Eradication Nation

New Zealand has an ambitious plan to wipe out invasive predators. Can the island country pull it off in time to save its iconic native birds? 

Briar Cook and Susannah Aitken crab-walk down a steep slope on the Bottle Rock Peninsula, a 400-hectare (1,000-acre) finger of forest pointing off the top of New Zealand’s South Island. I follow, grasping at branches and roots, trying to stay upright. “Scooch down on your bum,” offers Cook, whose field team has traversed this route hundreds of times before, setting and checking traps. I take her advice, sliding down the dark soil like a kid on a toboggan, wishing I had a toboggan. 

We bob and weave our way through the dense, temperate forest of manuka, tree ferns, beech, and rimu, rewarded with glimpses of the crystalline Marlborough Sounds beyond the shoreline. A ridge runs down the center of the peninsula, one side sloping toward Resolution Bay and the other toward Ship Cove. When Captain James Cook, the first European to trace the contours of New Zealand, arrived here in 1770, the expedition found itself enveloped by what his naturalist, Joseph Banks, described as “the most melodious wild music I have ever heard”—a riot of native birds among the trees.

Today, these forests are largely silent. Aside from the occasional fluting arpeggio of a bellbird (Anthornis melanura) or the chatter of a fantail (Rhipidura fuliginosa) bouncing behind us as we kick up delectable insects, all we hear are the hiss of palm leaves in the wind and the incessant surrender of waves to the shore below. 

Unbeknownst to Banks, the ships that brought him and other European explorers to New Zealand’s bird paradise also brought key agents in its destruction: Norway rats (Rattus norvegicus). While these were not the first rats to colonize the islands—kiore, or Pacific rats (Rattus exulans) arrived with the Polynesians 400 years earlier—they were particularly adaptable. Later, joined by even more voracious ship rats (Rattus rattus) that stowed away with waves of European colonists, the rats made short work of New Zealand’s naive native birds.

Until the arrival of humans and their invasive hitchhikers in the 14th century, the flora and fauna here had lived in a kind of biological Eden, relatively free from tooth and claw. The only creatures eating other creatures were a few aerial predators—harriers, the giant Haast eagle, and bats. With nothing on the ground to fear, many birds evolved to be flightless. When the toothed aliens arrived—first the rats, then Australian brush tail possums (Trichosurus vulpecula) in the 1830s, and finally European stoats (Mustela erminea) 40 years later—the birds didn’t stand a chance. 

In the centuries since, more than 70 native bird species have gone extinct. With more than 26 million native birds and other creatures killed by invasive predators across the country every year, others are headed in that direction. The stakes are almost unfathomably high in New Zealand’s predator wars. The loss of so many species threatens not only the country’s ecological riches, but also its economy. Invasive predators cost New Zealand more than NZ$3.3 billion (US$2.3 billion) in lost agricultural productivity every year. Possums, which can spread a form of tuberculosis to cows, are of special concern to the dairy industry—New Zealand’s largest export. The tourism industry has reason to worry, too: Tourists drawn by the country’s spectacular wildlife and ecosystems, from the Fiordland peaks featured in Lord of the Rings in the south to kiwi reserves in the north, contribute about NZ$35 billion (US$24 billion) to the economy—about 10 percent of the country’s GDP. The Great Predator Invasion is New Zealand’s Silent Spring: a quieting of the cacophony that, in no small part, makes New Zealand New Zealand. But instead of pesticides, the silencer of the birds in the U.S. in the mid-1900s, the killer here is invasive mammals.

My guides on the Bottle Rock Peninsula, Cook and Aitken, are agents of a bold nationwide plan announced by the federal government in July 2016 that aims to save the country’s native species by eradicating its worst offending mammalian invaders—rats, possums and stoats—by 2050. New Zealand is already a world leader in predator control, having waged serious battle with invasive species for decades, so global experts say if any country can pull off an eradication blitzkrieg, New Zealand can. But to rid the entire country of these invaders, even New Zealand will have to step up its predator-purging game. Winning will require better weapons and new strategies, and time is ticking on the Predator Free New Zealand countdown clock.






