Year of the RAT

Jamie Morton turns the spotlight on to New Zealand’s most ambitious pest-control operation, targeting a plague of rodents fuelled by the biggest mast-seeding event in nearly 50 years

The biggest pestbusting offensive mounted in New Zealand will likely blow through its $38 million budget as officials face the reality of a massive plague threatening our wildlife.

Even in the earliest stages of its sprawling operation, the Department of Conservation (DoC) is being forced to reapply 1080 poison in some areas, after finding that even small gaps were leaving enough surviving rats to quickly rebuild numbers.

“The rat numbers are at the other end of what we’d anticipated; we are really seeing high rates in the forests that we’re trying to protect,” DoC’s Tiakina Nga¯ Manu programme manager, Peter Morton, told the Herald.

“I can’t say that it’s worse than we thought — but it’s as bad as we thought.”

THE MEGA-MAST

Behind the plague is the largest mast-seeding event in nearly 50 years. Conservationists have dubbed it a “mega-mast”.

In mast years, trees like the beech, which make up much of New Zealand’s wilderness forest, produce an extremely heavy flowering and seeding.

Long in the past, before pest predator species such as rats and stoats arrived on our shores, mast seasons offered seed-eating birds such as kea and ka¯ka¯ a veritable buffet to make up for those lean years in between.

Other insectivorous birds such as robins and tomtits would have gorged on all of the insects also showing up for the feast.

But today, with enough predators lurking in our bush to lay waste to some 25 million birds each year, mast years spell even greater danger to those native species still hanging on.

The extra food boosted rodent numbers — and in turn stoat numbers. The plague turned on our native birds, bats, lizards and insects after the seed was gone.

In beech trees, mast seeding kicked off when the average summer temperature was more than a degree higher than the average temperature of the preceding summer.

But even as far back as last March, there were signals that this year’s mast would be significant.

Climate data — and later, reports of flowering trees all around the country — confirmed that DoC had on its hands the largest and most widespread event in 45 years.

Directly in the firing line are pockets of threatened mohua, orange-fronted parakeets, rock wren, and long and short-tailed bats.

To combat the threat, DoC has planned a huge programme, stretching over one million hectares, or 12 per cent of the conservation estate, and taking in biodiversity hotspots like Kahurangi, Arthur’s Pass, Westland, Mt

Aspiring and Fiordland national parks, the Catlins and Whirinaki.

The bulk — 900,000ha — is being managed with drops of 1080 poison, while a further 66,000ha is being covered with trapping on the ground.

That was an enormous operational area — even when compared with major programmes undertaken in 2016 (840,000ha) and 2014 and 2017 (600,000ha).

UPHILL BATTLE

DoC’s offensive has been split into two phases; the first, most intensive phase aims to break the back of the rat plague and knock down numbers to nothing, while giving protection to priority species such as mohua.

After that part of the operation wrapped up in December, the second phase, kicking off around February, would mainly target mustelids such as stoats.

But, at only three out of 20 aerial operations in — or about 15 per cent of the overall programme complete — it was clear that gains would have to be hard won.

“We knew we were facing an exceptional challenge this year so started monitoring earlier than usual to find out quickly if this mega mast might require new approaches to bait application,” said Morton. The latest monitoring from the Cobb area in the Kahurangi National Park showed more rats were surviving than had been hoped, with an estimated 20 per cent being left to breed and increase numbers until the seed began to rot in spring.

“We will have also killed possums and stoats in this operation, but we were aiming to knock down at least 95 per cent of rats to best protect our vulnerable native species such as tuke/ rock wren and Powelliphanta snails,” Morton said.

DoC was now reassessing predator control for the rest of Kahurangi National Park, of which the Cobb block was

just one fifth, and was planning to apply 1080 bait more intensively over a smaller area of the park than originally mooted to better protect high priority areas.

Bait drops over the next three target areas — Arthurs Pass, Clinton Eglinton and Te Maruia — would be upped from 1.5kg to 2kg per hectare.

“What we think is going on is the home range of rodents that we are targeting has shrunken right down — there’s so much food in the system that they don’t have to travel far to get what they need,” Morton said.

