Rob Suisted

Dead heat

Giant carnivorous land snails don’t ask for much: moist leaf litter to burrow into, earthworms to suck up like spaghetti. But if the lower layer of the forest is nibbled away, if sunlight reaches the soil, and if one month of drought follows another, molluscs relying on damp homes struggle to survive.

Written by       Photographed by Rod Morris

On a late February day last summer, the police told my wife and me to get ready to flee our Tasman home. According to the Ministry for Primary Industries (MPI), we were being evacuated because of a “medium-scale adverse event”. There was smoke in the air, and flecks of ash and soot settled on the car as we filled it with treasures. Not 10 kilometres away, a wildfire was gathered up, ready to hurdle the main road.

We were lucky. Because this was the third fire in as many weeks, a squadron of more than a dozen firefighting helicopters was stationed nearby, and volunteer crews hadn’t yet gone home after the first one, when 400 people had been evacuated.

It was New Zealand’s third-warmest summer, and it left swathes of the country in drought. Northland, Taranaki, Nelson, Tasman, Marlborough and the West Coast got less than half their normal rain. A weather station down the road at Appleby recorded 355 hours of sunshine in January, a South Island record that turned soil to dust. As fires raged up Pigeon and Redwood Valleys, volunteers and MPI staff snatched animals out of the way. A makeshift animal shelter at the Richmond Showgrounds took in more than 350 evacuated horses, pigs, goats, chickens—even a turtle. But no-one spared a thought for the snails.

Except for Kath Walker. In 40 years of studying native land snails, Walker, a Department of Conservation biologist, has seen a lot of dead ones. She can tell the crushing bite of a pig from the relative surgery of a possum’s nibble. She knows when a snail was beset by rats, or fell victim to a weka, or was battered to death on a thrush’s anvil.

Predation has long been the main cause of death for snails like the massive Powelliphanta, but a few years ago, Walker started finding shells undamaged but empty, as though their owners had simply moved on.

“There were no obvious signs of predation,” she says.

In the daytime, snails live hunkered under logs, or snuggled into forest moss or leaf litter. Some live above the bushline, among the tussocks. They can spend a whole winter under snow if they have to, because, for snails, surviving too much water is easy—what’s killing them could be its increasing scarcity.

Department of Conservation ecologist Kath Walker surveys the karst country of Canaan Downs, high in Abel Tasman National Park. The forest here should be a mecca for molluscs, but browsing deer, goats and cattle have left large tracts devastated, denying native snails the habitat, and the shade, they need.

We call them land snails, but they never really disavowed the water whence they came, 280-odd million years ago. Even today, a snail can’t breathe, move, feed, build a shell or reproduce without water.

“Most land snails have what’s loosely called a lung,” says Hamish Spencer, an evolutionary geneticist and professor at Otago University. “And like all gas exchange organs, it has to be kept moist. Their shell can protect them from the effects of drying out, but only up to a point. To adapt that body plan to something more resilient to drying, I think, would be really, really hard. It’s not a problem molluscs have been able to solve.

“And you might ask why they should—snails have succeeded in colonising those damp places really well. So they’ve stuck with their body plan.”

Until now, that’s been a good call. Some places host diverse communities of up to 70 species of tiny microsnails in just a few hectares. As for Powelliphanta, no-one’s sure exactly how many species there are, but there are at least 59 subspecies, and the architecture of their various shells is heraldry. Like any coat of arms, each shell tells something of its bearer’s history. Snails are, in that sense, a geological record—their subtleties are an account of change.

The largest powelliphanta species, P. superba prouseorum, can weigh 90 grams, with a shell nine centimetres across. But snails are deceptively delicate: without moisture, a shell dries, shrinks and cracks.

There is one form of water land snails can’t deal with. When the planet cooled, and glaciers came grinding down the valleys, they couldn’t flee. The ice created islands of snails, relict populations stranded in time and space. In warmer interglacial periods, as the seas rose and drowned the margins, more populations were lost. Over centuries, these enclaves—now hemmed in by shifting seas, or permanent snows—began to diverge. Darwin would have nodded at what biologists nowadays call ‘genetic drift’—the formation of new subspecies. Wait a few million years, and you have a solitary, separate bloodline that Walker calls a “spot endemic”.

Walker can tell whether a snail has come from Kahurangi, or Golden Bay, or the Marlborough Sounds, just by looking at its shell. But for years, shells have been turning up with features she can’t explain. She shows me a photograph: shells picked up in the Heaphy Valley, each circumvallated by mysterious, thickened ridges, deformities that speak to arrested development.

“There were live snails in that area, too, and they had that same growth check as well. It was in the same place on all the shells, meaning they were afflicted sometime between 2000 and 2002. Lots of snails died of it.”

