The natural born killers lurking in our forests

by Rebecca Macfie / 05 December, 2016
Beech forest in Fiordland National Park with a deadly stoat. Photo/One Shot; Getty Images

The tiny killers are still in their dens, suckling on mothers’ milk and primed for a season of slaughter.

In an abundant “mast” year such as the current season, each adult female stoat will have given birth in October to eight or 10 – perhaps as many as 15 – healthy kits. Within the babies’ first weeks, while they’re still blind, deaf and immobile, an adult male will have visited the den and impregnated all the female young.

By Christmas, they will be weaned and marauding close to the den, preparing for life as independent carnivores at the top of the perverted forest food chain. By January, they will be on their own, fanning out across large distances; in the Eglinton Valley in Fiordland, a tagged young female stoat was once found to have travelled 65km in less than a month.

Their entry into the ecosystem will mark a greater than fivefold increase in the stoat population. No source of protein will be safe from these athletic, intelligent and voracious hunters.

“First, they eat the kiwi chicks,” says Department of Conservation scientist Josh Kemp. “Take a Haast tokoeka – just beautiful prey for a stoat. They leave a big trail of scent along the ground. The stoat doesn’t have to climb a tree to get it. The kiwi’s natural response [to threat] is to freeze, and that’s 300-400g of meat for a 200-300g stoat – enough to last a few days.”

Up to 60% of kiwi chicks are eaten by stoats, but they take prey of all sizes: hole-dwelling birds such as mohua and kakariki, ground-feeding robins, bats, lizards, frogs and weta, as well as the mice and rats that are at plague proportions in this year of plenty. “The young stoats will just try everything,” says Kemp.

Larger birds such as kea, kaka and whio will be largely off the menu – until later. They will become prey when the onset of winter causes the rat and mice population to crash, depriving the stoats of a large source of nutrition.

“That’s when we see adult kea and even takahe being taken on,” says Kemp. “Kea and whio nest in August, and the stoat plague cuts their productivity to zero. We have footage from cameras inside a kea nest where she lays an egg, and the stoat nicks it; she lays another, and the stoat nicks it again … Blue ducks [whio] are the same. And for both those species, we also see an increase in adult mortality. During stoat plague events, takahe weighing 1.5 or 2kg have been taken.”

A possum tucking into fallen fruit. Photo/Alamy

Cycle of devastation

The stoat plague that is about to irrupt through our forests during the summer of 2017 will mark the final phase of a repeating cycle of devastation that takes two years from beginning to end. It kicks off with a heavy spring flowering of beech trees (called a mast, derived from the Old English word “maest”, meaning the nuts and seeds on the forest floor), producing an abundance of high-protein seeds and pollen, which supports fecundity among endemic birds and invertebrates, as well as introduced predators.

Mast years have been thought to occur every four to six years, although the 2016 mast has come just two years after the previous one, and there are signs that 2017 may be another. The years 1999 and 2000 brought two masts in succession.

From the January following a heavy spring flowering, rodent numbers explode. With rats and mice producing litters of six to eight, and young females breeding from about three months and yielding up to four litters a year, the population grows by 1.2% every day during a mast, says Kemp.

The forest floor remains a rich larder of nourishing seeds throughout a mast winter such as 2016’s, allowing continued exponential growth in numbers of rats and mice. But in late spring, the seeds germinate and the protein source vanishes.

“The rug is suddenly pulled out from under them,” says Kemp. “They’ve been living in the lolly jar all year, breeding up, living at very close quarters. Then suddenly – bam! – within a few weeks it’s all over.”

As plenty turns to want, “everything is at risk”, he says. The rodents’ highly structured social system comes under acute stress, with evidence of rats fighting each other for resources. “At that point, the rats stop breeding, but they don’t all die. So you have a population of big hungry rats … and this coincides with when a lot of birds are laying eggs. Robins, fantails, tomtits all start breeding around August and September. The mohua start nesting in late October and November, so they get an absolute pasting. Kakariki are nesting in their knot holes right through this, so they are heavily preyed upon.”

Also on the list of endemic species that are suddenly acutely vulnerable to ravenous rats are the brown creeper, grey warbler, kokako and long- and short-tailed bats.

A possum embryo in its mother’s pouch. Photo/Alamy

Zero productivity

Without human intervention to knock back rodent numbers, they cut like a scythe through native populations. “With no control, in a high-magnitude rat irruption, you can expect zero productivity. All the nests will be preyed upon, and you can expect some loss of adult birds,” says Kemp.

