Two conservation researchers have criticised New Zealand's mission to clear the country of rats, stoats and possums by 2050, calling current policy "badly designed and unachievable".

But the alternative they've proposed has itself been criticised by the Department of Conversation (DoC) as a "business-as-usual" approach that won't give our species the big boost they need to survive.

The Predator Free 2050 initiative aspired to rid New Zealand's 26 million ha of mainland of the three key pest predators by the middle of the century, while hitting four other targets by 2025.

Those were completing the purge of our offshore islands; driving predators from a million more hectares; showing this could be done across at least 20,000ha without having to put fences up; and making a scientific breakthrough that could take at least one of the big three out of the picture.

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Associate Professor Wayne Linklater, of Victoria University's School of Biological Sciences, and ecologist and Greater Wellington Regional Council senior biodiversity advisor Dr Jamie Steer, argued the mission was essentially based on three flawed assumptions.

Those were that killing pest predators was the best way to protect biodiversity; that this had to involve the extermination of every stoat, rat and possum in the country; and that complete eradication was possible.

Linklater viewed a total wipe-out as "technologically impossible", and maintained that biodiversity was affected more in some places by habitat decline and plant eaters than it was by predators.

Eliminating select predators from complex communities of other plants, animals and humans could also have negative social and ecological consequences, he argued.

"Eradicating some predators will cause populations of other introduced animals to erupt," he said.

"Many people also have valid concerns about the safety and cruelty of predator control methods, and the policy fails to take into account Maori views on predator management as well, particularly on Maori lands."

Linklater worried a mission failure could lead to reduced public and government support for future conservation policies.

He advocated a more varied approach that included building more biodiversity sanctuaries and restoring habitats in the landscapes surrounding them - something he said would create "a network" of populations of endangered species.

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New Zealand could also make cities and farms for biodiversity-friendly by ensuring this factored into development policy-making.

Steer claimed there was "widespread scepticism" of the 2050 policy within the sector, "but it is generally expressed in whispers".

DoC's Predator Free 2050 programme manager Brent Beaven, who was well aware of Linklater's views and had met with him to hear them some months ago, however wasn't aware of any such broad sentiment across the academic community.

Beaven considered their proposed alternative as based on the status quo.

"Doing more of the same is not going to achieve step change growth in protection."

He pointed out the 2050 initiative wasn't simply a policy in isolation, but one of numerous workstreams that needed to act in unison to meet the New Zealand Biodiversity Strategy's goals and reverse the decline of our native species.

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By leading the mission, DoC could roll it out alongside its usual work in weed control and threatened species management.

"Each of these work programmes would fail in isolation. The last few years have shown us that we aren't doing enough to save our native species – we need more than business as usual," Beaven said.

"Predators are one of the key causes of decline of many of our native species and PF2050 addresses this threat head on and lifts our collective sights to aim for more."

That point was emphasised by ecologist Dr James Russell, a globally-recognised expert in island conservation at the University of Auckland.

Russell said Linklater and Steer took "a very conservative approach" toward avoiding risk and uncertainty in policy, "and essentially argue to go back to tinkering around the edges of conservation".

"Concerningly, the work essentially argues for a 'business as usual' approach to predator control which the Parliamentary Commissioner for the Environment has already identified as a pathway to extinction for our native vertebrates."

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READ MORE: Can New Zealand really kill every possum, stoat and rat?

But Beaven acknowledged Linklater had raised some useful points which were now being accounted for in planning.

"We're building in collaboration with other biodiversity protection programmes to take a holistic approach which is needed to achieve the goals and targets of the Biodiversity Strategy," he said.

"Island eradications are a great example of what happens when we selectively remove predator species – these pest free islands are now our biodiversity hotspots."

Further, Beaven said, DoC was well aware that nationwide eradication was currently not considered feasible.

"This is not a weakness, but rather one of the strengths of the programme - by setting the goal early, we have seen strong alignment in science effort that is helping guide rapid progress toward eradication being feasible," he said.

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"In the meantime, we are seeing investment in new tools and techniques that are going to make predator management more effective in the short term."

Manaaki Whenua-Landcare Research ecologist Dr Andrea Byrom, who directs New Zealand's Biological Heritage National Science Challenge, also agreed Linklater and Steer raised some important points for discussion, not least of which was that the proposed impacts and benefits needed to be clearer.

She'd personally been amazed at the way the public had embraced the challenge - and the 2050 vision had been "fantastic" for raising awareness around the plight of our native biodiversity.

"However, as Linklater and Steer point out, now is the time to capitalise on that public enthusiasm by empowering communities to be a part of the discourse as to what we really want New Zealand's biological heritage to be like in future," she said.

"For exactly that reason, in the Biological Heritage National Science Challenge, we've recently refocused our own work around three common goals that we think are achievable for New Zealand."