Rodents and other predators are responsible for the destruction of tens of millions of native birds every year - and scientists say current measures simply aren't able to rid them from our country by 2050.
Rodents and other predators are responsible for the destruction of tens of millions of native birds every year - and scientists say current measures simply aren't able to rid them from our country by 2050.
Wiping out the millions of bird-eating rodents infesting New Zealand’s wilderness – a goal of our 2050 dream - simply isn’t possible with today’s measures.
Yet, as researchers also report in a new study, there are plenty more steps we could be making in the meantime toward a predator-free Aotearoa.
While the country has only four rodent species – the house mouse and the brown (Norway), black (ship) and Pacific (kiore) rats – the devastation they wreak on our cherished endemic fauna and flora is virtually incalculable.
They chomp through seeds, snails, lizards, weta, larvae, fruit, flowers and eat countless birds, from adult seabirds on offshore islands to native species nesting high in podocarp forests.
“Rodents generally are a key cause of impact nationally because they are omnivorous, abundant, ubiquitous, arboreal and recover rapidly after control,” Manaaki Whenua Landcare Research senior scientist John Innes said.
In a recently-published paper, Innes and colleagues set out several major regimes in which rodents might be controlled: island eradications, fenced and unfenced eco-sanctuaries, and large-scale 1080 drops.
The bad news, they said, was that entirely eradicating them from the mainland was currently considered “impossible”, given their bounce-back couldn’t be stopped with most measures.
“Rodents are not especially difficult to control in the sense that they can be eradicated from extremely large islands,” Innes said.
That had already been demonstrated with successful wipe-outs of targeted rodent populations on remote spots like South Georgia Island, Lord Howe Island and, closer to home, our subantarctic Antipodes and Campbell islands.
Our subantarctic Antipodes Islands - home to many colourful seabird species - were recently cleared of rodents in a pain-staking, years-long effort. Photo / James Russell
“The problem is reinvasion or how to sustain low numbers when surrounding land harbours re-invaders.”
Some projects funded by Crown-owned company Predator Free 2050 now instead aimed for “elimination”, where small numbers of animals could remain at a site until they were detected and removed.
Yet there were huge potential gains to be made by applying what conservationists already knew now, he said, instead of waiting for “silver bullets” that might arise with breakthrough technology.
That included targeting different approaches against different rodent species, which, despite their varying behaviour and impacts, tended to be “lumped together” under current control strategies.
On the mainland, the study pointed out that only ring-fenced ecosanctuaries had prevented most reinvasions after eradicating pest mammals – and in some of these, mice frequently remained or returned.
“In all other mainland rodent control regimes, reinvasion is rapid, currently unstoppable, and the key block to achieving a rodent-free mainland.”
Innes said these protected ecosanctuaries had been shown to clear nearly all 14 widespread pest mammals from sites – yet, largely due to their cost, no new ones had been built since 2017.
“They are, I guess, expensive, but what is the value of our declining wildlife?” Innes said.
“Where would you reintroduce our most vulnerable species – to an area made possum-free but where all the other pests persist, or to a fenced sanctuary where a multi-species eradication removed all invasives and therefore the forest recovered?
A kaka at Wellington's Zealandia - one of the country's few fully-fenced eco-sanctuaries. Photo / Wellington NZ
“A cost-benefit analysis of fences versus other regimes has never been attempted to my knowledge.”
The study authors proposed the best path to achieving known benefits from current tools was a network of pest-fenced ecosanctuaries, with surrounding “halos” of less intensive control.
Such a network happened to be suggested by the late Sir Paul Callaghan, who famously declared a predator-free New Zealand as a “crazy and ambitious moonshot” worth striving for.
Other strategies also had some strengths against certain species: for instance, 1080 drops valuably and routinely help knock down ship rat populations at large scales, but their numbers still bounce back quickly after – especially since operations leave fewer possums for them to compete with.
As for island eradications – 86 of our 320 islands larger than 5ha have now been cleared – the study said bigger, inhabited or remote islands remained a “frontier” for rodent-targeted efforts.
Pilot studies are now under way on Waiheke and Aotea/Great Barrier islands – while Predator Free Rakiura has the bold aim of removing all three rats, possums, feral cats and hedgehogs from our third-largest island.
The study concluded by highlighting the need to understand the ecology of target rodent species and their reinvasion risk – and define their management at each site.
It also flagged some systemic problems with New Zealand’s overall approach, pointing out that development and communication of best-practice pest control techniques was “inconsistent”, with advice dispersed among numerous agencies and websites.
Other recent studies have similarly concluded that our predator-free dream sat beyond reach with present measures.
Rats, possums and mustelids are the key target species of New Zealand's predator-free mission.
In another paper last year – and the first strategic assessment of how various predator-busting tools could be deployed across the country at scale over the next 10 to 15 years - the same researchers found toxins could be potentially dropped over more of the country’s land area.
At the same time, however, predator-proof “exclusion” fences were found to be only suitable for about 500sq km – or just 0.2 per cent of the mainland – and 29,000 sq km of pest-invaded land was likely unsuitable for any measures we currently had.
In these typically rugged and remote places, poisoning, trapping and fencing wouldn’t be enough to wipe out pest populations that had grown in the absence of intensive control.
While it singled out some genetic techniques, the most promising ones today might not be so a decade or two from now – and as it stood, the Government currently had little appetite in using them.
All the while, our biodiversity crisis continued to worsen, with the latest Stats NZ data showing more than three-quarters of native reptile, bird, bat and freshwater fish groups are threatened with extinction - or at risk of becoming so.
