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Introduced animal pests

by  Bob Brockie

Certain animals have done immense damage to the native New Zealand bush. Some, like rats, were stowaways on canoes or ships. But some were introduced on purpose – possums, stoats, deer and goats are among those which were brought over to solve one problem, but created another.


Impact of animals on the bush

People

People are thought to be the most damaging animal pest. A thousand years ago New Zealand was mostly forest. It was burned or milled by Māori and European settlers, and today only a quarter of the country remains covered in mature native trees. Remaining areas of bush are fragmented into hundreds of nature reserves, national parks, and smaller, privately owned patches. Most remaining bush is on land too steep to farm or mill.

Larger areas of bush contain many native plant and animal species; smaller patches have far fewer. Small outcrops are like forested islands in a sea of pastureland – many native animals cannot move between patches, and are unable to breed with each other. Almost all lowland forest has disappeared, and several bird species no longer have enough food to sustain them through winter.

Introduced animals

People have also introduced animals that damage the bush:

  • Possums feed mainly on leaves, from tree tops to the forest floor.
  • Possums and rats sometimes consume complete crops of forest fruits and berries (including seeds, which prevents regeneration).
  • Deer and goats eat most plants within their reach, including seedlings and saplings.
  • Pig rooting damages the forest floor.
  • In South Island beech forests, introduced wasps take up to 90% of honeydew, a vital winter food for many native animals.

Few of the bush’s 300–400 plant species can resist the combined attacks of possums, rats, deer, goats and pigs. Their continual browsing opens up the forest canopy, which causes the undergrowth to thin and makes the bush more vulnerable to wind and invading weeds. Because pests prefer to eat certain species, their browsing disrupts and redirects forest regeneration and succession. Palatable plants become rare, replaced by inedible species.

Loss of native animals

Humans, rats, mice, cats, stoats and wasps have dramatically reduced native animal life in the bush – they have eliminated 43% of native bird species. Between about 1250 and the arrival of Europeans in the late 1700s, Māori and the Pacific rats (kiore) they introduced are thought to have extinguished at least 28 species of native bird. This included 10 moa species, a hawk, an eagle, five rails, an owlet-nightjar, two crow and two adzebill species. Kiore probably killed off tuatara (a lizard-like reptile), two kinds of frog, and small petrels and seabirds on the mainland.

In the 19th century, European settlers introduced more rat species, mice, weasels, ferrets, cats, and stoats. These spread and bird numbers in the bush declined. A wave of extinctions followed, including huia, native quails, wrens, the laughing owl and a native thrush. Stitchbirds, saddlebacks and kōkako disappeared from the mainland. Predatory animals severely reduced numbers of native bats, lizards, frogs, many invertebrates (animals without a backbone), and petrels that were large enough to survive the predations of kiore introduced some 450 years earlier.


Possums

Introduction and establishment

Possums (Trichosurus vulpecula) were first brought from Australia in 1837, to Riverton in Southland, to form the basis of a fur industry. This initial attempt failed, and it was not until 1858 that they were successfully introduced at the same place. They are now found in about 95% of all farmland, scrubland and bush.

Most possums – estimated at over 50 million – live in farmland or scrubland. With at least five possums per hectare, another 16 million are reckoned to live in the bush. Their population density in New Zealand is much greater than in most parts of Australia.

Diet

They eat and damage the forest at every point, taking mainly leaves, but also buds, flowers, fruit and seeds from the tallest tree tops. On the ground they eat seedlings, saplings and sometimes bark. Possums also eat parts of perching plants, scramblers, climbers, some tree ferns and fungi. Favoured foods include mistletoes, tree fuchsias, kāmahi and rātā trees, which is why these are among the first species to be eaten out of the forest.

Damage to plants and animals

When possums eat a plant’s flowers, the nectar and fruit are reduced. The few berries that grow are also eaten. In one study, an entire crop of kaikōmako berries was eaten in a few nights. Another study revealed that possums ate some 60% of hīnau berries. They rob native birds, bats, lizards and insects of food.

