Invading hordes

Animal pests arrived in New Zealand alongside humans. Kiore (the Pacific rat) was introduced by Maori in the 10th century; since then it's been all downhill for New Zealand's ecosystems. Hundreds of other exotic species arrived on European ships in the 19th century and are now firmly established.

Our native plants and animals evolved without the threat of exotic predators and browsers. They're poorly equipped to deal with - for example - possums, wasps and stoats.

Some of our most destructive pests were deliberately introduced into New Zealand. Rabbits were released by Captain Cook in 1777 for food and fur. When rabbits got out of hand, 19th-century farmers released weasels and stoats to deal with them. Stoats kill up to 15,000 North Island brown kiwi each year, or 60 percent of that year's chicks.

Pest-free zones

But populations of our native species can regenerate quickly if they're given a chance. The Karori Sanctuary in Wellington is one of 10 essentially "mammalian pest-free" zones on the New Zealand mainland. The 225-hectare valley is ringed by a predator-proof fence that's 2.2m high. The fence is made of a woven galvanised wire mesh. In eight years, only mice and a solitary weasel have managed to get into the sanctuary.

Fifteen native species have been released back into the valley including kiwi, hihi, tuatara, and saddleback. These endangered New Zealanders are now re-established in the valley. Conservation scientist Raewyn Empson says that "many of these species are regarded as nationally endangered or threatened, and establishing breeding populations in the sanctuary has helped improve their conservation status."

The valley teems with bird life, and some of the success has flown beyond the sanctuary walls. "We've noticed an increase and spread throughout Wellington of birds like tui that were previously rare or missing from the area."

More information

• Department of Conservation: www.doc.govt.nz
• ERMA: www.ermanz.govt.nz
• Karori Sanctuary: www.sanctuary.org.nz
• Pest exterminators: www.pmanz.co.nz

Report by Luke Harrison