“A 20-50% loss of total population, or more, can be expected. In 2006, we left a portion of the Dart-Routeburn mohua untreated for scientific reasons. It basically went extinct in one season, as did the Eglinton mohua population when we left it to face the rat plague of 2001. In 2009, we had a partial mast in the Dart-Routeburn and left the whole mohua unmanaged, thinking the rat irruption would be relatively minor. We lost about half the mohua. We don’t need to repeat those lessons any more.”

And while the rats are satisfying their appetites with little birds and eggs, the female stoats – well-nourished on their plentiful mast winter diet of rats and mice – are giving birth to bumper litters, which will soon emerge to take their place in the cycle of destruction.

Jan Wright, the Parliamentary Commissioner for the Environment, described the modern-day mast phenomenon in a 2011 report as “death in a time of plenty”.

She described it as a “tragic irony” that the conditions to which our native species have evolved over tens of millions of years to boost their birth rates are the very conditions in which introduced predators commit their worst slaughter. The breeding patterns of many native birds have evolved around the mast cycle – the kaka and the kakapo will breed only in a mast year, and other species such as kakariki will triple their usual breeding effort in mast years because of the greater availability of food.

But evolution has been hijacked by the invaders: what ought to be a season of preservation and expansion for our wildlife has been transformed into a fast track to extinction.

Abel Tasman National Park: over the next 30 years, $25 million will be spent to restore its ecology. Photo/Getty Images

Bad news

The good news is that the mast phenomenon – particularly in South Island beech forests – is increasingly well understood. In 2012, scientists developed a model that helps predict mast years, which appear to be triggered by a summer that is a degree or so warmer than the previous summer, says DoC scientist Graeme Elliott.

But there is plenty of bad news, including the fact that climate change is bringing gradually increasing temperatures. Therefore the chances of one summer being hotter than the previous – triggering a mast – are also increasing.

And although ecologists’ understanding of mast-year devastation has increased, enabling more strategic aerial 1080 drops to cut down rodent and stoat plagues (the carnivorous stoats die when they eat the poisoned rodents), only a small fraction of the country is under any kind of systematic predator control.

Battle for Our Birds, the campaign launched by DoC to push back the plagues of invaders in the 2014 mast year, signalled a more focused approach to predator control by the department. It has been further expanded this year to cover 840,000ha, up from 694,000ha in 2014.

But that’s less than 10% of the total DoC estate, and Kemp says only a quarter of South Island forests with masting beech trees will receive any kind of predator control this year.

The department’s inadequacy at holding back the tide of extinction has been well-documented, with the annual death toll from stoats, rats and possums put at 26.6 million chicks and birds.

Wright, who researched the use of 1080 in 2011 and concluded that the department needed to use much more of it, was shocked to discover how little pest control was going on. In a follow-up report, she noted that the department allocated more funding to researching 1080 and its alternatives than it did to pest control using the toxin.

Without DoC’s concerted action, she foresaw a grim future in which surviving fragments of our unique biodiversity, which evolved in isolation over 80 million years without the presence of predatory mammals, would exist only on offshore islands and behind costly mainland sanctuary fences. Our national icon, the kiwi, was declining in areas with no pest control at between 2% and 6% a year, she reported – fast enough to be gone within a generation.

New Zealand conservationists and scientists have won global renown for bringing species back from the edge of extinction – among them the black robin, takahe and kakapo – yet this country has the dubious distinction of having one of the highest proportions of threatened species in the world.

And while native ecosystems continue to be devastated by predators, DoC – the country’s biodiversity guardian – has itself lived through waves of restructuring, shrinking research spending and the loss of core scientific expertise, including threatened-species and pest-eradication specialists. Under successive DoC management regimes, “science was simply seen as getting in the way of the politics”, says former DoC biodiversity expert Theo Stephens, who quit during the sweeping 2012 restructuring overseen by former director-general Al Morrison.

Few people exemplify the flimsiness of DoC’s thin green line of defence against invasive predators as vividly as park ranger Evan Smith. Working at Lake Mackenzie Hut on the spectacular Routeburn Track, he despaired at the damage stoats were doing to the birdlife in his area, which received no DoC predator control. So he started putting out his own traps, and in his evening hut talks he would explain the problem to trampers and ask for donations to buy more traps.

Four years since the start of Smith’s desperate campaign, 2000 trampers have opened their wallets to help out, his trap line extends 20km, and the department’s spin doctors have cast him as a backcountry folk hero. DoC director-general Lou Sanson credits him as a great storyteller, but admits he was “embarrassed” to find when he took on the department’s top job in 2013 that “we are begging for people to help bring back our birds”.

