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Brushtail possums are among the numerous invasive pests regularly culled in New Zealand.
Razza the rat nearly ended James Russell’s scientific career. Twelve years ago, as an ecology graduate student, Russell was releasing radio-collared rats on to small islands off the coast of New Zealand to study how the creatures take hold and become invasive. Despite his sworn assurances that released animals would be well monitored and quickly removed, one rat, Razza, evaded capture and swam to a nearby island.
For 18 weeks, Russell hunted the animal. Frustrated and embarrassed, he fretted about how the disaster would affect his PhD. “I felt rather morose about the prospects for my dissertation,” he says.
Although there was a lot of literature on controlling large rat populations, little had been written about tracking and killing a single rodent, which turns out to be rather important in efforts to completely eradicate a species. “It demonstrated how hard it is to catch that very first rat as it arrives on an island — or, conversely, the very last rat that you’re trying to get off,” says Russell, now at the University of Auckland.
Razza’s escape became the subject of a paper in Nature1 as well as a popular children’s book. And now, with more than a decade of successful pest-eradication projects behind him, Russell is taking on a much bigger challenge. He is coordinating research and development for a programme that the government announced last July to eliminate all invasive vertebrate predators — rats, brushtail possums, stoats and more — from New Zealand by 2050 to protect the country’s rare endemic species.
Joel Sartore/NGC
Stoats (Mustela erminea), along with their mustelid cousins weasels and ferrets, were introduced in the 1880s in an attempt to curb invasive populations of rabbits and rats, which had grown out of control. But stoats don’t just eat rabbits and rodents: they also prey on native birds such as kiwis, as well as on insects and fish, and have been implicated in the extinction of the bush wren and laughing owl.
The audacious plan is not as far-fetched as it sounds, says Josh Donlan, director of Advanced Conservation Strategies, a consultancy that has designed invasive-species eradication projects in Europe, South America and the United States. Around the world, more than 1,000 islands have been cleared of invasive species through ‘mega eradications’. And New Zealand, home to some of the leading experts in the field, carried out more than 200 of them. With enough money, time and political will, Donlan says, it should be possible to clear the entire country.
But the size of this latest target represents a tremendous leap. The largest island ever cleared is Australia’s Macquarie Island, which covers about 128 square kilometres. New Zealand’s total area is about 268,000 square kilometres, and the country’s cities and towns complicate eradication efforts and provide countless places in which animals can hide.
“With current techniques, it’s not feasible,” says Richard Griffiths, an ecologist based in Auckland with the environmental group Island Conservation. To scale up, new approaches will be required.

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New Zealand wants to rid itself of all its invasive pests on a tight deadline. Hear how it plans to do it.
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That’s where Russell and his colleagues come in. They are about to start a major research project to develop some of the necessary technologies, such as new baits, species-specific poisons and genetic tweaks that interfere with animal fertility. To succeed, the project will require public and political support — and money. In a 2015 paper2, the team estimated the entire cost at around NZ$9 billion (US$6 billion), arguing that the savings to pest-control programmes, and the reduction in environmental damage and crop loss, would more than cover the outlay. Their argument has been convincing. “Our government just grabbed that paper, and the surrounding evidence and public goodwill, and announced this policy,” Russell says. “It’s been pretty hectic here ever since.”

A dying wish

New Zealand is a poster child for the havoc wrought by invasive species. For millennia it was an island of small lizards and flightless birds, such as the iconic kiwi. Since land mammals, including humans, first arrived some 750 years ago, the number of species of native vertebrate fauna have nearly halved — at least 51 species of bird have disappeared in that time. Losses sped up dramatically after Europeans arrived in the late eighteenth century.

