Image: New Zealand Department of Conservation

It’s not often conservationists celebrate the mass killing of animals, but managing our relationship with nature these days requires creative, and often compromising, solutions. Such is the case in New Zealand, where leaders are determined to get rid of all rats, possums, stoats, and other invasive predators by 2050. A major step towards this goal was just achieved on one of the country’s subantarctic islands, where hundreds of thousands of mice have officially been eradicated.

New Zealand’s subantarctic islands include five groups of islands south of the mainland that have collectively been designated a UNESCO World Heritage Site. They are remote, undeveloped, and known for their unique biodiversity, especially when it comes to seabirds.

At nearly eight square miles, Antipodes Island is the largest in its group. Until recently, it was overrun a single only mammalian predator—some 200,000 mice. The mice were devouring invertebrate species and plant material, and potentially preying on bird chicks and eggs.

According to New Zealand Department of Conservation project manager Stephen Horn, eradicating those mice was one of the most complex efforts of its type anywhere in the world to date.

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“Mice had never been eradicated on this scale before, so the international community was watching the process very closely,” he told Earther. Horn said the remoteness, challenging weather, and minimal island infrastructure made the project especially hard, as it could only be achieved through an aerial baiting operation involving poison delivery from helicopters.

“Any other method risked leaving mice behind and with eradication, rather than control, the overall goal, we needed to be sure we could get every last individual,” he said.

Horn said that New Zealand is home to more species of seabird than any other country in the word, and that the Antipodes alone provide important breeding grounds for more than twenty-one species of seabirds that feed in the Southern Ocean encircling Antarctica.

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According to New Zealand Conservation Minister Eugenie Sage, the success of the five-year project will help more than 150 species of insects, 21 uncommon plant species, and four endemic land birds to thrive, along with the 21 seabirds.

The Antipodes island group is the fourth of New Zealand’s five subantarctic island groups to be declared predator free after a number of successful pest eradication efforts. According to The New Zealand Herald, “more than 100 of New Zealand’s 220 large islands are now pest-free.” Auckland Island, where pigs, cats, and mice all live, is the only subantarctic island with mammalian pests left—and which will present the most daunting extermination effort yet.

Graphic: Wikimedia Commons

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Dr. James Russell, a conservation biologist at the University of Auckland known for his work on remote islands, told Earther that mice are particularly hard to eradicate due to their small size, and in turn, high density, making it especially hard to make sure every last one is killed, “the cardinal rule for achieving eradication.”

He said while mice don’t necessarily have huge impacts on large animals, they can drive entire species and communities of invertebrates to extinction. Mice had already wiped out two taxa of insects from Antipodes Island.

“Hence, in order to guarantee complete restoration of entire island ecosystems, and not just large birds, we have to work that little bit harder to eradicate even the mice,” he said.

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Russell explained that the aerial baiting was achieved by spreading “off-the-shelf rat poison” consistently across every mouse stronghold on the island, even if that meant having bait placed every five or ten meters.

“This is achieved through precision GPS delivery from helicopters, which minimizes any toxin entering the marine environment, and has very few side effects on other animals because the poison is most strongly acting on land mammals, which aren’t normally found on islands,” he said.

Russel said the Million Dollar Mouse project, which is monitoring the recovery of the Antipodes ecosystem, has already noticed positive signs, such as a large increase in snipe calling—a shorebird—and many more pipits, a smaller bird that previously competed with mice for insects.

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An adult Antipodes Island snipe.
Image: New Zealand Department of Conservation

Russell said that the successful demonstration of mice eradication from a large islands clears the way for similar undertakings at other islands, such as Gough Island in the South Atlantic, where mice are demonstrably driving seabird species towards extinction.

The Million Dollar Mouse project was launched in 2014 to help fund the necessary eradication efforts. It has drawn in a number of partners including the New Zealand government, the World Wildlife Fund, and Island Conservation.

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Sally Esposito, with the non-profit Island Conservation, told Earther that island ecosystems are some of the vulnerable environments on the planet, and the sites of around three-fourths of known animal extinctions.

“Nearly half of our world’s most threatened species are found on islands, with invasive species as a primary threat,” she said, noting a recent paper by Island Conservation and partners that found that removing invasive species—including rats, mice, goats, pigs, and feral cats—from islands could protect 41 percent of all highly threatened species and 95 percent of all island species.

Update: A previous version of this post included a statement from an outside source that was not properly attributed. The text has been updated to clarify the source of the language and properly attribute it.