“Super-lure” quest: Tim Sjoberg (left), Tom Agnew. Photo/Martin Hunter

Allies in high places

Now, after years of being pushed closer to extinction by invading predators – defended by an earnest home guard of volunteer conservation groups, overstretched DoC front-liners and agricultural contractors targeting TB-carrying possums – our endangered species suddenly have allies in high places.

In July, Prime Minister John Key and four Cabinet ministers – including Economic Development and Associate Finance Minister Steven Joyce and Conservation Minister Maggie Barry – trooped up to Zealandia wildlife sanctuary in Wellington to declare that by 2050 the country would be “completely free” of rats, stoats and possums.

The announcement came out of the blue and received massive international pickup, with coverage in outlets ranging from the Economist and Al Jazeera to Vice News and Time.

But with an upfront Government investment of just $7 million a year over the next four years, is it to be taken seriously? In a recent New Zealand Journal of Ecology paper, former Landcare ecologist John Parkes, who has worked on pest-eradication projects here and around the world, calculated the price tag of ridding the country of every last rat, stoat and possum at $32 billion, based on the per-hectare cost of clearing pests off Rangi­toto and Motutapu islands in Auckland’s Hauraki Gulf.

Others have come up with lower – but still enormous – numbers: a paper by scientist James Russell in BioScience put the likely cost at between $5.5 billion and $26.2 billion in 2013 dollars. Tracking down and killing just three stoats that had made their way on to pest-free Kapiti Island in 2010 cost $600,000.

So, was the big Zealandia announcement just a cynical distraction from a winter of discontent that saw the Auckland housing market spiral out of control, with daily stories of families living in cars?

And if there is a genuine interest in saving our extraordinary biodiversity, from where did this unexpected rush of interest and enthusiasm come?

Sanson says the declaration of the Predator Free 2050 vision is the product of “a thousand small stories coming together in a wave. The country was heading down a wave, and the Government has come on to the wave.”

In Sanson’s telling, that “wave” began in 1988 when a group of then-Wildlife Service rangers eradicated a 170ha Fiordland island, Breaksea, of rats. “Then by 2002, we had done the largest island eradication in the world, Campbell Island [the remote 11,300ha island had been infested with up to 200,000 rats] … So in the space of 14 years, we had become world leaders, and there is scarcely an island eradication operation in the world that doesn’t have an ex-DoC ranger or a Kiwi helicopter pilot.”

The stoat enclosure at Lincoln University. Photo/Martin Hunter

Volunteers’ efforts

The concept of fenced mainland islands – exemplified by Zealandia and the Sanctuary Mountain Maungatautari reserve in Waikato – also started building momentum, and community trapping groups sprang up in their hundreds throughout the country. Friends of Flora, which formed after the devastation of the 1999 beech mast in Kahurangi National Park, is one of the oldest such groups and now traps 8000ha of the 452,000ha park. The volunteers’ efforts have restored the whio population in Flora Stream, the group are six years into a project to relocate great spotted kiwi into the area, and they have managed to hold the line against further loss of bird species.

More recently, business and philanthropic interests have been starting to come to the defence of threatened biodiversity. One of the most persistent voices for action was Les Kelly, a New Zealand businessman who returned home after years of living in Australia and was dismayed to discover the forests had fallen silent during his decades away. He began hammering on the doors of movers and shakers and, according to one of his key allies, DoC scientist Paul “Scratch” Jansen, helped build a “groundswell of key influencers”. Among those he rarked up were Forest & Bird, which in turn hosted a 2012 gathering of ecologists and predator experts from DoC, regional councils, Ospri [formerly the Animal Health Board], universities and Landcare Research.

By morning tea time on the first day of the meeting, each of the predator experts had agreed it was technically (if not financially or socially) possible to rid the country of invasive pests. “It was a wonderful moment,” recalls Kevin Hackwell, Forest & Bird advocacy manager. “You saw light bulbs going on around the room. Up until then, everyone had been in the control paradigm on the mainland – that pest control will be forever, that we’ll get better at it, but that it will be this constant problem that we will always be dealing with.”

Serious money

The notion that New Zealand could do better than waging an endless losing defence against the invaders was articulated most powerfully by scientist Sir Paul Callaghan shortly before he died in 2012. In a speech at Zealandia, he spoke of the country’s unique biota in the same breath as Britain’s Stonehenge, the Great Wall of China and the prehistoric cave paintings of central France. But this rich heritage was imperilled, he warned: “The situation has never been as bad as it is now. We are facing utter catastrophe in our forests.”

