Danielle Shanahan, Zealandia’s CEO, shows a visitor the innovative fence surrounding the ecosanctuary in Wellington, New Zealand.

Aren Elliott

A single mouse redefined the borders of Zealandia, a 556-acre ecosanctuary in Wellington, New Zealand.

Researchers testing prototype fencing watched a clever rodent wrap its tail around a screw, swing its body and pull itself over the top. Decades ago, that display of gymnastics helped engineers fine-tune the 5.3-mile engineering marvel that now encircles Zealandia.

Today, a fine-mesh fortress topped with a curved steel top protects Zealandia, a design proven to repel even the most athletic intruder.

“The story about how it was built is just mind-blowing,” says Danielle Shanahan, Zealandia’s CEO.

Here’s what success sounds like in Wellington

This experiment—the world’s first fully fenced urban ecosanctuary—is an attempt to turn back the clock to a time before mammals arrived.

For more than a century, the valley just above Wellington’s capital supplied the city’s drinking water. Before that, the land was scarred by unsuccessful gold mines and stripped for farmland, leaving the slopes bare. It was a place managed by civil engineers that accidentally preserved a pocket of land long enough for a bigger vision to take root.

Zealandia opened 26 years ago. For the roughly 140,000 visitors who trek to the sanctuary annually, it offers a surreal dislocation: the chance to spot a dinosaur-era Tuatara reptile or a flightless kiwi just a short cab ride from the city’s best coffee shops.

But for the scientists involved, the fence represents more than a way to protect one of the region’s biggest tourist attractions. It’s also a line in the sand, evidence that if you build a wall strong enough, the wild will return.

Wero, a German shorthaired pointer tracking dog, with his handler Brad Windust. The pair search for signs of unwanted stoats and weasels in Zealandia.

Scott LangdaleHow Zealandia purged itself of mammals

Getting here required a complete eradication of 15 different mammalian predators introduced by people. New Zealand’s wildlife evolved in the absence of mammals, with only two species – bats and seals – native to the island nation. This means they are particularly susceptible to predation and the browsing habits of introduced mammals.

The eradication efforts included fish. Shanahan, who ran the eradication project before becoming CEO, helped drain the lower lake by 20 feet, turning the floor into a gray moonscape to remove invasive perch.

They applied Rotenone, a toxin derived from the roots of a South American tree, which stops fish from metabolizing oxygen.

“It’s like applying a nuclear bomb,” Shanahan explains, recalling the intensity of the process. But nature is efficient; within hours, seabirds moved in to clean up the dead fish, and within a day, the lakebed was clear.

Conservationists carried out a tactical battle on land to clear out thousands of possums and rats, goats, deer, stoats, and eventually one final hedgehog, nicknamed Colonel Custer, who held out alone in the bush avoiding traps until he was finally caught.

A prehistoric soundtrack rises in the ecosancturary

Inside the perimeter, the din of the big city gives way to a prehistoric soundtrack.

Shanahan grew up in the region and recalls seeing perhaps 20 Tūī, or parson birds, in her entire childhood. Today, the valley is teeming with them. The air is thick with the complex, bell-like melodies of the Tūī and the screech of the Kākā, a large, olive-brown forest parrot with flashes of crimson under its wings that was once completely gone from the region.

“The soundscape has completely changed,” she says.

It’s a sensory assault that signals the health of the forest. The birds have become so boisterous that they now chase mallard ducks, a behavior the staff watches with amusement.

Volunteers patrol the perimeter with military precision. The 7-foot-tall fence has a skirt that also extends horizontally underground. Rabbits will dig to the fence line, hit the skirt, and stop, lacking the intelligence to back up and try again. When a breach is suspected—perhaps a tunnel rotted out by old pine roots—Shanahan calls in the cavalry. Specialized conservation dogs, trained to ignore native birds and fixate on the scent of rodents or stoats, sweep the valley before it can establish a foothold.

Silver Fern, also known as Mamaku, is a common feature of New Zealand indigenous forests.

