The four rodent species in New Zealand are introduced pests. The European house mouse and the three species of rats threaten the long-term survival of native species.
The three species of rat are the Pacific rat/kiore, ship or common rat, and Norway or brown rat.
Comparison of rat pelts, from top: Norway, ship, kiore
Kiore (Pacific rat)
Possum and rat both preying on a thrush nest
There are three types of rat in New Zealand.
Kiore is the Maori name for a species of rat (Rattus exulans). It is the world's third most widely distributed rat, found throughout the Asia/Pacific area.
Kiore are poor swimmers and have been introduced accidentally and intentionally by humans. Maori settlers introduced rats to New Zeaalnd in about the 10th century.
Of the many introduced mammals to New Zealand, kiore are unique because of their association with the migration of Polynesians throughout the Pacific, and because of their cultural and spiritual values to some iwi Maori.
Kiore eat a wide range of foods including seeds, fruits, insects, lizards, eggs and chicks. New Zealand's native plants and animals evolved without predators and are unable to compete with introduced species.
In 1995 the Department of Conservation (DOC) released a kiore strategy document. That strategy advocated the elimination of kiore and other rodents from islands administered by DOC.
At the same time the strategy recognised the cultural and spiritual value of kiore to some iwi Maori and acknowledged that kiore were likely to remain on the mainland and on some islands (outside of DOC's administration) for cultural or scientific reasons.
DOC administers approximately 230 of the estimated 600 islands around New Zealand. Fifteen of these islands have kiore.
Removal of kiore from all 15 of DOC administered islands would reduce their New Zealand range by only four per cent. DOC has successfully eradicated kiore (and other rodents) from a number of offshore islands and plans to eradicate rodents from all fifteen islands under its administration. Where they are removed from offshore islands, considerable conservation gains are possible.
Kiore are not protected under New Zealand statutes.
Recognising that iwi Maori sometimes have a cultural interest in kiore, as well as the ecological health and well-being of plants, forests and wildlife, DOC consults before starting eradication programmes.
Norway or brown rat
The Norway rat (Rattus norvegicus) is the largest of the two European rats. It is distinguished from the ship rat in that the tail, which is about 180 mm in length, is thick and shorter than the body, which is about 200 mm in length, and that the ear of this rat does not reach the eye when pressed forward.
Ship rat in Fantail nest
Ship or common rat
By contrast the ship rat (Rattus rattus), which is more common, has a tail larger than its body, and ears that cover the eyes when pressed forward. Both of the European species are associated with human activity and are found in houses, tips, waterways and cropland.
Rats have a major impact on New Zealand’s wildlife because they eat native animals and their eggs. They also eat a wide range of native fruit and plants, which puts them in competition with native wildlife for food.
Kiore, introduced by early Maori voyagers, predate rare weta, snails, frogs, lizards, tuatara, birds and bats, as well as other insects and the flowers, fruits and seeds of plants.
Norway rats are large enough to kill nesting adult seabirds and prey on animals that live, roost or nest close to the ground.
Ship rats are causing the most damage to our wildlife because they are good climbers, so they are able to get access to most bird nests high in trees.
Rodents and other predators have been eradicated from several offshore islands which, as nature reserves, are now safe havens for many rare and endangered species.
Various methods have been used to eradicate rodents from offshore islands including trapping, 1080 and talon-type baits. Recent advances, including the development of more effective anticoagulant poisons and new application methods, mean it is now possible to consider rodent eradication operations on larger islands.
On the mainland, six official "mainland islands" have been established. These are areas of intense predator management, making them also safe for vulnerable species. But pest control is on-going throughout DOC managed land.
DOC, health and regional authorities are involved in poisoning operations to ensure risk to humans, the environment and non-target species is minimised.
You can help
Signs that rats are present
Remains of food such as seeds or snail shells can be found under cover like hollow logs, or amongst tree roots. Sometimes nests of loosely woven twigs and leaves can be found in tree holes, or in burrows in the case of Norway rats. Droppings are cylindrical and stubby, sometimes tapering to a point. Norway rat pellets are about 16mm long, roughly twice the length of other rat pellets.
Norway rats excavate burrows 60-90 mm in diameter, often beneath cover such as rocks or tree roots. They can excavate large volumes of soil, and often hoard food in their burrows. Pathways 50-100mm wide link burrows with feeding sites, and these paths can become well worn trails.1
Methods of control
Rat with poison bait, Fiordland National Park
Rats populations fluctuate. Numbers will suddenly increase every five or seven years and it is at these peaks that major damage occurs. Regular control will keep rat populations at continuous low levels. Control methods include trapping, poisoning and the use of predator proof fences.
If you are planning a pest control operation enrol for the Animal Pest Control Methods field based course.
The course provides an overview of animal pests, their impacts and control methods (including the principles these are based on, and the task specifications DOC has developed).
The course covers all the legal requirements for animal welfare and handling toxins. Working within the law is vital to allow pest control agencies and community groups continued access to the full suite of animal pest control methods.
In particular, it describes the control methods most commonly used in DOC, and their advantages and disadvantages.