Once thought safe, South Island's giant wētā are disappearing

Conservationists fear the clock is ticking for New Zealand's "large and charismatic" Southern Alps giant wētā.

A new study shows numbers of the native insect have plummeted by more than 90 percent at historical strongholds high in the mountains - long thought to be beyond the reach of introduced predators.

Recently published in the New Zealand Journal of Zoology, the research was instigated by Wētā Conservation Trust chair Danilo Hegg, an avid outdoorsman, who was bewildered by his own lack of encounters with Deinacrida pluvialis.

"I have been tramping in the Southern Alps for decades... and I started asking myself questions. I said, if this species is not threatened, how come I never come across it?

"The problem was basically that we had one single survey in time done in '92... and because the species was assumed to be doing fine, nobody ever checked again for nearly 30 years."

Hegg and other researchers scoured a site in Westland, finding just three living Southern Alps giant wētā, where 36 of the insects were recorded in 1992.

They then searched a site in Fiordland, where two adult male were recorded in 2023.

Over the course of 36 hours, the researchers did not spot a single wētā - instead recording evidence of rats.

"What was written in the 90s was that this species was safe, because it was above the level of rat invasion," Hegg said. "Basically, they thought rats don't go that high."

Deinacrida pluvialis were unable to fly, too heavy to jump, too big to hide and did not have a strong bite, making them highly vulnerable to predation, he said.

The species was reclassified as 'Threatened - Nationally Endangered' in 2022.

Although there was at least one other known population in Mt Aspiring National Park, it was almost impossible to determine how many of the insects remained across the South Island, he said.

Hegg and other conservationists were keen to figure out where predator control work should be scaled up.

"The population number... eventually it will go to zero, so we do need to do something about it," he said.

Department of Conservation technical advisor Warren Chinn said the "large and charismatic" critters had long fascinated him and other entomologists.

Southern Alps giant wētā were remarkably resilient, slowing right down in winter and surviving under cold alpine rocks for extended periods, he said.

The nocturnal insects could measure up to 5.5cm long, not including their legs or antennae.

Chinn said the new study highlighted a collision between "two profoundly different evolutionary trajectories of animals".

"You've got the rodents and the stoats coming from the northern hemisphere... into Aotearoa, where we have a strongly avian and invertebrate fauna isolated since its break-up 65 million years ago from Gondwana," he said.

"Rare species in isolated lands like our country are extraordinarily valuable, as books of genetic and biological information that represent the diversity of all life on Earth."

Predator Free New Zealand chief executive Jessi Morgan said New Zealand too often lost species faster than expected.

"It's been such a long time between studies and we've seen such a massive decline - and if that study hadn't been done, we'd have no idea," she said.

Morgan said improved tracking technology showed rats, hedgehogs and feral cats all penetrating high-alpine areas above 2000 metres.

'We don't know whether rats are changing their behaviour due to climate change and different snow patterns... or it may have been that rats have always been there, but at levels that are too low for us to detect," she said.