, resembling a large rat print. Forefeet are much broader and shorter in length than the hind feet, meaning there are two distinctly different prints left by the one animal.

Hedgehog © Michael Gäbler / Wikimedia
House mouse
House mouse © 4028mdk09 / Wikimedia


Mice are a key food source for stoats and feral cats in the New Zealand bush. When conditions are right and there is plenty of food, their numbers increase rapidly.

Mice are omnivorous and get their dietary requirements from many sources. They feed on plant and animal material, and have very flexible feeding behaviour to take advantage of whatever food sources are available. Invertebrates are usually the main species eaten, but bird eggs, chicks and lizards may also be consumed. Mice have been recorded feeding on inanga eggs in estuaries, which impacts whitebait runs.

Their broad diet means that their impacts are equally broad. They affect nutrient cycling in ecosystems by eating the insect larvae that break down leaf litter, therefore preventing the nutrients in the leaf litter returning to the soil. Selective predation of certain seed types prevents regeneration and alters forest plant species composition.

In some seasons, plant communities flower and seed prolifically (mast years). This occurs both in beech forest systems and in tussock grasslands and results in rapid population growth in mice. This feeds growth in mouse predator numbers. These predators feed on native species as well as mice, and during high predator years predation on native birds increases dramatically and can cause local population collapse.

Feral Cats

Feral cats have a major impact on our native birds, bats, lizards, wētā and other insects.

Feral cats are different to stray cats. Neither are owned, but strays have varying interactions and dependence on humans while feral cats are wild.

Feral cats have the same appearance as some common, short-haired house cats such as tabby, tortoiseshell and black. They can grow to a much larger size than house cats if conditions are favourable, though they don’t live as long. Male feral cats captured in the South Island high country averaged a weight of 3.75 kg and the heaviest male weighed 7 kg.

Feral cats can travel long distances. Scientists tracked a feral cat in the South Island high country that covered almost 6 km in one night. Source: DOC.

Feral cat
© Ke Vin / Unsplash