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As Climate Change Bears Down, Do We Relocate Threatened Species?

Posted by Richard Conniff on December 26, 2017

(Photo: Frans Lanting)

by Richard Conniff/Scientific American

On a knob of rock in New Zealand’s Cook Strait known as North Brother Island, a population of the lizard-like creature called the tuatara is quickly becoming all male. When scientists first noticed the imbalance in the late 1990s, the sex-ratio was already 62.4 percent male, and it has rapidly worsened since then, to more than 70 percent. Researchers say climate change is the cause: ground temperature determines the sex of tuatara embryos, with cooler temperatures favoring females and warmer ones favoring males.  When climate pushes the sex ratio to 85 percent male, the North Brother Island tuataras will slip inescapably into what biologists call the extinction vortex.

So what should conservationists do? For the tuatara and many other species threatened by climate change, relocating them to places they have never lived before–a practice known as assisted colonization—is beginning to seem like the only option. “We’d prefer to do something a little more natural,” says Jessica Hellman, a lepidopterist at Notre Dame, who was among the first researchers to put the assisted colonization idea up for discussion. That is, it would be better for species to shift their ranges on their own, using natural corridors to find new homes as their old ones become less habitable. But for many island and mountain species, long distance moves were never an option in the first place, says Hellman. In other cases, old corridors no longer exist, because human development has fragmented them.

The idea of assisted colonization as a conservation tactic has elicited fierce criticism, however, because of its potential to wreak ecological havoc on both the relocated species and on the destination habitat. Conservationists have up to now also been inclined by their culture to be against assisted colonization. Many of them have devoted their lives to putting species back where they used to live 100 or 200 years ago—gray wolves in Yellowstone, or bison on the Great Plains. Imagining new places where they might live in some unknowable future can feel like heresy.

But as the likely devastation from climate change has become more apparent, criticism has given way to guidelines on how and when to move species–and to increasing, if uneasy, acceptance. A recent survey of 2300 biodiversity scientists in the online journal Elementa found that most supported assisted colonization under certain limited conditions—notably where it is the only way to prevent extinction, and where the risk to the destination habitat is small or nonexistent.


Gilbert’s potoroo

The need to plan what could be, in essence, emergency evacuations became painfully evident this past December when a wildfire devastated the habitat of one of the world’s most endangered mammals–a small, kangaroo-like marsupial called  the Gilbert’s potoroo–in drought-stricken Western Australia. The fire killed 15 of the 20 potoroo in the reserve where the species had been rediscovered in 1995 after having previously been considered extinct for more than a century. Loss of that habitat would have been an automatic sentence to extinction–except that, in the aftermath of the rediscovery, conservationists had established a separate colony on a nearby island.

The potoroo was translocated within its original home range, which is much less controversial than moving a species into a new area.  Conservationists elsewhere have also begun buying time in that fashion: In the Florida Keys, for instance, researchers have already relocated populations of key deer and the tree-like key

cactus to upland areas, to give them a few more decades of suitable habitat as sea levels rise. For Australia’s Bramble Cay melomys, a small rodent, it is already too late for that kind of delaying tactic. In June, University of Queensland researchers announced that the species had vanished after repeated inundations of its island home.  They described the event as  probably “the first recorded mammalian extinction due to anthropogenic climate change.”

Western swamp tortoise

Thus, the best hope for other species may lie beyond their traditional home range. For instance, the marsh where Australia’s critically endangered western swamp tortoise lives outside Perth faces the triple threat of climate change, urban expansion, and the city’s relentless drawing down of the underlying aquifer. The University of Queensland’s Tracy Rout and her colleagues have used a supercomputer to sort through 13,000 potential relocation sites around the region. Further work on the ground has narrowed the list to several sites a few hours south of the city with hydrology and other conditions likely to remain suitable in the drier climate 30 or 50 years from now. Assuming they receive the necessary permissions, researchers will drive south with a load of captive-reared tortoises as early as August 2016 to begin introducing them to their new homes. (UPDATE: Done.]

Other researchers are debating where to move Australia’s critically endangered mountain pygmy possum. It’s a measure of the complexity of such moves that they might also have to relocate its preferred food, the Bogong moth. The alpine habitat for both species is warming so fast that simply moving uphill will no longer be possible.