Southern Brown Kiwi

Northern Brown Kiwi

Okarito Brown Kiwi

Great Spotted Kiwi

Little Spotted Kiwi



New Zealand Kaka

Historically, New Zealand’s predator control has relied on a combination of conventional traps and toxic baits. While those ground-control methods have proven effective in wiping out invaders from small islands and a few fenced sanctuaries on the mainland, they are labor-intensive and expensive to maintain at large scales. Traps must be checked and reset constantly, and bait stations must be refilled. The larger and more inaccessible the area, the more impractical those methods become. Eradication of the millions of rats, possums, and stoats still at large in New Zealand today (no one knows for sure how many there are) will require cheaper, easier, and more efficient tools.


The public-philanthropic partnership that Cook and Aitken work for, Zero Invasive Predators (ZIP), is a key player in developing and testing those tools—from more attractive lures to electronically monitored traps. And Bottle Rock Peninsula, surrounded by water on three sides, with only one way in or out, is an ideal outdoor laboratory for testing the most promising among them. The site is designed for rapid trial and error. Experimental trap lines span the width of the peninsula, from its neck to its tip. The lines contain various types of traps, and the traps contain various types of lures. If a trial trap or lure proves to be of little interest to invaders, it is quickly relegated to the ZIP scrapyard, and replaced by a new or refined model, minted at ZIP’s R&D labs outside Christchurch.

So far today, Cook and Aitken have checked four kinds of pest-control devices. Our first stop is a prototype rectangular wooden box trap, open at each end, that houses two powerful spring traps with lure smeared on them. (The researchers suspect that rats, like people, have varying tastes, so they vary their lures; while many rats go for Pic’s peanut butter, others prefer Nutella.) When the animal steps on the plate to get the lure, a flat metal bar slams down, killing it instantly. “It’s the same mechanism as a standard mouse or rat trap, but with a whole lot more force,” Aitken explains. “You don’t get those kills where it just breaks the back; we don’t want prolonged suffering.”

Several knee-busting skids away, another trap aims to attract rats with a different scare resource: company, in this case, rather than food. It contains a scent lure: a tea strainer stuffed with rat bedding from ZIP’s research facility that gives off an odor of “eau de toilette de rat.” Upslope, we inspect a third type: a metal leg-hold possum trap nested in a wooden platform; the whole apparatus is attached to a tree at about chest height. Its lure is nothing more than a small white plastic square nailed just above the trap, which draws possums like moths to a light. “There’s no food in it,” Cook says, but possums don’t have a strong sense of smell anyway. They are naturally curious about novel objects, though, which may explain why they find the plastic square worth investigating. A helpful new onramp leads from the ground to the trap—a modification that, the team has found, makes it easier for possums to reach the trap. A small electronic node hangs below it, ready to alert rangers when the trap is sprung. Here and there, we encounter a fourth variety: white plastic bait stations nailed to trees at the height of predators’ heads. They’re filled with dime-sized rat pellets laced with a toxin called pindone, accessible via a small trough at the bottom.

We bushwhack to a trap line called D3, one of seven parallel defense lines stretching across the peninsula’s neck. Together, these trap lines and electronic detectors form a virtual barrier between the peninsula and the mainland. Cook stops at a pongo, the Maori name for tree fern. Nailed to the tree a few feet off the ground is a shiny orange-and-black contraption called the GoodNature A24. Powered by a gas-fired piston, it delivers a quick, fatal blow to an animal’s head as it tries to snatch the bait inside. The device can kill 24 rats or stoats with a single canister of gas, requiring fewer of these strenuous, time-consuming trap line tromps, thereby saving on labor costs (and laundry loads). Fastened to the tree just below the trap is a black plastic flowerpot, a 50-cent DIY addition that Aitken had suggested to collect the GoodNature’s bounty in order to measure its effectiveness. Cook peers inside.

“There’s a rat in there,” she says, scrunching her nose. “It’s not pretty.”

I lean in. Down at the bottom of the pot is brown goop, the consistency and color of melted dark chocolate.

“That’s a rat?” I ask.

