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Exotic species brought to New Zealand by humans have had a dramatic impact on New Zealand’s native biodiversity.

More than 25,000 plant species, 54 mammal species, and about 2000 invertebrates have been introduced. Introduced birds such as the Australian magpie and Asian myna compete aggressively with native species, and 22 species of introduced freshwater fish are now in New Zealand waterways (by comparison New Zealand has just 36 native freshwater fish).

Every year, several hundred more plant species arrive in New Zealand. Nearly 2000 exotic plants species are now established in the wild – in urban Auckland, four new species go wild each year. More than 200 introduced plant species have the potential to displace native plant ecosystems.

Of the mammals introduced to New Zealand, a group of 31 species now dominate the environment. Not all are bad – for example, sheep and cattle are a cornerstone of New Zealand’s primary industry, but rats and possums are serious threats to our native biodiversity.

Because mammals are bigger, hungrier and more active than other animals, their effect has been huge. Humans, grazing animals, domestic cats and dogs have had the biggest impact. Together they dominate the land – forests have been felled and wetlands drained to accommodate them.

On land, alien species threaten a third of our protected forests (1.8 million hectares). In the sea the amount and rate of marine biodiversity loss is largely unknown, though we are lucky to have fewer exotic organisms than many other countries.

Part of the problem is that New Zealand’s native plants and animals were completely unprepared for the sudden arrival of competitors and predators. Their long isolation and slow evolution meant they were especially vulnerable to change.

When not being smothered or overshadowed by exotic weeds, native plants are being eaten by browsing and grazing animals. The most damaging are possums, goats and deer.

Native birds, reptiles, frogs and the larger invertebrates (including weta) fall prey to mammalian predators such as stoats, rats and cats. German and European wasps can kill nesting birds, and compete with them for food.

Several aquatic species appear to be displaced by trout. New Zealand’s native biodiversity is also continuously under threat from new invaders, whether naturally, deliberately or unintentionally released. A recent example is Undaria, an unintentionally introduced seaweed first detected in the 1980s which threatens to overtake native seaweeds and threatens the seaweed communities they support.

Biosecurity measures to protect us from new exotic pests and weeds are important for our remaining native biodiversity, as well as for introduced species on which our economy depends. This includes blocking exotic marine species that could slip into New Zealand waters through the discharge of ballast water (carried in the base of ships for stability) or as fouling on vessel hulls. The Protect New Zealand website contains biosecurity information for gardeners, producers and growers, importers, seafarers and aquaculturists, schools and travellers and tourists. Introduced biodiversity is neither all “good” nor all “bad.” The threats or benefits of individual exotic species often depend on the situation in which they arise.

Possum on lacebark. Photo: Rod Morris/DOC.
Possum on lacebark.


Beech forest understorey damage caused by deer. Photo: DOC.
Beech forest understorey damage caused by deer.


Volunteer removes Pinus contorta from Tongariro National Park. Photo: Harry Keys/DOC.
Volunteer removes Pinus contorta from Tongariro National Park.

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