Possum cacophony

With a repertoire of at least 20 different sounds, a possum’s calls can be startling:

‘There are screeches, grunts, growls, hisses, and chatters (mostly used in aggressive interactions); zook-zooks and squeaks (dependant juveniles and pouch young); and shook-shooks and clicks (males during courtship). The shook-shook sound of the male, resembling the call of juveniles, may serve to reduce female aggression.’ 1

By feeding selectively, possums have radically altered the makeup of forests. In the 1960s, possums opened up large tracts of forest in the Ruahine Range to invading scrub. They have eaten nearly all of the rātā trees from the Aorangi Range, and have almost wiped out mistletoes in the North Island. Large stretches of Westland forest are dotted with possum-killed rātā and kāmahi. When possums are eradicated, as they have been on Kapiti Island, the bush regenerates.

Possums also eat insects, bats, birds and their eggs and nestlings. They drive native animals out of their dens and nesting sites.

Controlling possums

Traps, poisons and guns can control possum numbers. However, these are expensive methods, and only practical in easily accessible bush or in bush that is valuable to conservation or farming. This leaves possums uncontrolled over large areas of hinterland. Scientists are searching for new methods of control – the most promising are ways of making possums less fertile.

Footnotes
  1. Carolyn King, ed., The handbook of New Zealand mammals. 2nd ed. Melbourne: Oxford University Press, 2005, p. 85. › Back

Rats and mice

New Zealand has no native rats, but three kinds came with early sailors.

Kiore

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.

House mice

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.

Footnotes
  1. Transactions and Proceedings of the New Zealand Institute 12 (1889): 301–302. › Back

Stoats and cats

Stoats

Large numbers of stoats (Mustela erminea) were brought from Britain in the 1870s to control ‘verminous rabbits’. They immediately spread to the bush, where they preyed on native animals. Stoats are energetic, bold and versatile hunters, foraging in every hole, under any cover and up the tallest trees. They are also good swimmers.

By 1910, many native birds had disappeared. Together with rats and cats, stoats have contributed to the extinction of huia, bush wrens, native thrushes, laughing owls and quails. They also exterminated stitchbirds, saddlebacks, kākāpō and little spotted kiwi from the mainland.

Today, stoats live from North Cape to Bluff, on mountains, in farmland, scrub and bush. In the bush, they eat more birds than any other food. Rats and mice, rabbits, possums, hares, lizards and insects form the rest of their diet. In one study, stoats were recorded as robbing just over half of 149 birds’ nests in bush near Kaikōura. They make off with most bush pigeon and kākā eggs and nestlings. Stoats have short lives and highly variable birth and death rates – their population can rapidly increase when food becomes abundant.

Feral cats

Captain James Cook and later whalers and sealers introduced cats (Felis catus), but these did not become common until the 1830s. In the 1870s, rabbit numbers were driving farmers from their land, so quantities of cats were released to control them.

Rat-catchers

Cats were carried on ships to control rats. When James Cook’s Resolution was tied to trees in Dusky Sound, Fiordland, in 1773, one of the cats ‘regularly took a walk in the woods every morning and made great havoc among the little birds, that were not aware of such an insidious enemy.’ 1

The cats went into the bush, joining rats and stoats as predators. Soon after cats appeared on Little Barrier, Cuvier and Stephens islands, saddlebacks and other native birds disappeared. Introduced to forest-covered Herekopare Island, cats quickly extinguished parakeets, robins, fernbirds, brown creepers, snipe and a native bat.

Wild cats live high in the mountains and along the coast, as well as in bush, scrub, and on farmland. They feed mainly on young rabbits, rats and mice, but also on native birds, lizards and large insects such as wētā, cicadas and dragonflies. In mainland bush, birds make up some 15% of their diet but, with most native birds gone, they usually eat blackbirds, chaffinches, silvereyes and hedge sparrows.

It is difficult and expensive to rid the bush of cats. It took 128 people almost 400 days to remove 100 cats from Little Barrier Island. In some cases they can be seen as a lesser evil, as they prey on rats, which cause even more damage.

Other pests

Other introduced pests such as weasels, ferrets and hedgehogs also live in the bush but make lesser inroads into the native animal populations than cats and stoats.