“There are pockets of survivors that are hunkered down into tiny areas — even just a few square metres.”

In previous operations, these gaps — created by everything from winds moving bait away to wobbles in flight lines, amid all the natural other imperfections of aerial bait drops — wouldn’t have been a problem.

“But it’s not tolerable this time around, so by flying each area twice, we’re just trying to pick up any little gaps that occurred the first time.

“Once we have the results from these operations, we will evaluate and adjust our approach as necessary.”

Morton acknowledged the change in strategy could also mean delays in getting to other areas.

“Some of the jobs, we are thinking about whether we can safely delay them. There are some sites where our main species is kiwi, and although it would be better if we could deliver protection now, it would still be worthwhile if we end up having to wait until late in the season.”

RISING COSTS

Earlier this year, DoC scientist Dr Graeme Elliott warned that some local extinctions may be unavoidable.

Highly vulnerable birds such as orange-fronted parakeet and mohua would likely get the best protection, but another tier, made up of slightly more resilient species such as kiwi, ka¯ ka¯ , kea and whio, would still take a hit.

Last month Debs Martin, from the Pelorus Bat Recovery programme, and Gillian Wadams, from Ark in the Park in the Waitakere Ranges, presented official maps at the Forest & Bird’s annual conference.

They showed the extent of the mast — and those vast tracts of land where 1080 wasn’t being used.

Despite intensive trapping over 250ha in the Pelorus Bridge Scenic Reserve, Martin said, rats had been “pouring out” of the 166,000ha Richmond Ranges, where there was no predator control at all.

Still, DoC was striving for an ambitious, and perhaps unlikely, goal of no local extinctions.

“Our goal has never changed — we are still going to protect as many threatened species populations as we can through this mast,” Morton said.

“It’s just that, unfortunately, we are going to have to work more intensively and pour more resources into it than we would hope.”

DoC now expected to spend more than the $38m it had originally set aside — and the overflow would have to be drawn from an $81m pool earlier allocated over four years by the Government for the wider “Battle for Our Birds” effort.

“While that has given us flexibility, we don’t want to spend everything in one year,” Morton said.

“So we’ll be dipping into more of that $81m total this year, but it means we will do less in subsequent years.”

Forest & Bird has made repeated calls for more funding and resources to deal with this and future masts, and pointed to a mohua wipe-out at Marlborough’s Mt Stokes during a 1999 event as a lesson in what happened when populations were poorly protected.

DoC has told the Herald that it wouldn’t be widening this season’s coverage area.

However, it was planning to expand the area under sustained predator control — where predators were kept to low levels under a regular cycle of control work — from 800,000ha to 1.85 million ha, or to 20 per cent of conservation land, by 2022.

50 MILLION YEARS

This year’s mega mast comes as scientists have estimated it would take millions of years for New Zealand’s birds to evolve to fill the gaps left by those who have gone extinct.

Their study, published in the journal Current Biology, used specially developed computer models to simulate a range of human-induced extinction scenarios.

In New Zealand over the past 800 years, humans and their accompanying pests have brought about the extinction of 32 per cent of indigenous land and freshwater birds, 18 per cent of endemic seabirds, three of seven frog species, at least 12 invertebrates, possibly 11 plants, a fish, a bat and perhaps three known reptiles.

Today, about 1000 of our known animal, plant and fungi species are considered threatened.

Study co-author Dr Juan Carlos Garcia-Ramirez, of Massey University, said while that human impact was well known, no study to date had measured our effect on the evolutionary history of New Zealand.

“This study reveals how conservation decisions we take today will have repercussions for millions of years.”

They ultimately found it would take approximately 50 million years to recover the number of species lost since humans first arrived here.

And if all species currently under threat were allowed to go extinct, it would require about 10 million years of evolutionary time to return to the species numbers of today.

But the study’s lead author, Dr Luis Valente, an evolutionary biologist at Berlin’s Natural History Museum in Germany, said there were some positive takeaways.

“The conservation initiatives currently being undertaken in New Zealand are highly innovative and appear to be efficient and may yet prevent millions of years of evolution from further being lost.”