(CNN) — For the past quarter century, a remote corner of southwest New Zealand has provided a predator-free sanctuary for threatened species, including the world’s only flightless parrot and a lizard that’s found nowhere else on Earth.

Chalky Island, a rugged yet lush 2-square-mile outcrop in the Pacific nation’s Fiordland, is home to the endemic Te Kākahu skink, the iconic little spotted kiwi and the kākāpō, the only parrot that can’t fly and of which fewer than 250 are believed to remain in the wild.

So in August 2022, when conservation workers on the island identified a single male stoat, a weasel-like mammal native to Eurasia and North America that preys on a variety of animals and birds, they knew they had to act to save its delicate ecosystem — even if it cost a small fortune.

The country’s Department of Conservation (DOC) launched a major biosecurity response involving trapping experts, dogs, trail cameras, helicopters and boats that took eight months to finally trap and kill the mustelid in what one official claimed as a big victory.

“This is a huge win — but we can’t take our foot off the pedal now,” DOC incident controller Rebecca Teele said in a news release hailing the stoat’s capture last April. “This is one of the highest priority sites for biodiversity in Fiordland and it’s crucial we do everything we can to protect the vulnerable species living there.”

Last month, an annual review published by the New Zealand Parliament’s environment select committee revealed the price of the mission to catch that stoat: nearly half a million New Zealand dollars (about $300,000).

The figure raised eyebrows on social media, with one user on X saying: “I’m all for protecting endangered animals but bloody hell.”

Meanwhile, right-wing pressure group the New Zealand Taxpayers’ Union asked: “What were they using to kill it — missiles?”

But officials defended the cost.

“Inaction would have been more costly — with a potentially devastating impact for our kākāpō population,” said Aaron Fleming, DOC operations director for Southern South Island.

“We would have been faced with flying kākāpō off the island at huge expense. And we have nowhere else to put them. The opportunity cost of not catching the stoat would have been in the millions.”

Invasive predator

Along with fellow mustelids weasels and ferrets, stoats were introduced to New Zealand in the late 19th century to control rabbits destroying sheep pasture — but they have had a devastating impact on the country’s unique birdlife, according to the DOC, implicated in the extinction of several subspecies.

Introduced predators kill about 25 million native birds in New Zealand annually, with about 4,000 native species threatened or at risk of extinction, according to the DOC.

In an effort to protect them, New Zealand has spent more than $300 million since 2016 pursuing its goal of a predator-free country by 2050, CNN affiliate RNZ reported last month.

Under the program, the government plans to eradicate rats, possums (a marsupial native to Australia), weasels, ferrets — and the pesky stoat.

“(The stoat) is a small, energetic and very efficient killer of native bush birds and lizards,” said Carolyn M. King, emeritus professor at the University of Waikato, who has written about stoats and the threat they pose to inshore sanctuaries.

“They are small enough to enter the burrows of rabbits and rats, or even the dome nests of small birds, and sinuous enough to turn round inside.”

They are also capable swimmers.

King said one study of a small group of stoats found nearly half of them swam non-stop “for more than an hour,” implying a “permanent risk of periodic visits or invasions by stoats” to islands within a range of 3 to 5 kilometers from the mainland shore.

That includes Chalky Island.

“It’s possible it swam to the island or hitched a ride on driftwood,” said Fleming, from the DOC, of the now infamous intruder.

Chalky Island and the neighboring Passage Islands have been predator-free since 1999 following the first ever stoat eradication campaign, according to the DOC.

And for Fleming, the recent stoat incursion only underscores the importance of the Predator Free 2050 plan.

“If we eradicate stoats from Aotearoa (New Zealand) completely we remove the costs of incursions, and our wildlife can thrive alongside us,” he said.

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