Footnotes
  1. J. G. A. Forster, A voyage around the world in his Britannic majesty’s sloop Resolution. Vol 1. London: B. White, 1777, p. 128. › Back

Deer, goats, pigs and wasps

Deer

Red deer (Cervus elephas) are the most abundant deer, but seven other species – wapiti, fallow, white-tailed, sika, sambar, rusa deer, and possibly moose – also live in the bush. Chamois and tahr (thar) occasionally stray into the bush from the alpine zone.

Between 1851 and 1923, about 1,000 British red deer were released to provide sportsmen with game. At first the deer multiplied rapidly, and then there was a period of sustained high density. By the 1940s they had colonised most of the bush in the southern and central North Island and most of the remote areas of the South Island. They ate out many of the palatable plants within reach, and began to damage the forests irreversibly. Their diet included large-leaved plants such as broadleaf, five-finger, patē, coprosmas, fuchsia, tītoki, heketara, and hen-and-chicken ferns. They also browsed any grasses or tussock in the bush, scrub or alpine zones.

Several studies have shown that deer may take 90% of seedlings, preventing the regeneration of damaged forest. After eating the palatable plants, deer leave those that are inedible, such as pepper trees, which grow in their place. If deer are excluded from a patch of bush, plants regenerate spectacularly.

Some 40 years after their establishment, deer numbers fell because of government deer-culling operations. Numbers declined further when helicopters were used to shoot deer or to capture them for farming.

Today, red deer inhabit forests from East Cape to Stewart Island. Other deer species have more restricted ranges.

Goats

Captain James Cook gave goats (Capra hircus) to Māori in the 1770s and the first sealers and whalers brought more. Early farmers used goats to control blackberry, gorse and briar. But many of the goats escaped and they became populous in the bush. In 2006, goats were scattered at low densities, with concentrations in scrubby hill country in both islands. Most lived in patches of rough, upland, scrubby or bushed country, with the largest groups in Taranaki, the top of the South Island and inland Otago.

Goats are mobile and inquisitive, grazing on plants up to 2 metres high, although they can browse higher by climbing on leaning tree trunks. In the bush they eat mainly broadleaved trees and shrubs, but also ferns, grasses and unlikely plants like tree nettle, bush lawyer and lancewood.

Browsing goats change forests because the plant species they eat are replaced with inedible ones. Continued grazing thins undergrowth and, as each generation of seedlings is destroyed, the course of plant regeneration and succession is disrupted and redirected. On some offshore islands, goats have reduced forest to rank grassland.

A shilling for a pig

In 1925 the New Zealand government introduced a bounty scheme in which one shilling or three rounds of ammunition would be paid for a pig’s snout and tail. During their most populous year, 1947, there were an estimated 123 pigs per square kilometre of bush.

Pigs

Pigs were introduced to New Zealand by early explorers in the late 1700s. Wild pigs (Sus scrofa) plough up large areas of forest floor, uprooting supplejack, bracken and many other plants. They feed on any forest berries, fruit or succulent stems, and any surface-dwelling animals, such as wētā, litter hoppers, earthworms and centipedes. With up to 40 pigs per square kilometre of bush, large volumes of food are taken which would otherwise feed native species.

Wasps

German wasps (Vespula germanica) arrived in New Zealand in 1945. They now occur throughout the country and have reached plague proportions in South Island beech forests, especially in north-west Nelson. In the late 1970s European wasps (Vespula europaeus) arrived and quickly became established.

In spring and summer, German and European wasps are carnivores, eating mainly flies, caterpillars and spiders. They threaten the survival of some rare native insects. In late summer and autumn, wasps switch to a diet of nectar. In South Island beech forests they feed on the honeydew produced by scale insects (Ultracoelostoma species). These suck the sap of beech trees and produce up to 4 tonnes of honeydew per hectare of forest each year. Wasps, which have up to 34 nests and 27,000 individuals per hectare of beech forest, take up to 90% of the honeydew, depriving native birds, bats, lizards and insects of this vital winter food.


External links and sources

More suggestions and sources


How to cite this page: Bob Brockie, 'Introduced animal pests', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/introduced-animal-pests/print (accessed 7 December 2019)

Story by Bob Brockie, published 24 Sep 2007, updated 1 Jul 2015