“There’s two, actually,” Cook says. “Look for the tails.”

I look again. Sure enough, near the side are two pale pink tails. They decompose more slowly than the rest of the body, Cook explains.

Two rats down; countless millions to go. 

New Zealand, like most islands, owes its spectacular biodiversity—as well as its vulnerability—to its 60-million-year isolation. A 2016 paper co-authored by James Russell, a predator expert with the University of Auckland, includes a disquieting statistic: Two-thirds of recent extinctions have occurred on islands—the most biodiverse ecosystems in the world. And that, the researchers wrote, makes islands “critical targets of conservation.”

Few nations have fought as long or as hard as New Zealand to save its native wildlife. The country’s biologists and wildlife managers started doubling down on invasives in the mid-20th century, and by 1963 had managed to eradicate rats from a small offshore island—proving, to the surprise of many, that full eradication was possible. Over the following decades more islands were purged of predators, each larger than the last. By 1988, ship rats were eradicated from the 170,000-hectare (420,000-acre) Breaksea Island in Fiordland National Park, allowing biologists to reintroduce saddlebacks, a small black-and-orange bird with wattles dangling on either side of its beak. Today, that population is growing enough to seed new saddleback populations elsewhere. The 1,965-hectare (4,855-acre) Kapiti Island was cleared in 1996; then came the 3,080-hectare (7,610-acre) Little Barrier Island in 2004 and the 4,040-hectare (9,983-acre) Rangitoto in 2011. In almost every case, biologists owed those successes to traps and the aerial application of a pesticide called brodifacoum. 

Emboldened by the successes on the offshore islands, biologists in the late 1990s began to eye the mainland. The New Zealand Department of Conservation, university scientists, and communities set out to establish “mainland islands” of predator-free reserves, with fences offering the protection normally afforded by water.

The first of ten fully fenced predator-free ecosanctuaries, Zealandia, opened in 1999. The 200-hectaure (500-acre) native bird paradise on the outskirts of the capital city of Wellington is better fortified than some prisons. A 2-meter- (7-foot-) high fence encircles the sanctuary, capped to keep would-be invaders from climbing over the top. To get in, I have to open one gate, close it behind me, then open and close another one before passing through the third and final barricade. The fortification hints at the value of what’s inside: a living example of New Zealand’s biological past—and, if Predator Free New Zealand succeeds, its future.

Brownish-red feathers flash as kaka (Nestor meridionalis) fly from branch to branch; takahe (Porphyrio hochstetteri), purple chicken-like birds with large legs, waddle through the grass near the river. A pair of North Island saddlebacks in a tree fern scold passersby. They represent some of the 18 species reintroduced to the site after the removal of invasive predators through poisoning and trapping; six of those native species hadn’t seen the mainland for more than a century. This, the world’s first fenced urban ecosanctuary, remains the most successful mainland ecological restoration effort in New Zealand to date. The clean austral winter air pulses with the songs of a thousand birds. This, I think as I close my eyes and listen, is the “music” naturalist Joseph Banks must have heard back in 1770.

Zealandia is inspiring backyard conservation throughout Wellington, too. Following the city’s example, all of its suburbs have committed to being predator-free, and all but two have achieved it. Across New Zealand’s many islands, local hardware stores now sell predator traps. Schools host predator-control events. Communities have set up “trap libraries” that lend traps to residents for free.

In the past few years, what began as a bold idea has become a bold plan. Encouraged by an ever-increasing tally of successes, scientists, policy-makers, and communities across the country decided it was time to go big. Following a workshop that explored the potential for a predator-free New Zealand, in 2015, Russell and other leading New Zealand predator experts penned a clear-eyed analysis in the scientific journal Bioscience of what it would take to pull it off, and warned that inaction was not an option if the country wanted to keep its native birds. The next year, in July 2016, the center-right federal government announced the Predator Free New Zealand plan: to rid the entirety of New Zealand of all rats, stoats, and possums by 2050. “This is the most ambitious conservation project attempted anywhere in the world,” said then-Prime Minister John Key. “But we believe if we all work together as a country, we can achieve it.”