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The mammalian pests are a drain on New Zealand’s economy. The government spends around NZ$70 million each year on pest-control programmes for animals, and invasive predators cost the country an estimated NZ$3.3 billion a year in lost productivity3. Most of the losses come from agriculture, but government officials also worry about the hit to the country’s reputation as a destination for unspoilt natural beauty. “Last year, tourism overtook agriculture as our biggest revenue earner,” says Maggie Barry, the minister of conservation. “Our environment is what attracts people here.”
Although the environmental and economic arguments had been around for some time, many people credit physicist Paul Callaghan with getting the public to back eradication plans. Callaghan was an eminent scientist and a household name in New Zealand — a sort of Kiwi David Attenborough — writing popular books and presenting television shows about science and innovation. In a public address in 2012, he encouraged New Zealanders to save the nation’s native fauna by eradicating its invasive pests. “It’s crazy and ambitious but I think it might be worth a shot,” he said. The address would be his last; he died of cancer a few months later.
Callaghan’s plea caught the public’s imagination, tapping into a groundswell of support for local conservation programmes designed to protect native birds and other animals.
Joel Sartore/NGC
The Australian brushtail possum (Trichosurus vulpecula) was introduced to New Zealand in 1837 to enable settlers to trade the fur, and quickly spread to most of the forests in the country. It destroys crops and competes with native birds, bats, lizards and insects for food, but will also eat adult birds and bats. It can carry and transmit the agricultural disease bovine tuberculosis.
A lot of the techniques for clearing an island are well established. The standard practice for killing rats and other invaders is to lace bait stations with a poison — usually sodium fluoroacetate, known as 1080, or the anticoagulant brodifacoum — and to spread the poison across the landscape by helicopter. The few animals that survive the chemical onslaught are caught in traps or shot. The active phase of eradication is very quick. It takes just a few days to spread the bait, and within a few weeks all the invaders are gone, says Griffiths. Most of the time is spent on preparation. “You generally have just one chance to get it right,” he says, mainly because of the high cost. “So 90% of the work is planning and logistics.”
In 2011, Griffiths reached the end of a four-year project costing NZ$3.5 million4 to eradicate all invasive mammals from Rangitoto and Motutapu, two inhabited islands with a combined size of 38 square kilometres. After two years of planning and consultations with local people, rats were wiped out in 3–4 weeks; conservationists then moved on in stages to deal with rabbits, stoats, hedgehogs and feral cats. The effort was complicated by the presence of human inhabitants, and by the islands’ proximity to Auckland, New Zealand’s largest city, which provides a deep pool of potential reinvaders.
“The ferry goes there six times a day, with hundreds of people, and boats pull up every weekend,” says Russell. Hitchhiking rats and mice are intercepted about once a year, but the island has remained pest-free for the past five.

New technologies

Tackling all of New Zealand isn’t just about scaling up efforts. “We’re good at killing things,” says Barry, “but we’ll rely on scientific breakthroughs to get us over the line.” Some of the first innovations that Griffiths would like to see are new baits, poisons and traps, as well as tools for detecting invaders. The poison 1080 has been in widespread use since the 1950s and is an effective pesticide, but it can kill game animals such as deer and pigs (which are also introduced species, but not the target of eradication efforts); it also threatens the kea (Nestor notabilis), a native alpine parrot. Many hunters and animal-rights groups oppose use of the chemical, especially when it is sprayed from helicopters.
“Something that targets only rats or mice would be wonderful,” Griffiths says. For possums, Russell and his colleagues plan to sequence the creature’s genome in the hope of identifying targets unique to its marsupial biology.
Traps could be improved by developing devices that need minimal human intervention. A New Zealand company called Goodnature already makes rat and possum traps with a skull-crushing piston that is powered by compressed gas. It can reset itself 24 times (clean-up is provided by scavenging birds and cats). Russell’s colleague Andrew Kralicek is working on wireless electronic biosensors that can detect species-specific molecules given off by a pest. Such devices could be used to monitor traps or send warnings about new invaders.
Joel Sartore/NGC
New Zealand has three invasive species of rat. The Pacific rat, or kiore (Rattus exulans), was introduced from Polynesia in about the twelfth century; the ship rat (Rattus rattus) arrived in the late 1700s; and the Norway rat (Rattus norvegicus) became established in the 1860s. All three prey on native birds, insects and lizards, and have been blamed for the decline or extinction of a variety of species.
And drones, which have already been used to monitor sheep herds in the country, could be fitted with those biosensors to sniff out targets and quickly drop a precise dose of poison. This could be useful in areas where releasing tonnes of laced bait by helicopter is not feasible. “That’s kind of a Skynet future,” says Russell, “but it could work in pest control.”
The ideas that are generating the most excitement in conservation circles are genetic biocontrols that might be able to suppress invaders by introducing harmful traits. The powerful gene-editing tool CRISPR–Cas9 could be used to disrupt a gene that is vital for survival or reproduction or that makes an animal more susceptible to a certain poison. Then, using what is known as a gene drive, scientists could engineer that gene to spread through the population. “It can go from 1% to 100% of the population in around 10 generations,” says Ethan Bier, a geneticist at the University of California, San Diego, who is using gene drives to engineer mosquitoes that are resistant to the malaria parasite5.
So far, gene drives have been used only in the lab and mostly with insects, but there is nothing to suggest that they wouldn’t work in the wild on possums or rats. The problem, says Bier, is that once you start introducing harmful traits, you’re fighting against evolution, which tends to eliminate problematic mutations. There is also the danger of reverse invasions. The possums that have become invasive in New Zealand originated in Australia. If some sort of gene-driven ‘suicide possum’ made its way back there, it could wreak havoc on the native populations.
Another genetic technique, being developed in New Zealand as part of Russell’s project, could avoid some of these difficulties. The Trojan Female Technique targets mitochondria, the tiny power plants inside cells. Mutations in mitochondrial DNA can seriously impair the ability of sperm to swim. Because these mutations affect the fitness only of males, and because mitochondria are passed down only through the female line, these traits can survive natural selection. Females carrying the mutations would have sterile male offspring, but their daughters would be able to breed, producing yet more sterile males.
Daniel Tompkins, an ecologist in Dunedin, New Zealand, and his colleagues have already shown in computer models and lab experiments that this technique can work in fruit flies6: a single release of Trojan females kept population numbers low over ten generations, with no sign of natural selection fighting back.
Of course, when it comes to mammals, releasing thousands of Trojan female rats would be counterproductive: those rats would be just as much of a threat to the ecosystem as the ones you’re trying to get rid of. So Tompkins, who works for Landcare Research, a government research institute, sees it more as a coup de grâce — a way to prevent pest populations from recovering after they have been cut back by conventional techniques. Once the numbers are small, releasing a few Trojan females would cap population regrowth, he says. Those small populations might then simply die out naturally, or survive at such low levels that they would no longer pose a threat to native species.