Marbled white butterfly

The use of assisted colonization as a tool for addressing climate change isn’t entirely new. Stephen J. Willis, an ecologist at Durham University in northern England, and Jane K. Hill, now at the University of York, tried it experimentally beginning in 2000. “We had been looking at climate change impacts” on British butterfly species, including the relatively common marbled whites and little skippers, said Hill, “and we saw there was some suitable climate north of their normal range they hadn’t reached.”

That’s because of a phenomenon called “migration lag.” Even when natural corridors survive intact, species tend to lag behind the pace of climate change. That kind of delay might be predictable for trees and other less mobile species. But studies in Europe have found that even birds and many mammal species also lag behind changes in climate, perhaps because they depend on slower-moving vegetation and habitat types. The gap between “climate velocity” and “biotic velocity” can be insuperable. Joshua Lawler at the University of Washington projects, for instance, that South America’s yellow-banded poison dart frogs will need to hop hundreds of kilometers to the southwest to find suitable habitat later in this century.

When Willis and Hill noticed that marbled whites and little skipper butterflies were climate laggards, they set out to help them catch up. “We did it as a demonstration, as a good case study,” said Hill. They obtained the necessary permissions because the habitats for the proposed relocations were relatively restricted, in quarries and urbanized areas, and because other species there were already known to be compatible. They released the marbled whites 65 kilometers north of their traditional home range, and the little skippers 35 kilometers north. Both populations seem to be thriving in their new homes, Willis said recently. But he added that the developing guidelines for assisted migration “are all saying the right thing, that you need to take a cautious, reserved approach. You don’t want to be introducing the next rabbit into Australia.”


Translocating any species is inevitably fraught with risk. In a 2009 critique, McGill University’s Anthony Ricciardi and Daniel Simberloff of the University of Kentucky urged conservationists not to play “ecological roulette” and warned that proponents have “grossly underestimated” just how difficult it is to forecast the impacts of introducing species to a habitat, even with the most cautious and nuanced analysis.

Ricciardi pointed to Newfoundland’s 1963 decision to introduce red squirrels into its black spruce forests, with the idea that they would provide a new food source for pine martens. The martens, weasel-like creatures then in decline, turned out to have no appetite for squirrels. The spruce cones, which had evolved squirrel-free for 9000 years, had no means to protect themselves from the squirrels. And the Newfoundland crossbill, a subspecies that had evolved to depend on those same cones, collapsed in the face of new competition. It’s now endangered–and a case study of how the best intentions can go horribly wrong when people move species outside their historic range.

(Photo: Eric Wilson)

There may be ways to minimize the likelihood of such disastrous outcomes, however. Nathalie Pettorelli of the London Zoological Society and her colleagues set out to exactly that in a 2013 study of New Zealand’s hihi. It’s beautiful yellow-and-black bird, with a hovering flight, and 34 million years of evolution separate from its nearest living relative. But it survives in just five isolated habitats on the North Island, where conservationists provide support with the sort of sugar feeders commonly used in North America for hummingbirds. Even so, Pettorelli and her co-authors found that climate change in the coming decades will make that northern habitat largely unsuitable for the hihi.  On the other hand, the shift will open up habitat in the South Island, outside the hihi’s historical range.

“We were not looking to say when or how or where to move the hihi,” said Pettorelli. That’s the job of local managers. But the researchers thought they could provide a methodology for making such decisions carefully. They started by breaking out a half-dozen separate ways things can go wrong for a translocation habitat—including negative effects on other species (“ecological risk”), introduction of new pathogens (“disease risk”), the possibility of spreading beyond the intended range and outcompeting native species (“invasive risk”), hybridization with a related species (“gene escape risk”), and costs to human residents (“socio-economic risk”). They also factored in a host of climate factors, such as how dry it gets in the dry season and how rainfall varies over the course of the year,  in old and potential new habitats, to make their models as precise as possible.

“We need to increase collaboration between people on the ground making the decision and the scientists,” says Pettorelli. “A lot of people want to work together but don’t know how to do it, don’t have connections to work together.” Even now, “a lot of management decisions are taken without consideration of what science is available and how to make use of it.” The point of the exercise was to show them how. As a result, conservationists have now established a new hihi population on the South Island.


