New Zealand has no native rats, but three kinds came with early sailors.
Polynesians introduced the rat around 1250–1300 AD, when they settled in New Zealand. Kiore are thought to have wiped out snipe-rails, owlet-nightjars, some small petrels, some native frogs, and all tuatara on the mainland.
Very few kiore now survive on the mainland as more aggressive European rodents have replaced them.
Under the floorboards
European settlers were surprised by occasional swarms of kiore, which were familiar to Māori:
‘In Picton, during the swarm of 1884, the stench becoming unbearable in one of the houses, the floor of the sitting room was removed, when forty-seven rats were found lying together dead near the fireplace. ... Indeed, the whole town was pervaded with the odour of dead rats. It took the place of pastille in the drawing rooms, and overpowered that of sanctity, even, in the churches.’ 1
Norway and ship rats
Norway rats (Rattus norvegicus), also known as brown or water rats, were on the ships of the first explorers, who arrived in New Zealand in the late 1700s. These rats quickly spread. Europeans brought ship rats (Rattus rattus), also known as black or roof rats, but these did not become established until after the 1860s.
Today, ship rats occur throughout the country, and are abundant in kauri and rimu–rātā forest. They mostly live and nest in trees. The spread of ship rats caused the sudden decline of many native bird and bat species. For example, when they were accidentally introduced to Big South Cape Island (off Stewart Island) in 1964, they quickly eliminated five types of native bird, one bat species, and a large flightless weevil. In a study at Kōwhai Bush, near Kaikōura, ship rats ruined and robbed eggs and nestlings from 16% of small birds’ nests.
In the bush, ship rats feed mainly on fruit, berries and fallen seeds in autumn and winter, and on other animals in spring and summer. Among these are wētā, stick insects, cicadas, beetles, caterpillars and grubs, spiders, native slugs, snails and lizards.
Norway rats live from North Cape to Stewart Island. They are ground dwellers, usually living near wetlands or in damp lowland bush. They threaten animals living, roosting or nesting near the ground, taking birds’ eggs and nestlings, native insects and lizards.
Every bit of bush in the North and South islands harbours house mice (Mus musculus), sometimes in plague numbers. By eating insects and fallen seeds and berries, mice deprive many native ground-feeding animals of food.
Beech trees produce heavy seed crops every two or three years (‘masting’ years), providing a bonanza for mice whose numbers often explode in response. Mice in turn are food for stoats, whose numbers may also surge. Months later, when the seeds have germinated, rotted or been eaten, and many of the mice are also eaten, a hungry, increased stoat population turns its predatory attention to native birds.