During glacial periods, when the sea level was lower, most nearshore islands were connected to the mainland. When the sea rose to its present height about 6,000 years ago, isolated hills became islands. Today, their plants and animals generally resemble those of the nearby coast, but few remain in their original state. Māori brought the kiore or Polynesian rat – a deadly threat to many native creatures – to mainland New Zealand and some of these islands. Europeans cleared the land and brought new plants and a host of predators.
Unique plants and animals
The more isolated islands still have some unique species. One of the world’s rarest trees grows on Great Island in the Three Kings group – a single specimen of Pennantia baylisiana. The Poor Knights Islands, with caves, sea arches and prolific marine life, were cited by marine explorer Jacques Cousteau as one of the world’s 10 best subtropical diving locations.
Stephens Island in the Marlborough Sounds was the last home of the tiny, nocturnal, flightless Stephens Island wren. Discovered in 1894, it became extinct in 1895. It was once thought that the wrens were all killed by Tibbles, the lighthouse keeper’s cat, but feral cats and tree clearance may have also played a part.
Islands as refuges
Some islands became refuges for animals that were once abundant on the mainland. For example, by 1910, there were only 500 surviving North Island saddlebacks, all on Hen Island, off the Whāngārei coast. By 1964 Big Cape Island, off Stewart Island, was the only home for the 36 South Island saddlebacks that remained. When rats invaded, the birds were taken to rat-free islands close by. At this time, Little Barrier Island, in the Hauraki Gulf, was the only place you could find stitchbirds.
Nearshore islands host all of New Zealand’s tuatara, with the largest population on Stephens Island. That island is also home to Hamilton’s frog, possibly the world’s rarest amphibian. Its habitat once consisted of a single boulder bank near the island’s summit. Maud Island has pakeka frogs, which are almost as rare.
As naturalists noted with alarm the disappearance of many native species, the government started purchasing islands as sanctuaries. The first nature reserve was Resolution Island in Fiordland, set aside in 1891. Richard Henry, the curator, was aware of the carnage that predators caused among flightless birds. He pioneered capture techniques and transferred more than 700 kākāpō and kiwi to the island. But stoats eventually swam to the island and killed the birds.
Clean sweep of brushtail
In 1893, four years before the government bought Kāpiti Island for a nature reserve, a few Australian brushtail possums were released there. They soon multiplied, despite early efforts at trapping. They preyed on native birds, and their huge appetite for leaves and fruit curbed regrowth of the forest. Eventually, between 1980 and 1986, the entire possum population of 21,000 was eradicated.
Soon after, Little Barrier and Kāpiti islands were purchased. The Department of Conservation now manages or has an interest in more than 220 islands larger than 5 hectares in area.
Some islands are being restored to something resembling their original ecology, with unwanted animals entirely eradicated. Larger browsing animals such as wild cattle and goats are the first to go, followed by smaller predators – cats and possums, stoats and weasels, and eventually rats. Existing patches of native vegetation are enlarged, often with the help of planting programmes.
Some sparse populations have become abundant once again. Other species were reintroduced to their former homes, while some islands became sanctuaries for animals never previously found there. Kākāpō were once plentiful, but when only a few remained in Fiordland and on Stewart Island, they were moved to Whenua Hou/Codfish, Maud and Little Barrier islands, and in 2003 to Fiordland’s Chalky Island.
Most conservation islands restrict access to prevent disturbance or the arrival of unwanted pests. But several are open to the public. Kāpiti Island near Wellington and Tiritiri Matangi near Auckland have similar histories. Once cleared for farming, they now have no animal pests, and native forest has been re-established – over 250,000 native trees were planted on Tiritiri Matangi. Both are refuges for saddlebacks, kōkako, stitchbirds, little spotted kiwi and takahē. Not far from Kāpiti is Mana Island, also a bird sanctuary, and noted for its ‘colony’ of concrete gannets, built to tempt real gannets to breed there.
Betty gets her goats
In the 1970s the government decided to kill wild goats and sheep on Arapawa Island, in the Marlborough Sounds. Protesters claimed the animals were descendants of those left by James Cook. DNA tests showed some were genetically unique. Many were culled, but others were sent to farms and wildlife parks. Betty Rowe led the rescue campaign and established a sanctuary on Arapawa Island.
To create areas where people could not disturb the ocean, seabed and seashore, the Marine Reserves Act was passed in 1971. The first reserve was at Goat Island, north of Auckland (now Cape Rodney–Ōkakari Point Marine Reserve). The second was the ocean around the Poor Knights Islands, noted for subtropical fish. Others include islands off the Coromandel Peninsula, Tūhua (Mayor Island) in the Bay of Plenty, Kāpiti Island, Long Island in the Marlborough Sounds, Tonga Island in Abel Tasman National Park, Elizabeth Island in Fiordland and Ulva Island near Stewart Island.