During the announcement, held at Zealandia, Key was flanked by ministers from the nation’s conservation, environment, primary industry, business innovation and employment, and tourism departments. “I can’t think of another announcement that would bring together so many ministers,” Maggie Barry, then New Zealand’s Minister of Conservation, told me in July. That was largely her doing; Barry led the charge for a nationwide effort, showing that eradication is possible and worthwhile. “When the Department of Conservation started 30 years ago, everyone said, ‘You’re joking if you think you can ever get the predators off the offshore islands,’” said Barry, sitting in her spacious office in the Beehive, New Zealand’s federal building in downtown Wellington. Since then, more than 100 offshore islands have been deemed predator-free.

At 38 years old, James Russell, born and raised in Auckland, has become one of the world’s leading invasive predator control experts. With a slight build and a mop of brown hair and glasses, he looks a little like a grown-up Harry Potter. Russell’s easygoing manner belies his fierce dedication to reversing New Zealand’s decline in biodiversity. He has his fingers in almost every aspect of the campaign to rid the country of its most dangerous invasive predators: He co-authors scientific papers; he serves as an adviser to ZIP, ecosanctuaries, and community predator-control programs; he oversees efforts to develop new high-tech eradication methods across public and private institutions; and he co-leads a bioethics committee grappling with how best to address public concerns. 

Russell, like others involved with Predator Free New Zealand, acknowledges the challenges they face in achieving the project’s ambitious goals. Rats turn up in the vast majority of traps during surveys of the 55 percent of New Zealand without some kind of predator control. Stoats, now considered predator enemy number one, are hard to trap — they roam huge ranges and are whip-smart and wary of anything new, notes Elaine Murphy, an ecologist with the Department of Conservation and ZIP, who is developing new scent-based stoat lures. Still, Russell says he has zero doubt about where the country will be in 2050. “We can do it, and we will do it,” he says. “We’ve shown we can do it on large islands and in fenced sanctuaries,” Russell says. “We just need to scale it up.”

His confidence comes, in part, from recent advances in eradication technologies. Since ZIP began experimenting with traps and lures in 2015, the rat population on the peninsula has dropped from about 2,000 to between 20 and 50. Those experiments have involved developing evermore attractive baits and traps that have a higher probability of taking out their targets. Self-setting traps like the GoodNature A24 are more accurate and can cut down on labor. GPS-aided aerial application of a pesticide called 1080 now allows for more targeted and efficient use, reducing the likelihood of collateral damage. (Kea and robins sometimes perish after eating poison baits, though it’s rare, Russell says.) ZIP is now testing species-specific estrus lures for stoats, which use the scent of a female in heat to entice males into a trap.

But for the country to slay its invaders nationwide, Russell and most other predator control experts agree that it won’t be enough to poison and trap large areas of the landscape repeatedly. Using current methods, the Department of Conservation can only afford to sustain predator control on approximately 10 percent of conserved lands. Complete removal of invasives will require refined methods and new approaches. ZIP has found that the stragglers—those last few dozen that remain on the Bottle Rock Peninsula—are the hardest to catch, and it only takes one pregnant female to create a new population. Reinvasion from outside the defended zone is a persistent worry as well; at Bottle Rock, even with traps set 10 meters (33 feet) apart across the defense lines, a few rats inevitably make it through.

If the pressure is weighing on Alistair Bramley, ZIP’s CEO and a former manager with the New Zealand Department of Conservation, he hides it well. Encouraged by the rapid advances ZIP has seen in trapping and monitoring in just two years, he believes the 2050 goal is entirely achievable. In fact, some people think the timeline may be too conservative. ”These new techniques for removal are only now emerging, but they’re starting to look more and more feasible,” he says, sitting in the kitchen in ZIP’s offices, which are perched high on one side of the valley that cradles Zealandia. Through the large picture window, its bright green tree canopy stretches out below; kaka and other native birds occasionally fly by. 

