ENTRY to Zealandia is past a checkpoint where all bags and pockets are turned inside out to stop unwanted stowaways, and then through a double gate. Inside the 2.2-metre-high fence is an ancient world, completely unlike the humming urban environment we just left behind. Bird song soon takes over, the tracks narrow and the forest closes in.
We are inside the old water reservoir for New Zealand’s capital, Wellington. Over the past two decades, it has undergone an extraordinary transformation, from urban utility to ecological haven. During the day, large forest parrots called kaka swoop over tuatara, the only survivors of a prehistoric group of reptiles. Night-time visitors have a good chance of crossing paths with a little spotted kiwi. Hihi – small black, white and yellow birds that had once disappeared from New Zealand’s main islands – are flourishing.
What you won’t see are many mammals: virtually all have been eradicated. Mice (and humans) are the only exception and pest control keeps mouse numbers low.
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The sanctuary is a pocket version of something the government would like to see rolled out nation-wide: a step into New Zealand’s rich and unusual ecological past. Apart from bats, all terrestrial mammals in New Zealand are invasive species, introduced by humans in recent times. They pose a real threat to native animals, so in July last year, the country’s then Prime Minister, John Key, announced an audacious plan. , it seeks to rid the country of three major alien pests – rats, European stoats and Australian possums – by mid-century. Nothing on this scale has ever been attempted before. But is it feasible?
New Zealand’s wildlife is particularly ill-equipped to deal with competition from invasive aliens. When it broke off from Gondwana 80 million years ago, mammals hadn’t spread that far. It drifted off into the South Pacific with a cargo of birds, amphibians, invertebrates and a few reptiles.
For millions of years, they evolved without predators, bar a few birds of prey. Evolution veered off in unique directions. Huge cricket-like weta and flightless kiwi scurried around on the forest floor. Giant flightless birds called moa – now extinct – browsed on vegetation like deer. Unfortunately, the process also left these unusual animals with few defences against new competitors and predators.
When humans finally made it to New Zealand, starting 750 years ago, they brought Polynesian and Norway rats, all manner of exotic birds and game like the Australian brushtail possum, imported in 1837 to start a fur trade. The invaders soon got to work. Possums now munch through vast quantities of vegetation each year, prey on nestlings and carry bovine tuberculosis. European stoats, introduced in the 1870s in the hope they might control rabbits, rapidly switched to eating nesting birds and eggs instead.
Ravenous
A single stoat demonstrated the power of mammalian invaders in 2015. Like Zealandia, the Orokonui eco-sanctuary in the South Island is fenced in, but that winter one stoat snuck in. By the time it was caught weeks later, the sanctuary’s entire population of a rare bird called the saddleback was gone.
Predator Free 2050’s initial focus will be to eradicate invasive predators from all offshore island reserves. By 2025, the hope is to have added a million hectares to the 100,000 hectares of existing pest-free land, and to demonstrate that it is possible to keep 20,000-hectare swathes of land pest-free without fences. And by then, the government also wants to have developed the means to remove at least one alien invader entirely.
“If the proposal was to start working now on the single target of total eradication across all of New Zealand, I would say there was no hope,” says Chris Johnson, at the University of Tasmania. “But the concept is scaled in a sensible way, to start with discrete large-island eradications that are highly ambitious but feasible, and move on to still more ambitious projects as new methods are developed.” Each of these steps, he says, would be worthwhile even if total eradication is not reached. And the scale of the overall effort is warranted: “Previous success in eradicating predators has saved quite a few species from extinction,” he says, “but many of those species persist in numbers that are too small for survival over long periods and they have almost no evolutionary potential.”
Piero Genovesi, the chair of the IUCN’s Invasive Species Specialist Group, likens the goal to global efforts to eliminate infectious diseases like smallpox. “There is no technical impediment to achieve the targets of the campaign, if adequate resources are invested,” he says.
Offshore islands and conservation areas and sanctuaries like Zealandia offer test beds for solutions, and the country is building on a long history of successful eradications. Off the coast of Auckland, for instance, is the 1-hectare Maria Island. Invasive rats threatened the storm petrels that bred there until, in 1964, the Royal Forest and Bird Protection Society got a £5 grant to buy anticoagulant poison. Two years after the poisoning operation, the island was rat-free. “Every decade since then, the size of an island that we have been able to eradicate rats from has gone up by a factor of 10. We are on track to do 100,000 hectares successfully,” says Kevin Hackwell, a campaign manager for the Society. “In two more decades we can do the North and South islands.”