And yet even proponents of assisted colonization worry they may be getting ahead of themselves. Sometimes the “do no harm” option can work out just fine. Species can sometimes adapt surprisingly quickly. In the Rocky Mountains west of Denver, for instance, certain flowers with deep pollen tubes have become scarcer as temperatures have risen. So bumblebees that had evolved long tongues to feed on them are now reversing the process, losing a quarter of their tongue length over the past 50 years to feed on the flowers that are still there.

Species can also turn out to be resilient in ways we might not expect. In southern Australia in 2010, for instance, researchers working on a commercial lobster fishery translocated 10,000 southern rock lobsters from deep water. But instead of moving them poleward to establish outpost populations in colder waters, they moved them closer to the equator, to see how the species would handle the warmer conditions predicted in the near future. Counterintuitively, the lobsters went on to grow at four times the rate seen at their site of origin and to boost their output by 35,000 eggs a year. They were more adaptable to temperature change than expected, and there was more for them to eat.

Predicting that sort of resilience is a challenge. When researchers recently examined how 155 species of British butterflies and moths fared over 40 years of climate change, they found that roughly half seemed to do better, and half worse. Different factors mattered for different species, with some sensitive to summer temperatures, others to winter temperatures, some to spring rainfall, and so on almost ad infinitum. “It turns out that these 155 different species of butterflies and moth have almost 155 different ‘opinions’ on how much the climate has changed, and whether it has got better or worse,” says Chris D. Thomas of the University of York


Where does all this uncertainty leave a species like the tuatara? Tuatara males can breed every year, while the North Brother females can produce a clutch of eggs only once every nine years. That means the females suffer constant mating harassment, which rapidly erodes their ability to stay healthy–a problem that is worsening as the sex ration of the population skews toward males.

Because North Brother Island offers no shade, and hardly any nooks and crannies, to reduce this effect, the 500 or so tuatara there have become a bellwether for how a rapidly warming planet will affect the entire species. Already pushed off the mainland onto a handful of islands, the 100,000 remaining tuatara are the last survivors from 200 million years of evolution.

Nicola Mitchell of the University of Western Australia recently co-authored an article listing the various management options on North Brother. All the parties that care about the tuatara—scientists, government managers, and the Maori, for whom it is a cultural totem—could band together to remove unneeded structures and open up nesting sites on cooler faces of the islands. Or they could send in researchers to find eggs and use captive incubation to achieve the right temperatures for an equal gender mix. Alternatively, they could restore gender balance by protecting female hatchlings and adding them to the population while removing excess adult males.

“But these are all really difficult things to do,” said Mitchell, who spent two summers on the island searching for nests. “There are so few females nesting each year, and they’re very secretive and hard to find.” Plus every trip to North Brother Island involves a helicopter from downtown Wellington, a budget killer. Moreover, there are already insurance populations of the same subspecies, though a different genetic group, on nearby islands. So the most pragmatic solution may be to regard the North Brother Island tuatara however reluctantly as a sacrifice population. That is, scientists might just want to wait and watch how things work out on their own.


Not so long ago, conservation biology was about trying to put together a jigsaw puzzle with some of the pieces missing, and that was hard enough. Now it’s as if someone—all of us—have kicked the jigsaw puzzle into the sky, and the trick for biologists is to put it together again while it is not just airborne, but caught in shifting winds. Also it’s starting to rain. Or, wait, maybe it will never rain again. And here is a species you care about passionately. Maybe it’s some lizardy thing, or an insect, or even a snake, but you care as if it were a big-eyed puppy at the pound with no time left, and nobody else to take it home. So here are the five things you could do to make the difference between survival and oblivion. Only one of them will save the day. It’s not an emotional decision, but time is running out.

Quick! Choose.


NOTE:  Scientific American chose a different ending. I think I prefer the one above, but again, it’s a bit of a toss-up.  Here’s how the published version ended:

Ultimately such decisions will boil down to how comfortable conservation biologists and society as a whole feel about meddling with nature to decide which species survive and which species die out. “When does it feel like you’re working with natural processes? And when does it feel like gardening?” muses Notre Dame’s Hellman. “You can’t garden all of biodiversity.”










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