Walker thinks these growth checks may be the result of the stresses of dry seasons. NIWA records confirm that Tasman’s second-worst recorded drought, ‘the big dry’, struck in 2001.

A Powelliphanta snail engulfs an earthworm. A meal like this, says Kath Walker, can keep a snail going for as long as a month.

While counting snails on Mount Robertson, near Picton, in 1999, Walker found some—both alive and freshly dead—with the same aberration. Again, the climate record supports her hypothesis: the summer of 1998-99 dealt Marlborough its second-worst drought in 40 years.

According to Gary Barker of Landcare Research, any month delivering less than 30 millimetres of rain is a tough one for snails. Make that two months back to back, and it’s a catastrophe, “producing mass snail mortality”.

Northwest Nelson is still the epicentre of snail diversity, but NIWA’s climate records show that the region routinely produces Barker’s definition of catastrophe. The weather station at Appleby considers 33 per cent soil moisture to be normal for January. When that desiccates to 21 per cent, NIWA calls it a “wilting point”. Since 2003, soils have hit wilting point at Appleby every year but one. Last summer, Nelson stifled in its fourth-longest dry spell since 1862—40 days with less than a millimetre of rain on any one day.

Powelliphanta hochstetteri hochstetteri keep it local. Radio-tracking studies at Canaan Downs showed the snails travelled just 1.3 metres a night, and then only in damp or humid weather. When the temperature climbs, they rarely venture out.

Thirty years’ worth of NIWA rainfall and soil moisture data at important snail sites shows an incontrovertible trend. The Marlborough Sounds, Horowhenua, Golden Bay, even the famously pluvial West Coast—all are now getting drier every summer, a crunch time for snails. Readings show that soils at those sites are dehydrating as snail numbers are falling.

“Connecting the two is supposition,” says Walker. “It’s a correlation, but it’s a very strong correlation. All the other causes of mortality we’ve been measuring don’t account for this trend. When we looked at predator numbers, they weren’t as strongly associated with the trend as was summer soil moisture deficit.”

It’s happening across many species, and it’s on the rise: “The numbers of intact, empty shells have been increasing all over the place, while live snail numbers are going down, and we’re wondering whether that’s down to drought.”

Every year brings more and more sunshine. That wouldn’t be such a problem for snails in healthy habitat, but our forests are ailing. Almost everywhere, they look, and function, nothing like they should.

We have a false ideal of forest: we see open glades, easily walked, and our conditioning perceives everything in balance. Those sunbeams casting like searchlights across the forest floor shout that it is not.

“It should not be possible to see far, or walk easily, within a healthy temperate rain forest,” says ecologist Geoff Walls. “From the outside they look great—healthy canopies and diverse vegetation on the edges—but the hollow interior tells a different story. The lower tiers of ferns, shrubs, seedlings and saplings are depleted or missing.”

That’s because European settlers bent fervently to the quest of populating New Zealand forests with edible ungulates from overseas: goats, pigs and seven species of deer. To the uplands, they introduced chamois and thar.

“Many of the smaller trees have their bark stripped by deer,” says Walls, “and are dead or wounded. In the upland forests, where the giant land snails are, the ground is extensively bared and ploughed up by pigs. It’s not just the direct damage that ungulates do; once it’s done, it only takes a few animals to prevent any recovery.”

Walls is describing D’Urville Island, home to Powelliphanta hochstetteri obscura, a threatened subspecies that’s also been found at Tennyson Inlet and Maud Island in the western Marlborough Sounds. On D’Urville, it clings to existence mostly on a handful of peaks, likely because the lower forests have been trashed by browsers.

Rats are a major predator of land snails, and numbers of rodents rocket at precisely the time snails are most debilitated: after prolonged warm conditions.
A Powelliphanta snail engulfs an earthworm. A meal like this, says Kath Walker, can keep a snail going for as long as a month.

Walker shows me a photo from the forest interior on D’Urville. It’s a bomb site. Most of the understory, save for a few unpalatable leftovers, is gone, and that’s a train wreck for snails. The lack of vegetation means fresh litter is no longer being made on the forest floor. With the shade gone, humidity at ground level is fatally diminished. That makes it poor habitat for snails, for their moisture-dependent eggs and hatchlings, and for the animals they like to hunt there. A forest interior should be damp, dim, the litter soft and deep. On D’Urville, it’s bright and dry. The litter is shallow and firm, compacted by the passage of hundreds of hooves. Where the pigs have rooted, the litter is non-existent.

