The impacts of these introductions were significant. Habitats were modified and some indigenous species were hit hard. A few species were saved from extinction only by their survival on small adjacent islands.
Many common pests of mainland New Zealand, including stoats, ferrets, weasels, red deer, ship/black rats and kiore/pacific rats, were never introduced into the subantarctic. However, a long list of alien animals were.
- Possums were introduced to the Auckland Islands in 1980 but luckily failed to establish.
- Pigs were introduced at the Auckland Islands from the early 1800s as a food resource for shipwreck victims. Goats joined them, once again as a food source for castaways. In fact, goats were released at more islands than any other animal: Auckland, Enderby, Ewing, Ocean islands in the Auckland group, as well as The Snares, Antipodes and Campbell groups.
- Sheep were also common on New Zealand's subantarctic islands where they had been introduced to the Auckland, Enderby, Rose and Adams islands in the Auckland group, as well as to the Campbell and Antipodes groups.
- From 1850 through to the 1990s cattle roamed around Enderby Island, first as castaway food and later as farm stock. There were also cattle at Rose, Campbell and Antipodes islands.
- French Blue rabbits were released at Enderby and Auckland islands in 1840 followed by mixed-breed rabbits at Rose Island ten years later.
- Brown (Norway) rats jumped ship at Campbell Island and mice invaded Auckland, Enderby and Antipodes islands.
- Cats were released at Auckland and Campbell islands, and they reached Masked Island in Carnley Harbour.
Many introduced plants have the potential to become ecological pests on the islands too.
Some of the introduced plants, such as gorse on Auckland Island and dock on Enderby, have been eradicated while other species are being monitored in case they start to spread and become an ecological problem.
This is of special concern with climate change possibly making the harsh subantarctic environment more suitable for a wider range of species.
By the 1980s the presence of alien animals was accepted as inconsistent with the national nature reserve status of the islands and moves were made remove the invaders and restore the islands to what they once were.
- Sheep were systematically removed from Campbell Island in 1970, 1984 and 1990-91. Cattle were removed in 1987.
- The removal of the sheep, and the consequent recovery of the vegetation, had an unexpected spin-off: the cats disappeared, possibly because the vegetation reclaimed the grazed open areas and created a damp and closed habitat unsuitable for cats.
- By the early 1990s all goats had been removed from Auckland Island. Many were transferred to the mainland, as stock of possible genetic benefit to goat farming.
- By the early 1990s rabbits, cattle and mice were cleared from Enderby.
- Norway rats were eradicated from Campbell Island in July 2001 (declared rat free in May 2003). The previous New Zealand record for island eradication was held by the Kapiti Island project. Campbell was more than five times the size.
- Mice were eradicated from Antipodes Island in winter 2016 (declared mouse free in March 2018)
Where introduced animals have been removed, such as on Campbell and Enderby islands, the ecosystems are showing positive signs of recovery. Birds that sought refugee on other predator-free offshore islands are returning, and many of the mega herbs and other plants, once nearly decimated and restricted to inaccessible cliffs, are now widespread.
Looking after these island ecosystems is of immense value to conservation and science. Experience has shown that plants and animals that have evolved in isolation on such islands, in the absence of terrestrial mammals, are highly vulnerable and sensitive to disturbance.
It is important we do not repeat the mistakes of our the past. A new incursion onto the islands could easily and quickly undo the recovery efforts. This is why strict measures are in place to prevent seeds, disease, rodents and insects from reaching the islands. It only takes one seed, or one insect, to establish a new species which could inflict untold damage on these fragile islands.