Aren ElliottFaking ancient history in Zealandia

The sanctuary still requires human intervention. Because the forest is regenerating from cleared farmland, many of the trees are only about 30 years old—too young to have the natural hollows birds need for nesting.

“We’re working to re-indigenize the space,” Shanahan explains.

In some areas that involves drilling holes or simulating storm breaks to invite rot, fungi, and insects, in the non-native pine trees. It’s a necessary paradox: vandalize nature to accelerate its wildness, forcing the young forest to act like an ancient one. Until the trees catch up, the staff maintains about 60 nest boxes to support the breeding populations.

The Takahē miracle chick

The safety of the valley allows for biological anomalies that would be impossible elsewhere. The Takahē, a flightless, prehistoric-looking swamp hen once thought extinct, wanders the tracks near the lake. The birds are large, vividly blue and green, with beaks that look like a pair of red pliers.

Recently, the sanctuary witnessed the birth of what it called a “miracle” chick. A pair of resident Takahē, thought to be too old or infertile to breed, suddenly produced an offspring.

“No one expected it,” says Shanahan.

Another prehistoric resident is the Tuatara. Often mistaken for a lizard, it is actually the last surviving species of a distinct order of reptiles that walked the earth with dinosaurs.

“It’s the only one of its kind in the world,” says Shanahan.

These creatures, with their spiny backs and slow metabolisms, bask in the sun, living artifacts of a world that existed before mammals evolved.

Kākā parrot sitting in a tree branch at Zealandia, an ecosanctuary in Wellington, New Zealand.

Aren ElliottAn urban spillover of birds

The wall works so well that Zealandia has become a pump, pushing rare species out into the capital. The kākā parrots have multiplied so successfully—starting from just 14 birds—that they now fly over the fence into the suburbs.

“You can see them in the tree opposite Parliament,” Shanahan says. But this migration exposes them to a new danger: urban affection. Well-meaning residents often feed the charismatic parrots seeds and nuts, human foods that cause metabolic bone disease. The birds become “junk food junkies,” according to Shanahan, suffering from nutritional deficiencies because they are too loved by their human neighbors.

Her vision is a city where the fence is eventually obsolete. She is currently advocating for a cultural shift in pet ownership, pushing for a future where domestic cats are kept safe at home. It’s a challenge for a nation of pet lovers that is used to roaming cats, but she draws a parallel to dogs.

“We keep our dogs on leashes, and close within our care” she says. “I think that’s the pathway that we’re on for cats as well.”

In Wellington, a human prescription

The restoration of the valley is not just for the wildlife. Shanahan, who also researches the connection between nature and human health, views the sanctuary as a medical necessity. Her research suggests there’s a minimum dose of nature required to reduce blood pressure, anxiety, and stress.

“You don’t even need to go into the most natural of environments,” she says. “Just a simple urban green space can help.”

For the 550 volunteers who check the fence and traplines, monitor the birds, weed, plant, maintain the tails and bridges, help with after-hours tours, share information with visitors and a host of other invaluable tasks, the sanctuary is a place of optimism in a world of bad news. It is a place where tangible progress can be measured in the volume of birdsong.

Perspective of the valve tower at the Karori reservoir dam.

Aren ElliottA park with a 500-year plan

Despite the visible success, the sanctuary sits on precarious ground. Shanahan surveys the earth dam, looking across the water.

“We’re standing on one of New Zealand’s major faults right now,” she points out. The Wellington fault line runs directly through the lower lake.

If a massive earthquake were to strike, the fence could be compromised, and the 5.3-mile barrier that keeps the predators out could fall.

Yet, Shanahan operates on a timescale that dwarfs seismic cycles. She works toward a 500-year plan. That is the time required for the original canopy of rimu (red pine) and rātā (a massive flowering tree) to return to its full glory. She knows she will not see the end result. She is merely one of the architects of the beginning.

“We can do really difficult things if we have a bit of vision,” she says.

Above her, a kākā launches itself from a pine tree. It ignores the engineering of the fence, crossing the perimeter without a glance, flying toward the city skyline.

The mesh may stop the mice, but these birds are Zealandia’s the real owners.