“In New Zealand 40% of native birds & 85% of reptiles are threatened or at risk of extinction”
Poison, dropped by helicopter, is standard practice for clearing rats from offshore islands. The poison is incorporated into baits like flavoured cereals, designed to be attractive to pests but not native wildlife. Aerial drops are effective but not an option in populated areas. So on the mainland, good old-fashioned traps and bait stations are frequently used, including by hundreds of pest-busting community groups. They target invasive species in halo zones around fenced sanctuaries and build corridors between protected areas.
In Dunedin, for instance, volunteers hope to establish a pest-free corridor from the Orokonui sanctuary to the Otago peninsula, where residents are using traps that can take out a dozen possums without having to be emptied or reset by people. Once it has removed all possums, the Otago group will run a dense line of traps across the peninsula’s narrow neck to keep them from coming back. The government plans to build similar networks to help achieve the national goal.
Controlling pest populations outside sanctuaries is one thing, but exterminating them is another. The hardest part is finishing the job. For this, New Zealand is going to need new solutions. “We’re thinking about what rats do when they are lonely,” says Al Bramley, CEO of Zero Invasive Predators, a start-up that is developing ways to find and kill mammals. His company is testing a scent-based lure in the hope of attracting lonesome rats. In field tests, they found that hanging 1 gram of used rat bedding inside a trap allowed them to catch 50 per cent more rats than if they use food. At Victoria University of Wellington, ecologist Wayne Linklater is hoping to identify the chemical substances in rat urine that are such a draw to them. His research could then be used to make synthetic lures. James Russell of the University of Auckland and a lead researcher on the government project says biosensors that could detect rare predators better than dogs are another focus.
Elsewhere, teams are turning to the pests’ genomes to develop species-specific toxins that wouldn’t pose a threat to native wildlife. Stoats are notoriously hard to catch and poison. Using their genetic sequence, a team at Lincoln University has developed a stoat-specific poison that stops their red blood cells from carrying oxygen. It was approved for limited use in 2011.
Finding ways to kill pests isn’t the only issue. With any large-scale ecological project there is the chance of unwanted repercussions. Take the effort to remove invasive feral cats from Macquarie Island in the South Pacific in the 1990s. Once the cats were gone, rabbits bred out of control, devastating the vegetation. The subsequent rabbit eradication cost $23 million.
These sorts of links between pests have to be taken into account when deciding how and in what order New Zealand’s predators are wiped out, says ecologist Andrea Byrom, a lead scientist for the project. For example, if you started by only killing possums, you would risk giving rats a boost, since they eat the same plant seeds. And stoats eat rats as well as native wildlife. So if only the rats were taken out, native species would probably suffer. “There are going to be complex ecosystem-level responses,” says Byrom.
Her favoured approach would be to use poisons, traps, lures and genetics to eradicate several species simultaneously. If that’s not possible, she believes it would be best to take a bottom-up approach, removing prey first. “Then you would move really fast to take top predators out as quickly as you can,” she says, “accepting that in the short term you’ll see prey switching, possibly to native species.”
Paul Jansen, of the government’s Department of Conservation, believes the benefits will be seen relatively quickly.”If you suppress pests below a certain level, wildlife and plant life recover, so even if it takes 50 years to eradicate, we’ll start to see the benefits within the first decades,” he says.
Another ecological consequence could well be that some native birds do too well. “We’re already starting to see tui, kaka or pukeko becoming a nuisance to people in urban gardens and horticultural areas,” says Byrom.
The fight for New Zealand’s native species may be won or lost in the backyard. Here, rats are the primary target, and every citizen will have to be on board. Ecologists not involved in the project have called this an enormous and largely unconsidered cloud. Success, Russell concedes, will hinge on a “virtuous social tipping point where trapping in your backyard is considered absolutely normal and there’s something wrong with you if you don’t do it”.
“2-6% per year:
Rate at which kiwi are declining in areas without pest control – fast enough to disappear within a human generation”
It’s a big ask, but cities are on-board. Wellington has declared its own goal to become the world’s first predator-free capital. Neighbourhoods and schools are setting rat traps and there are already a few rat-free corridors. Whether or not that attitude will prevail, only time can tell.
Costing up the kill
Extermination doesn’t come cheap. The New Zealand government has pledged NZ$28 million ($20 million) towards the goal of eradicating rats, stoats and possums from the entire country. An associated joint venture aims to triple that through investments and donations, but that is still only a small portion of what is needed. Ecologists and economists put the overall cost at NZ$9 billion.
But the financial cost of not doing it is also large. Invasive predators damage crops and both possums and stoats carry bovine tuberculosis, itself a target of national eradication at an annual expense of NZ$40 million. The Department of Conservation already spends NZ$20 million each year keeping possums, stoats and rats at bay – more than that during “mast” years, when unusually high beech seed production fuels rodent plagues.
This article appeared in print under the headline “Murder most foul”
