North brother island lies at 41° south and some 35 km north-west of Wellington, broadside to the full wrath of the Cook Strait seas and the glare of the northern sun. The strait is famously quick to anger, gathering up gales and hurling them at the steep cliffs of this sparse 4 ha outcrop.
As tuatara habitat, it’s what an ecologist might call “suboptimal”. Salt-scorched shrubs give up somewhere around waist-height, their roots gripping for dear life in shallow, spent soils against the rampaging northerlies. All the more surprising, then, to find tuatara here. North Brother’s shallow soils have always made life difficult for the burrow-nesting “living fossil”, but in a warming world, they might just finish it off altogether.
As the mercury passes 21.7°C, something fascinating happens inside a tuatara egg; the developing embryo stops becoming a female and pursues masculinity instead. Under artificial incubation at 22°C, says Nicky Nelson, a tuatara researcher at Wellington’s Victoria University, “we got 100 per cent males. At 21°C, we got three males out of 80 eggs.” Between lies the pivotal equipoise of 50/50 sex ratio, and the future of the tuatara.
But on North Brother, that balance has been lost. Sixty per cent of the island’s 350 tuatara are male, and researchers believe that under an ever-warmer sun, every egg on the island will hatch a male by the mid-2080s. That’s because the animals can’t dig any deeper to find cooler soil temperatures; the island bedrock lies just five to 25 cm down. Nor can they seek out forest shade. Only the island’s south-facing cliff-tops, where few tuatara venture, offer the cooler temperatures necessary to restore gender balance.
More males, says Nelson, might even drive the extinction vortex faster, as competition between them for food and mates becomes more pitched. Studies on lizards have shown such aggression harms females as well, and breeding success falls. Nelson says there are signs that may already be happening on North Brother, where females are only nesting once every seven to nine years. On the western horizon, on Takapourewa, or Stevens Island, females are laying eggs every four years on average.
Tuatara have been through hard times before. Their ancestors walked the ancient supercontinent of Pangaea 200 million years ago. They watched the dinosaurs rise, and survived their cosmic oblivion. They endured ice ages and heat waves, saw mountains come and go. But now, as little as a 1.5°C temperature rise could tip them into extinction on this, the most southerly outpost of their relict range.
That’s because warming is happening so rapidly, says Nelson, who points out that tuatara can live for more than a century. “So these temperature changes are happening within the lifespan of a single generation. That means that if they are going to deal with it, it has to be behaviourally, not genetically. We’re not talking adaptation; we’re talking about the ability of individuals to survive.”
Once, tuatara were widespread throughout the country, but rats and fires and axes drove them to the lifeboats of offshore islands, where they are now effectively marooned. In times past, when the climate changed, the creatures simply moved. But now, Nelson says, “They’re restricted to islands, which means they don’t have the options they once did. They can’t change their behaviour; they can’t go uphill, they can’t move further south to cooler temperatures.”
A decade ago, Victoria University researchers and DOC translocated North Brother tuatara to other islands; Titi in the outer Marlborough Sounds, and Matiu/Somes in the middle of Wellington Harbour, where they’re doing well. The idea was to establish insurance populations to guard against some catastrophic loss on North Brother, and to boost the animals’ failing genetic vigour. Tuatara may have lived in the much cooler environs of Otago as little as a century ago. Now, researchers are talking about sending them back, ahead of the warming front. At Dunedin’s Orokonui predator-proof mainland island, biologists are investigating soil temperatures and their implications for southern sex ratios in a bid to help the tuatara regain its gender balance.
By December, the black rocks of Penguin Bay, near the western tip of Campbell Island, should be ringing with the clamour of a million rockhopper penguins. But nowadays, a few scattered huddles remain where, just 50 years ago, there was a vast throng of birds. Beyond, a silent waste where tussocks, still flattened and stunted after the millennial patter of webbed feet, now swirl empty in the southerly.
Nobody knows for sure how many rockhoppers once lived here, but photographs taken by coastwatchers during WWII show penguins to every horizon. “The colony below Mt Paris was absolutely massive,” says DOC seabird scientist Graham Taylor. “The ground was totally covered in birds. When you go there now, there are none; absolutely zero.”
Comparisons show that rockhopper numbers on Campbell have crashed by 95 per cent in the last 50 years, and they’re not the only example. Not far away, grey-headed albatrosses nest along windswept cliff-tops. The coastwatchers photographed those colonies too; there may once have been as many as 43,000 nests in those early photos. A survey between 1995 and 1997 found 7800.
A half-century ago, those coastwatchers would have had to step carefully around the hundreds of giant elephant seals that came to Campbell to breed. When Taylor visited in the 1980s, they had all but disappeared. “There were once hundreds and hundreds of pups being produced there, [but] when I was there, that had dropped down to about half a dozen.”