He conceived of a New Zealand-style “Apollo mission” – a reference to the US’s galvanising vision to put a man on the Moon. Describing a predator-free New Zealand as this country’s “moonshot”, he said: “We can do this.”

At about the same time, philanthropists started pouring serious money into the war on pests. At Separation Point in Abel Tasman National Park, a population of plastic gannets suddenly appeared three years ago – a bid to lure real gannets, and one of the early tangible signs of an ambitious $25 million project to restore the park’s ecology over the next 30 years. Funded by Northland industrialists Neal and Annette Plowman, Project Janszoon has reintroduced kaka and kakariki, has knocked back wilding pines and by next year will have the entire park stoat-trapped alongside an Air New Zealand-backed trapping campaign in the northern area of the park.

Through their Next Foundation, the Plowmans are also funding (alongside others, including local iwi, Shell and technology entrepreneur Sam Morgan) the Taranaki Mounga project to clear invasive pests and weeds from 34,000ha around Mt Taranaki. Next is also a foundation funder (along with the Morgan Foundation and a clutch of dairy companies) of Zero Invasive Predators (Zip), which is focused on developing new techniques for monitoring and trapping rats, stoats and possums, and has achieved the total eradication of pests from the 400ha Bottle Rock Peninsula in the Marlborough Sounds without erecting a predator fence.

In Hawke’s Bay, wealthy philanthropist Julian Robertson is backing the 26,000ha City to Cape ecological restoration project, and the Reconnecting Northland biodiversity project has funding from Sir Stephen Tindall. The Morgan Foundation is funding projects such as Predator Free Stewart Island/Rakiura and the Million Dollar Mouse project to rid the Antipodes Island of mice.

Standing up for native species is no longer the preserve of “greenies” and beleaguered DoC rangers. The most influential conservationists now wear suits and have direct links to the Beehive. Sir Rob Fenwick, the founder of the Living Earth composting business, and chairman of the Predator Free New Zealand and Kiwis for Kiwi trusts, recalls taking John Key to a kiwi release in late 2014 and telling the Prime Minister that the national icon was declining at 2% a year.

“I said, ‘Look, Prime Minister, these birds will be extinct in your grandchild­ren’s lifetime’ … He was visibly shocked and said, ‘Well, what would plus 2% be like?’ You kick yourself for not asking the question yourself,” recalls Fenwick. “But we went away and got Landcare [Research] to do a report that answered that question, and actually, it’s not that hard. There was a manageable way through this.”

The Predator Free New Zealand trust was set up by Fenwick with DoC backing in 2013 to help co-ordinate voluntary, iwi and landowner conservation efforts around the country. So far, it has mapped 800 community trapping and pest-control groups. “We reckon 200,000 New Zealanders have done some kind of voluntary work for conservation in the past 12 months,” says Fenwick.

The final push that hoisted New Zealand’s endangered species on to the political agenda came with the development by DoC of a business case tailored to appeal to Cabinet dealmaker Steven Joyce. The document totted up the ongoing cost of predator control ($94 million a year, 70% of which is spent killing TB-carrying possums that threaten agriculture) and agricultural losses caused by pests ($52 million a year). It pushed the idea of an “investment approach” to conservation, with “business funding and entrepreneurial pace” augmenting the country’s long history of predator control. By setting up a public-private partnership named Predator Free New Zealand to act as “part promoter, part broker and part investor”, philanthropic and corporate funding and community effort could be leveraged to develop faster and more efficient methods of purging the land of pests.

The plan envisages that for every $1 of Government money, $2 of private-sector money will be stumped up. US-based businessman Chris Liddell, who chairs the Next Foundation, supports the proposed model and hopes it will draw more private philanthropic money out of the woodwork. And he says the predator-free goal has been “noticed” in the US, where he has had preliminary conversations with two or three potential funders.

“I have started talking to foundations and saying, ‘If you want to spend some money in a country which has got a big national goal, where … there is no corruption and a high degree of integrity, and create a very positive environmental impact, then you should look at New Zealand.”

A monumental

“I think every New Zealander who wakes up and hears a tui in the back garden thinks, ‘Wow, I’m proud to be a Kiwi’ … This is about our identity,” says Sanson. “We are the only country in the world that’s a land of birds, and in 1880 when we brought those stoats in, we just did so much damage. People want that country back. We will be ultimately the Galapagos of the world if we can do this.”

But it’s a monumental “if”.