Back-yard battles

All these techniques are several years away from large-scale deployment, and none is a silver bullet, cautions Russell, who says that the answer will be to use a mixture of methods, staggered over a long period. “What might be the cheapest or most appropriate in the forest won’t be the most appropriate in someone’s back yard,” he says.
And getting access to those back yards will make or break the project, says Donlan. An eradication has to be close to 100% successful for it to work, and that means getting buy-in from almost everyone concerned. If any large groups of people refuse to cooperate with the plan, areas could be left uncleared, providing havens for invaders. “The all-or-nothing nature of eradication makes social issues more important and challenging,” says Donlan. “Support has to be greater than just a simple majority.”
That’s an area where New Zealand is relatively lucky, says Russell. The country is already home to thousands of volunteer community groups that spend their free time setting and checking traps. People in the Wellington suburb of Crofton Downs, for example, think that the region is already free of predators after they managed to get a trap placed in every fifth back yard. “We’re in a relatively unique position in New Zealand, where people are really, really willing to kill for conservation,” Russell says. “It’s kind of a national pastime.”
“People are really willing to kill for conservation. It’s kind of a national pastime.”
Nevertheless, some aspects of the project could test the limits of public support. Biocontrol techniques for mosquitoes, for example, have faced stiff opposition from residents in Florida and Brazil. New Zealanders may be heavily in favour of conservation, but they are generally suspicious of genetic engineering7. And gene-drive technologies are controversial throughout the world.
Then there’s the money. The government and philanthropic groups have committed to donate about NZ$3 billion by the 2050 deadline — well short of the NZ$9 billion that Russell estimates would be needed. But the government hopes that further scientific breakthroughs will bring the cost down.
Russell is sure those breakthroughs will come. He points out that the first rat eradication was achieved on a 1-hectare island off New Zealand in 1963, at a time when no one thought it would be possible. “We don’t know how we’ll do it in 2050, but back in 1960 we didn’t know we’d be doing what we were doing in 1980 or 2010,” he says.
In some ways, it is his experience with Razza that gives him hope. Although the rat was eventually caught in a decidedly low-tech way — a convenient penguin carcass proved to be irresistible bait — the hunt forced Russell’s team to refine cutting-edge techniques that are still in use. For example, biosecurity dogs can hunt down individual hold-outs, and genetic sequencing of faeces can identify remaining populations.
“I am proud to look back and see how far we’ve come in just ten years,” he says.