Researchers suspect that the rockhoppers, albatrosses and elephant seals have all fallen foul of warming sea temperatures. As a general rule, the warmer an ocean gets, the less food it contains. That’s because minute floating plants, or phytoplankton, thrive in colder seas. As the marine equivalent of the Serengeti grasslands, phytoplankton are the feedstock for the entire ocean food web.
Scientists needed a way to compare the productivity of surrounding seas then with now, and David Thompson, an ecologist with NIWA, found one. Poring through museum collections, he analysed the carbon signatures of long-dead rockhopper specimens. Stable isotopes are slightly different forms of fundamental chemical elements. They differ from standard forms by the number of neutrons they carry; for instance, all carbon isotopes have six protons, but carbon-12 has six neutrons, while carbon-13 has seven. “The carbon signature—the ratio of carbon-13 to carbon-12—is a proxy measure of marine productivity,” says Thompson. “It’s actually a measure of how fast phytoplankton grows.”
When he compared carbon signatures from museum penguin feathers with fresh ones, Thompson found a dramatic drop in the ocean’s bounty. Commercial fishing is not to blame, says Taylor. The birds and seals started disappearing long before trawlers arrived in the subantarctic. “There’s something else going on in the ocean environment.”
“If you look at the general patterns of seabird and marine mammal distributions,” says DOC officer Colin Miskelly, “they’re very much dictated by oceanographic features.” He says many species breed as close as they can to what we used to call convergences, broad fronts where ocean currents meet, bringing waters of different temperatures together. Often, one current will be forced downwards, while the other wells up, drawing with it vast quantities of plankton and other food.
Such fronts are oases of productivity in otherwise comparatively lifeless tracts of ocean, and marine species have based breeding strategies around them for millennia. But if those fronts were to shift south, says Miskelly, creatures would have a much harder time finding enough food. “That puts extra energy constraints on them, and we suspect that’s particularly important to penguins, because, while an albatross could easily cope with an extra 100 kilometres, a penguin has to swim every extra metre.”
There are no islands further south for the birds to relocate, and all agree that we can’t hold back the tide of warming water. “Realistically,” says Miskelly, “DOC can only look at other things that are also impacting on them like by-catch and introduced predators… things that we can actually manage.”
Ben bell has studied native frogs along the Tokatea Ridge for more than 30 years. When he first started walking through this podocarp and broadleaf forest high on the Coromandel Peninsula, where vaporous clouds wrap themselves around tall kauri, he had no trouble finding his subjects. “Archey’s and Hochstetter’s frogs were everywhere,” he recalls.
Sometime in 1996, Archey’s frogs all but vanished from this, their former stronghold. Autopsies delivered the news everyone was dreading; chytrid fungus, a lethal pathogen responsible for frog extinctions around the globe, had spread from introduced Australian frogs into our native species. Bell, a herpetologist at Victoria University in Wellington, says chytrid may have wiped out 85 per cent of the Archey’s population.
Three native frog species went extinct after the Polynesian rat arrived here; four survived. The most common, Hochstetter’s frog, is found throughout the northern half of the North Island, while Archey’s is restricted to the Coromandel Peninsula, and one site near Te Kuiti. The other two cling to existence on craggy Cook Strait islands; Hamilton’s frog is found in a single jumble of boulders on Takapourewa, or Stephens Island, and a closely related neighbour lives on nearby Maud Island.
Like so many of our creatures, New Zealand’s native frogs are highly unusual, stemming from an ancient lineage almost indistinguishable from the fossilised remains of frogs that lived 150 million years ago. They don’t have a tadpole stage, hatching fully formed, lack webbed feet, don’t croak and most don’t require standing or flowing water to reproduce. Due to their genetic distinctiveness, Archey’s frog is the number one conservation priority in the international Evolutionary Distinct and Globally Endangered (EDGE) programme in a list of 100 endangered amphibians worldwide.
“To date, only Archey’s has tested positive for chytrid among the native frogs,” says Bell, but he’s keeping a nervous eye on the others; Archey’s and Hochstetter’s co-exist at Tokatea and one other site.
Chytrid’s modus operandi is baffling; on the Coromandel, the fungus killed mostly smaller males, while many larger females escaped unaffected. But the biggest mystery is why frogs, some of which have co-existed with chytrid for millions of years, are only now succumbing.
To varying degrees frogs live in two worlds, air and water, so we call them amphibians. They were the first to leave the ancestral womb of the ocean and take those earliest gasping steps onto land. But their bodies are a compromise—they can’t withstand the rigours of a totally terrestrial life. They will always need water, or at least moisture, close by. With only rudimentary lungs, frogs still do most of their “breathing” through their permeable skin, which has to stay moist to let that happen.
And because of this dependence on clean air and water, every breath is a health check, every swim a litmus test. When pesticides, agrochemicals and heavy metals find their way into ponds, streams and lakes, bad things happen to frogs.