It’s the same story on the sunlit flanks of Mount Evans, the highest point of Abel Tasman National Park. This is the abode of what’s left of Powelliphanta hochstetteri hochstetteri, a close relative of the D’Urville snails. Walker has been spotting empty shells ever since we left the carpark. Some were victims of weka: another, a possum. As we leave the kaikawaka and dracophyllum forest of the limestone country and venture up onto the granite dome of Evans, we enter silver beech forest. Or rather, a threadbare version of it. Even I can spot the shells here that have been “pigged”, as Walker puts it. They’re shattered. The forest floor is pockmarked with the bulldozings of foraging pigs, and Walker finds a well-used wallow in a ssphagnum bog.

There’s a paltry understory: mainly pepper tree and stinkwood, which goats and deer reject. Everywhere, the sun is dappling the ground. P. hochstetteri’s abode of millennia is now a hostile place.

“This population is at death’s door,” says Walker.

We know how bad it’s gotten, because for decades, forest ecologists have been maintaining ‘exclosure plots’ around the country: areas surrounded by fences high and solid enough to keep browsers out. Over time, they’ve monitored what’s happened inside and out. The contrasts have been stark. In 1978, three 30 metre by 20 metre plots were built on Arapawa Island, in Queen Charlotte Sound. Within a few years, ecologists reported: “A deep layer of leaf litter had accumulated inside the exclosures, soil was beginning to be rebuilt from the litter, and the ground was covered in masses of ferns and tree seedlings. Many of the trees had produced new shoots from their bases. Now, what was completely devoid of undergrowth has become a dense thicket, with canopy species well on the way to taking their place alongside the old trees.”


Back on D’Urville, in the late 1990s, numbers of Powelliphanta hochstetteri obscura doubled soon after pigs and deer were fenced out of two small plots. The adult snails were spared the scoffing of pigs, and their eggs and young had a much better chance of survival, too. More ground-level humidity protected the vulnerable, thin-shelled hatchlings. With the litter no longer being rooted up, there were more earthworms to eat.

What’s more, studies show that four-footed browsers can and do trample snails unwittingly. In 2014, a team including Gary Barker built a “mechanical hoof”—a pneumatic ram with a cow hoof attached, driven by a compressor. They placed it in a forest remnant in Waipa, in the Waikato, and fired it up. The idea was to gauge the impacts on litter communities from the incessant trampling of stray cattle. They found that even the casual passage of stock had big consequences for land snails: under the lightest trampling simulation, densities plummeted by an average of 42 individuals in every square metre.

Barker says it’s a mistake to assume that today’s four-legged browsers are simply doing what flightless birds would do anyway.

“Our diverse bird fauna meant ‘raking’ and ‘scratching’ forces, rather like that performed by the brush turkeys of eastern Australian rainforests,” he says.

That would’ve helped to keep the litter loose and full of spaces for snails: “New Zealand forest litter is naturally quite fluffy and deep, with a humus base.”

The intrusion of sunlight onto the forest floor not only kills adult snails, but leaves their eggs and thin-shelled hatchlings vulnerable to dessication. Most Powelliphanta species are found in the northwest of the South Island, around Nelson and at the top of the West Coast.

Saving our snails comes down to a very few options, and Barker says captive breeding isn’t one of them: “It’s worked in some circumstances, but it’s fraught with difficulties. Powelliphanta in particular are hard to rear in the lab; they get fungal infections, and you can very easily lose the lot, as we’ve discovered. Captive breeding has a place, but it should probably be the last resort. Short of pest control and fencing, there’s not much else you can do.”

Barker says Powelliphanta and Placostylus, the flax snails of the north, should be the urgent priority, “simply because the bigger snails are so much more vulnerable to habitat destruction and predation, but predator-proof fences are hard to maintain, and the longer they are, the harder it gets. Peninsulas are obviously one option, and forest fragments are another. The question is: what’s the minimum area you need to fence to maintain a population of snails?”

Kath Walker points out that there are more marine reserves than genuine terrestrial sanctuaries for snails: there is practically no unbrowsed forest on mainland Aotearoa.

“We need to get on with fencing off the biggest blocks practically possible, while leaving enough places with deer and pigs that hunters aren’t tempted to reintroduce them back into the fenced sanctuaries,” she says. “There are easy, simple things we could be doing to protect those snails in situ. How hard is possum control? It’s not hard at all.”

Harder, perhaps, to get people to care about something like a snail, when much of conservation in New Zealand is lavished on rock-star birds.

“People have long pondered,” observes Barker, “how you might get the public to appreciate things like snails—the diversity in biodiversity—but it hasn’t been very successful. Birds always seem to win
the day.”

Which is why, he says, protecting habitats, rather than species, is the way to go. “By protecting habitat for charismatic taxa like birds, you’re also doing the snails, the frogs and everything else a favour, too.”

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