In the days—perhaps weeks—it had spent in the trap, the stoat had lost most of its fur, so it looked as if it had been flayed. Its exposed skin was the deep, dull purple of a bruise, and it was coated in an oily sheen, like a sausage. Stoat traps are often baited with eggs, and this one contained an empty shell. Kevin Adshead, who had set the trap, poked at the stoat with a screwdriver. It writhed and squirmed, as if attempting to rise from the dead. Then it disgorged a column of maggots.

“Look at those teeth,” Adshead said, pointing with his screwdriver at the decomposing snout.

Adshead, who is sixty-four, lives about an hour north of Auckland. He and his wife, Gill, own a thirty-five-hundred-acre farm, where for many years they raised cows and sheep. About a decade ago, they decided they’d had enough of farming and left to do volunteer work in the Solomon Islands. When they returned, they began to look at the place differently. They noticed that many of the trees on the property, which should have been producing cascades of red flowers around Christmastime, instead were stripped bare. That was the work of brushtail possums. To save the trees, the Adsheads decided to eliminate the possums, a process that involved dosing them with cyanide.

One thing led to another, and soon the Adsheads were also going after rats. With them, the preferred poison is an anticoagulant that causes internal hemorrhaging. Next came the stoats, or, as Americans would say, short-tailed weasels. To dispatch these, the Adsheads lined their farm with powerful traps, known as DOC 200s, which feature spring-controlled kill bars. DOC 200s are also helpful against ferrets, but the opening is too small for cats, so the Adsheads bought cat traps, which look like rural mailboxes, except that inside, where the letters would go, there’s a steel brace that delivers an uppercut to the jaw.

The Adsheads put out about four hundred traps in all, and they check them on a regular rotation. When I visited, on a bright blue day toward the end of the Southern Hemisphere winter, they offered to show me how it was done. They packed a knapsack of supplies, including some eggs and kitty treats, and we set off.

As we tromped along, Kevin explained his trapping philosophy. Some people are fastidious about cleaning their traps of bits of rotted stoat. “But I’m not,” he said. “I like the smell in there; it attracts things.” Often, he experiments with new techniques; recently he’d learned about a kind of possum bait made from flour, molasses, and cinnamon, and Gill had whipped up a batch, which was now in the knapsack. For cats, he’d found that the best bait was Wiener schnitzel.

“I slice it thin and I tie it over the trigger,” he told me. “And what happens with that is it starts to dry out and they still go for it.”

I’d come to watch the Adsheads poke at decaying stoats because they are nature lovers. So are most New Zealanders. Indeed, on a per-capita basis, New Zealand may be the most nature-loving nation on the planet. With a population of just four and a half million, the country has some four thousand conservation groups. But theirs is, to borrow E. O. Wilson’s term, a bloody, bloody biophilia. The sort of amateur naturalist who in Oregon or Oklahoma might track butterflies or band birds will, in Otorohanga, poison possums and crush the heads of hedgehogs. As the coördinator of one volunteer group put it to me, “We always say that, for us, conservation is all about killing things.”

The reasons for this are in one sense complicated—the result of a peculiar set of geological and historical accidents—and in another quite simple. In New Zealand, anything with fur and beady little eyes is an invader, brought to the country by people—either Maori or European settlers. The invaders are eating their way through the native fauna, producing what is, even in an age of generalized extinction, a major crisis. So dire has the situation become that schoolchildren are regularly enlisted as little exterminators. (A recent blog post aimed at hardening hearts against cute little fuzzy things ran under the headline “Mrs. Tiggy-Winkle, Serial Killer.”)

Not long ago, New Zealand’s most prominent scientist issued an emotional appeal to his countrymen to wipe out all mammalian predators, a project that would entail eliminating hundreds of millions, maybe billions, of marsupials, mustelids, and rodents. To pursue this goal—perhaps visionary, perhaps quixotic—a new conservation group was formed this past fall. The logo of the group, Predator Free New Zealand, shows a kiwi with a surprised expression standing on the body of a dead rat.

New Zealand can be thought of as a country or as an archipelago or as a small continent. It consists of two major islands—the North Island and the South Island, which together are often referred to as the mainland—and hundreds of minor ones. It’s a long way from anywhere, and it’s been that way for a very long while. The last time New Zealand was within swimming distance of another large landmass was not long after it broke free from Australia, eighty million years ago. The two countries are now separated by the twelve-hundred-mile-wide Tasman Sea. New Zealand is separated from Antarctica by more than fifteen hundred miles and from South America by five thousand miles of the Pacific.

As the author David Quammen has observed, “Isolation is the flywheel of evolution.” In New Zealand, the wheel has spun in both directions. The country is home to several lineages that seem impossibly outdated. Its frogs, for example, never developed eardrums, but, as if in compensation, possess an extra vertebra. Unlike frogs elsewhere, which absorb the impact of a jump with their front legs, New Zealand frogs, when they hop, come down in a sort of belly flop. (As a recent scientific paper put it, this “saltational” pattern shows that “frogs evolved jumping before they perfected landing.”) Another “Lost World” holdover is the tuatara, a creature that looks like a lizard but is, in fact, the sole survivor of an entirely separate order—the Rhynchocephalia—which thrived in the early Mesozoic. The order was thought to have vanished with the dinosaurs, and the discovery that a single species had somehow managed to persist has been described as just as surprising to scientists as the capture of a live Tyrannosaurus rex would have been.

At the same time, New Zealand has produced some of nature’s most outlandish innovations. Except for a few species of bats, the country has no native mammals. Why this is the case is unclear, but it seems to have given other groups more room to experiment. Weta, which resemble giant crickets, are some of the largest insects in the world; they scurry around eating seeds and smaller invertebrates, playing the part that mice do almost everywhere else. Powelliphanta are snails that seem to think they’re wrens; each year, they lay a clutch of hard-shelled eggs. Powelliphanta, too, are unusually big—the largest measure more than three and a half inches across—and, in contrast to most other snails, they’re carnivores, and hunt down earthworms, which they slurp up like spaghetti.