Bell, like many others, suspects that frogs are succumbing not to a single blow, but a combination punch. Environmental pollutants may have so weakened the frogs’ immune system that they can no longer keep fungus at bay.
A tiny group of Archey’s frogs survived the initial chytrid onslaught, and Bell hopes they may have developed an immunity. “We’ve found chytrid in what appear to be healthy Archey’s frogs, he says, “and the survivors are doing better, because there’s more food per capita now.”
But our native frogs now face a new bane; warming temperatures will put still more stress on animals already reeling from the ravages of introduced predators, land clearance and habitat modification. “The main risk is to terrestrial species—Archey’s, Hamilton’s and the Maud Island frog—as they’re largely restricted to higher altitudes,” says Bell. “One could predict that over time, with global warming, the availability of those cool, montane habitats—all those cool ridge tops—is going to change.”
Predictions of more rain in western regions could be a mixed blessing, he says. While frogs need that moisture, “it could help chytrid fungus spread more easily, so there may be a cost to more precipitation, as well as a benefit”.
Because of that decline in the Coromandel, Archey’s frog has gone from near-threatened to critically endangered. “If something was to further impact those remnant populations, then it could be at risk of extinction.”
Look at the latest climate prediction models for New Zealand and you’ll see a mean temperature increase by 2080 of between 1.8°C and 2.2°C, with localised highs spiking to 5°C and higher under “fossil-fuel intensive” models. Perversely, that warming could run still higher in alpine zones—as much as 6°C, says NIWA’s climate scientist Brett Mullan, “because you’re losing permanent snow cover in the Alps. The moment you lose snow cover, it exposes bare rock, so the ground can warm up a lot more.”
Snow isn’t the only thing set to vanish under a warming scenario. In 2003, botanists Stephan Halloy and Alan Mark pondered what a 3°C temperature hike might mean for our alpine plants. By their prediction, it spelled oblivion for between 200 and 300 species.
During the Pleistocene, glaciers ground through large tracts of New Zealand’s high country, obliterating some mountain plant communities and leaving many more stranded on alpine “islands”—isolated refuges, many of them home to their own endemic species or varieties. Those plants have the most to lose, says Mark. “It’s reasonable to predict that if global warming is for real, then plants with limited altitudinal distribution are clearly threatened.”
While lowland plants can shift higher in response to rising temperatures, many alpine plants are already topped out, and while safe, cooler sanctuary could be just kilometres away, they have no way of getting there. DOC botanist John Sawyer says that some South Island plants “at least have a little more mountain to climb, but throughout the North Island, they may go extinct quite quickly because they have nowhere to go.”
Halloy and Mark calculated that New Zealand’s alpine habitat—currently reckoned at some 30,000 sq km—could shrink to just 6700 sq km within a century. The pair counted 441 alpine “islands” above 1000 m and 364 above 1500 m in New Zealand, and calculated that 93 per cent of North Island alpine vegetation zones and 77 per cent of South Island ones would vanish as a result of a 3ºC warming. The remaining sanctuaries would fragment into much smaller relicts.
But that’s just the beginning. Even as the mountain environment becomes less suitable for endemic plants, says Halloy, exotic weeds will find it increasingly attractive. “A lot of invaders are by definition pioneering species. And from a physiological point of view, there’s no doubt that CO2 enrichment enhances growth rates and the competitive capability of those species.”
Already, DOC reports that heather, an introduced weed, has now been found above 1200 m on the western flanks of Tongariro, beyond what was considered its altitudinal limit. Invasive contorta pines are now growing at 2400 m on Mt Ruapehu.
Sawyer’s talking about nothing short of an evacuation. “We need to ensure that alpine plants are held ex-situ.” That’s already happening at Percy Scenic Reserve, just north of Petone, Wellington, where nursery staff are holding more than 500 threatened alpine species for safe-keeping, seed collection and propagation. The New Zealand Plant Conservation Network is looking at setting up a national seed bank. And where that can’t happen, says Sawyer, “We could probably get some 90 per cent of the national flora into some sort of cryogenic storage. Preferably you’d use both—the more options the better.”
But Mark is sceptical of any rescue attempts. “Retaining them that way isn’t practicable. A lot of our plants don’t grow well from seeds; they’re very slow to establish. You’d be much better, I think, shifting plants to a better [wild] location. But that’s playing God, and it would have its supporters and critics.”
And Halloy wonders whether there would be any point. “When you conserve a tiger in a zoo, the idea is to preserve enough genetic material so you can eventually re-introduce it to the wild,” he says. “But when you do that as a response to climate change, the scenario is completely different, because you’re not going to have any suitable habitat left to reintroduce anything to.
“So you’re looking at keeping them ex-situ permanently, which is probably not a real option—they would become in effect museum exhibits.”