South of Australia, down under and off to the right, if you will, is New Zealand. These islands are so far from any other landmass (Australia is more than 1,200 miles away) that many of the creatures there resemble nothing else on Earth. Conversely, many creatures found virtually everywhere else don't exist in New Zealand. For instance, most mammals.

Or at least, most mammals were nonexistent prior to the arrival of humans about 1000 years ago. "There are no native land mammals in New Zealand except bats," says Jim Becker, a biologist at Pacific Northwest National Laboratory. But beginning with the Maori, and escalating with the Europeans in 1642, humans have introduced furry, warm-blooded milk-makers to New Zealand's ecosystems. And those species have pushed many native birds and giant insects out. But modern members of New Zealand's largest invasive species (that's humans) are woke enough to see the havoc their ancestors wrought. On July 25, the nation's prime minister announced that the country hopes to be free of all invasive rats, short-tailed weasels, and possums by 2050.

New Zealand's purge plan is pretty ambitious. The country is huge, relative to its population. It's also mountainous and forested—basically, full of useful hiding crannies for unwanted vermin. The government committed $20 million to the initial effort, creating a company called Predator Free New Zealand Limited. This is tacked on to the $42 to $56 million it already spends on pesticides, bait, traps, and poison-dropping helicopters each year.

Making this a nation-wide effort circumvents one of the first problems in dealing with invasives: Not everyone wants to play latter day St. Patrick. "One landowner wants them gone, and the next thinks they are kind of cute and lets them stay on her land," says Rick Boatner, invasive species coordinator for the Oregon Department of Fish and Wildlife. "This creates reservoirs for the population to replenish itself all the time." The best counter to this, says Boatner, is public education. New Zealand—where nature organizations teach kids that adorable hedgehogs are serial killers—seems to be doing pretty well on that front.

But even species that don't get public sympathy can be hard to nab. In Boatner's territory in Oregon, one of the most troublesome invasive species is feral swine. "Trouble is with a big landscape, you can work one area pretty well but the species will find a reservoir that they will get back into," he says. In New Zealand, the fugitive rat population is probably the best candidate for this cancer-like regrowth. And like real cancer, there's really no way to deal with it head on. New Zealand's approach is going to involve lots and lots of money and a concentrated, nation-wide effort. "This is probably the most daunting eradication program in the history of the world," says Becker.

And to New Zealand, worth it. The invasive critters kill tens of millions of rare, native birds each year. But the country still has to avoid collateral damage from the campaign. Traps, poisons, and other methods of vermin control can be pretty indiscriminate. New Zealand did numerous studies to make sure the benefit of killing off the invasives outweighed the impact on native species. Plus, eradicators can take extra steps to make sure the bait used in traps and poisons doesn't attract native birds—for instance, by adding cinnamon.

So let's say New Zealand is successful. Thirty-four years from now, native birds and bugs are no longer hunted by, nor do they compete against, rats, possums, and weasels. Huzzah! But like King Théoden, many New Zealanders will still have a threat purring softly by their sides. Domesticated cats—and their feral cousins—are responsible for a sizable pile of dead Kiwi fauna, too.

New Zealand's politicians are too savvy to take this one head on, but local nonprofits are attempting to confront it by urging cat owners to keep their pets inside, walk them on leashes (who does this?!), and—most important—not replace them when they go to kitty heaven. Boatner says this is probably the best approach. "From my perspective, sometimes euthanization is the only solution," he says. "But even us cold hearted biologists can’t do it very often."