As Russell wrote in Bio­Science, “50 years of pioneering and persistent effort” in which New Zealand has eradicated all introduced mammals from 100 of its offshore islands “has still only increased the pest-free island area from 0.5% to just 10%”. Eco-sanctuaries account for just 0.2% of New Zealand’s mainland, and rely on intensive trapping and poisoning.

For now, the only method available for large-scale predator control is dropping 1080 from helicopters over remote unpopulated areas of bush. It is highly effective, but it generally doesn’t achieve 100% eradication and predators reinvade over time – although a trial is planned by Zip to see if a complete knockout can be achieved by enticing the animals with two non-toxic pre-feeds followed by heavier sowing of the poison baits.

The anticoagulant brodifacoum is used in offshore islands to achieve total elimination of rats, but it builds up in the environment so can’t be used repeatedly and is not permitted aerially in unfenced areas on the mainland. A recently developed poison, PAPP (para-aminopropiophenone), is an effective and humane killer of stoats and cats, but it is not registered for aerial distribution over large areas.

High-tech pied pipers

Innovative Wellington company Good Nature has developed self-resetting traps – but a quick glance at a map of Fiordland or Kahurangi National Park is a sobering reminder that even such a highly efficient trap is no match for the predators that rule our vast back country.

If the war on pests is to be won, new weaponry is essential. Some of the leading work to improve the armoury is being done from a cramped office near Lincoln University by scientists Elaine Murphy, Tim Sjoberg and Tom Agnew of Zip.

They are experimenting with an array of methods that could lead to the development of “super-lures” that would work as high-tech Pied Pipers, drawing pests from across a wide area to traps. They have discovered, for instance, that a tiny piece of dacron taken from the bedding of a captive stoat will draw other stoats. Other scientists have found that female possum urine attracts other possums.

The scientific challenge is to identify the volatile compounds that make up those smells, then synthesise them cheaply so that they can work as long-life lures in combination with self-resetting traps.

Smell is just one type of lure under investigation. The Zip team is also studying the potential of using the pests’ own sounds to draw them to traps. “I have no doubt these guys are all talking to each other right now,” says Sjoberg, as he stands in a seemingly silent room full of caged rats kept for animal-behaviour analysis. The rodents communicate constantly at an ultrasonic level that’s inaudible to humans, he says.

Behavioural lures, such as luminous 3D objects nailed to trees, are being tested to see whether they will attract curious possums. Working in a 2ha fenced enclosure, the scientists are also exploring how light can be used to direct or herd rats in a particular direction, and experimenting with a small adjustable predator fence to figure out just how high such a barrier needs to be for stoats and rats.

They are also probing the dietary preferences of stoats to determine the best bait to use with an aerially delivered PAPP poison. It turns out the carnivorous predators like fresh rabbit – but to poison 100,000ha in this way would take about three tonnes of rabbit meat, says Murphy. “And you just can’t get three tonnes of rabbit.”

Targeted efforts

The challenges are as prosaic as they are complex and immense. Zip boss Al Bramley says much depends on improving the monitoring of pests in real time so that eradication efforts can be more targeted. “At the moment, we are still heavily reliant on someone going for a big walk to tell you something was there three weeks ago.”

He also sees promise in the potential of artificial intelligence and low-cost radio transmitters in efficiently eliminating predators over big areas.

Species-specific toxins

Another key line of research is the development of species-specific toxins. “A compound that is toxic only to rats, for example, would be a really big breakthrough,” says Andrea Byrom, head of the Biological Heritage National Science Challenge. “I would say we would get there in two to five years, and that’s huge because it would mean we could apply those toxins in places where we can’t right now, like farmland. It would be a massive opportunity to be able to throw out predator baits on farmland without harming dogs and livestock.”

For those who share a desire to see our ancient biodiversity restored and made safe from invaders, these emerging techniques offer the tantalising hope of a more ecologically healthy future. But many experts believe that even these advanced methods won’t be enough, and that achieving a predator-free New Zealand won’t happen without the use of fast-emerging gene-editing and gene-drive technologies.

“To think we are going to become predator free without poisons distributed from aircraft and/or genetic engineering could be viewed as overly optimistic,” says Josh Kemp.

In a country that took genetic modification off the agenda more than a decade ago, and in which determined pockets of resistance remain to the use of 1080 (despite compelling evidence that, for now, it is the singular tool keeping species from extinction), the biggest obstacle to rescuing kiwi, kokako, kea and their endemic cousins may not be a lack of money or know-how, but a lack of will.

This article was first published in the November 26, 2016 issue of the New Zealand Listener.

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