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.
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.