Ensuring fewer stragglers are left behind is a current priority, Bramley says. To this end, ZIP is working with Christchurch-based entrepreneur and engineer Grant Ryan to trial “smart” cameras that can detect and home in on particular species. Andrew Kralicek, who heads the Molecular Sensing Team at the New Zealand Institute for Plant & Food Research, is attacking the problem a different way. He’s developing inexpensive biosensors that can pick up the unique chemical compounds produced by a specific species’ urine or feces. “You’ll be picking up a particular biomolecule that your pest releases—like a fingerprint,” Kralicek explains. When the device senses one of these compounds, it will trigger an electrical signal, which is sent via satellite to a computer to pinpoint a rogue predator. Such specific location information could soon aid the ZIP team in targeting aerial applications of 1080 to finish off the few remaining predators in an area, and to prevent reinvasion once it has been cleared. “A year ago, we would not have been able to tell you we’ve got an aerial technique for eradicating possums,” Bramley says. “Now we’re very close to having that technique.”

The biggest technological breakthrough, Russell and other predator eradication visionaries believe, will come in genetics. Tweaking a gene so that females produce only males, for example, could winnow down the females enough to eventually eliminate a population. At this stage, scientists in New Zealand and Australia are developing a possum gene map to pinpoint potential target genes on which to make such a tweak, called gene drive. The technology graduated from fruit flies to a proof-of-concept for mice earlier this year, but it’s still a huge leap from mice to possums or stoats, neither of which have ever been genetically modified. In terms of the technology alone, “it’s probably five years away,” Russell says.

That lag time could be a good thing, though, because this kind of unproven, novel technology must be vetted carefully, says Neil Gemmell, a geneticist at the University of Otago in New Zealand. As gene drive technology has advanced, he has become increasingly concerned that a predator programmed to die out in New Zealand could spread.

If rats and stoats and possums got into New Zealand, they can also get out, he notes. And these species aren’t exotic invasive species everywhere; they have home ranges and niches in local ecosystems that could suffer if self-destructing individuals arrive and spread. “If you’d asked me a year ago, or maybe even six months ago, I would have told you gene drives are great, and they’re probably the best way to control pests in New Zealand,” says Gemmell, who co-authored a paper on the subject due out in the journal PLOS Biology in November. “Now I’d say, gene drives are great, but we need to figure out how to turn them off and how to regulate them.” Otherwise the impacts—both positive and negative—could reach far beyond New Zealand’s borders.

In addition to pushing scientific boundaries and spurring debate among scientists, these genetic manipulations also are likely to test the public’s tolerance. So far, New Zealanders have been largely supportive of traditional eradication efforts. Many people have traps in their own backyards. “There’s this huge growth in support for the work,” Bramley says. In Wellington, he notes, a public survey found that only 1 percent of respondents opposed the rat eradication on a nearby peninsula. But as the technological advances become increasingly high-tech—and in some people’s minds sci-fi—securing public support may prove more complex. “We would never act on this without having absolute social license,” says Barry, the former conservation minister. “And I acknowledge we don’t have it currently. We need to bring the public with us throughout the process.”

To that end, Russell and social scientist Emily Parke, also at the University of Auckland, have convened a bioethics panel whose members include an ecologist, a psychologist, a philosopher, a wildlife manager, and a Maori leader. The panel is drafting a report that will lay out ways to involve the public early and often as things like gene drive are developed. To assess what the public knows about new predator control technologies and how they feel about them, Edy McDonald, who leads the social science team at the Department of Conservation, recently conducted a public survey. Preliminary results released in November show that of the 8,000 New Zealanders who responded to the survey, only 14 percent thought enough pest control was being done. But the use of gene drives and other new technologies to address the problem received mixed reviews. “We really want to have the conversation with the country, so that bumpers can be put in the road if there are any concerns about any of the technologies that come up,” she says.

On a cool, sunny afternoon in the austral winter, Russell and I head to Ark in the Park, a predator-controlled oasis within Cascades Kauri Park, a half hour west of Auckland. This is predator-control season, the time of year when natural food sources are scarce and a hungry rat or possum or stoat is more likely to take the bait. In Russell’s blue Subaru, we rumble along the two-lane road that winds into the mountains, past small bedroom communities and thick forests of pine and palm. Over the whine of the engine, Russell is telling me about the new technologies that will be needed to wipe out New Zealand’s predators—technologies that would finally allow the Ark to live up to its name.

Suddenly he swerves to the right, just missing something small and fast darting into the road from the woods on the left. 

“What was that?” I ask.

“It was a rat—a ship rat, I believe—in the middle of the road,” he says, jerking the car back into the proper lane.

One of the world’s foremost experts on killing rats just saved a rat’s life. And a ship rat at that, the Terminator of New Zealand’s invasive rat species. I can’t help but ask: “Why swerve? Wouldn’t that have been one less predator to get rid of?”

“If I had killed that rat, I might feel good,” Russell says, “but another rat is going to replace it. You’ve taken a life and not added to the bigger picture.” Both ethics and science demanded that he swerve, he explains; if you’re going to kill rats, you have to kill them en masse. “It isn’t just about killing something. It’s about having a plan and making sure those deaths actually create the biodiversity outcome I want.”

When we arrive at Ark in the Park, manager Gillian Wadams—one of only two full-time employees—greets us outside the Ark’s offices, a low-slung weathered wood building next to a golf course. The ethos here contrasts with Zealandia’s, which is evidenced by the fact that Ark in the Park is fence-free. Although the Ark is nestled within protected parkland, it is vulnerable to constant reinvasion and reclaiming its native species remains a struggle. Every year since its creation in 2002, the Ark has relied on a small army of volunteers to purge invaders, growing from 5 volunteers to some 300 today. Trapping is constant. Three times a year, they set out bait; it takes three months to cover all 2,270 hectares (5,610 acres), Wadams says. “The bush here is really, really dense. It can take two hours to go a kilometer,” she says, leading us across the sunny fairway and into the dark woods. By the time one full round of on-the-ground baiting is completed, it’s time to start all over again.

“It’s a bit of a treadmill,” Wadams says. “There are no fences, so we can’t be no-predator, but we can be low-predator.” The Ark’s Sisyphean effort to keep invaders at bay shows the challenge that the Predator Free New Zealand visionaries are up against. At this point, after 15 years of battle, “it’s just about managing reinvasion,” Russell says. Wadams and her volunteer predator defense team have managed to reduce the rat population to the point where they show up in just 5 percent of the detection tunnels the Ark uses in monitoring surveys—numbers non-threatening enough to reintroduce North Island robins (Petroica longipes), which research has shown can withstand low levels of the rodents.

Once the coast is clear, or clear enough, there’s more planning to do to figure out which native species can be reintroduced next, and when. While kiwis are native to this forest, it would be too risky to bring them back; they’re likely to be killed by dogs or cars, Wadams explains. “We’re staying away from the ground-nesting ones, because we’re unlikely to be successful with them.”

Along the trail, past box traps similar to some of the ones at the Bottle Rock Peninsula, Russell eyes something along the trail’s edge and bends down to take a look. It’s a patch of delicate green seedlings. Not so unusual a find along a park trail, but here, where protecting park resources requires a special vigilance, the shoots are small spoils of victory. “These are all New Zealand native palm trees,” he says, running his fingers over the leaves appreciatively. “They have luscious, fat seeds, and if there were rats here you’d see none of that, because rats would eat every last seed.”

April Reese

April Reese is a freelance science writer and editor based in Portugal. Her reporting has appeared in Scientific American, Discover, bioGraphic, Science magazine, Aeon, and many other outlets. She holds a master's degree in Environmental Studies from the Yale School of the Environment.

Rod Morris

Rod Morris has made a career out of telling stories about the natural world. He wasa wildlife film-maker for 25 yearsand co-authoredmore than 30 natural history bookson New Zealandand the Pacific. He now spendsmuch of his time travelling, lecturing, writing, producing field guides, and doing nature photography.He is current Chairman of the Otago Peninsula Biodiversity Group, a trust involved in removing introduced predators from Otago Peninsula, where he lives.

bioGraphic is powered by the California Academy of Sciences, a renowned scientific and educational institution dedicated to regenerating the natural world through science, learning, and collaboration.