On a knob of rock in the Cook Strait known as North Brother Island, a population of the lizardlike creature called the tuatara is quickly becoming all male. When scientists examined 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 tuataras will slip inescapably into what biologists call the extinction vortex.

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 conservationists have for saving them. “We'd prefer to do something a little more natural,” says Jessica Hellman, an ecologist now at the University of Minnesota, who was among the first researchers to put the idea of assisted colonization 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, Hellman explains. 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 the destination habitat. In addition, many conservationists 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 of assisted colonization. A recent survey of 2,300 biodiversity scientists in the online journal Elementa: Science of the Anthropocene found that most supported the idea under certain limited conditions, notably when it would prevent extinction and when the risk to the destination habitat is small or nonexistent.

Out of Bounds

The need to plan what could be, in essence, emergency evacuations of species became painfully evident last November, when a wildfire devastated the habitat of one of the world's most endangered mammals—a small, kangaroolike marsupial called the Gilbert's potoroo—in a drought-stricken area of Western Australia. The fire killed 15 of the estimated 20 potoroos in the reserve where the species had been rediscovered in 1994 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 of the species nearby.

JASON EDWARDS Getty Images (possum); ROBIN BUSH Getty Images (bird); GETTY IMAGES (deer)

The potoroo was translocated within its original home range, which is much less controversial than moving it 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 treelike 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.”

Thus, the best hope for other species may lie beyond their traditional home range. For instance, the marshes where Australia's critically endangered western swamp tortoise lives outside Perth face the triple threat of climate change, urban expansion and the city's relentless drawing down of the underlying aquifer. Tracy Rout of the University of Queensland 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 to 50 years from now. After obtaining the necessary permissions from wildlife and environmental authorities, researchers drove south with a load of captive-reared tortoises this past August to begin introducing them to their new homes.

Other researchers are debating where to move Australia's critically endangered mountain pygmy possum. It is 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.

The use of assisted colonization as a tool for tackling climate change is not entirely new. Stephen G. Willis, an ecologist at Durham University in England, and Jane K. Hill, now at the University of York in England, tried it experimentally beginning in 1999. “We had been looking at climate change impacts” on British butterfly species, including the relatively common marbled whites and small skippers, Hill says, “and we saw there was some suitable climate north of their normal range they hadn't reached.”

The absence of the butterflies from that area stemmed from 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, say. But studies have found that even birds and many mammal species lag behind shifts in climate, perhaps because they depend on slower-moving vegetation and habitat types. The gap between “climate velocity” and “biotic velocity” can be insuperable. For instance, Joshua J. Lawler of the University of Washington projects that as its rain-forest home becomes drier and warmer, South America's yellow-banded poison dart frog 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 small skipper butterflies were climate laggards, they set out to help them catch up. “We did it as a demonstration, as a good case study,” Hill says. They were able to obtain 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 range and the small skippers 35 kilometers north. Both populations seem to be thriving in their new homes, Willis reports. But he adds 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, Anthony Ricciardi of McGill University and Daniel Simberloff of the University of Tennessee, Knoxville, urged conservationists not to play “ecological roulette” and later 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.

RUNE MIDTGAARD (tortoise); BUDDY MAYS Getty Images (frog); GETTY IMAGES (butterfly)

The two authors point to Newfoundland's 1960s decision to introduce red squirrels into its black spruce forests, with the idea that they would provide a new food source for the area's 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 9,000 years, had no means to protect themselves from the transplants. And the Newfoundland red crossbill, a bird subspecies that had evolved to depend on those same cones, collapsed in the face of new competition. The crossbill is now endangered and has become a case study of how the best intentions can go horribly wrong when people move species outside their historical range.

There may be ways to minimize the likelihood of such disastrous outcomes, however. Nathalie Pettorelli of the Zoological Society of London and her colleagues set out to do exactly that in a 2013 study of New Zealand's hihi, a beautiful bird with yellow and black plumage, hovering flight and 34 million years of evolution separating it from its nearest living relative. The bird survives in just five isolated habitats in and around the country's North Island, where conservationists provide support with the type of sugar feeders commonly used in North America for hummingbirds. 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 appropriate habitat in South Island, outside the hihi's historical range.

“We were not looking to say when, or how, or where to move the hihi,” Pettorelli recalls. That is the job of local managers. But the researchers thought they could provide a methodology for making such decisions carefully. They started by breaking out half a 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 (socioeconomic risk). Then they considered a host of climate factors, such as how dry it gets in the dry season and how rainfall varies over the course of 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,” Pettorelli says. “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 are considering establishing a new hihi population on South Island.

Uncertain Outcomes

And yet even proponents of assisted colonization worry they may be getting ahead of themselves. Sometimes the “do no harm” option—leaving species to adapt on their own—can work out just fine. In the Rocky Mountains west of Denver, for instance, alpine flowers have become scarcer as temperatures have risen. So bumblebees that had evolved long tongues to specialize on flowers with deep pollen tubes have become less discriminating. That is reversing the evolutionary process, and the bees have lost a quarter of their tongue length over the past 40 years to feed on the flowers that are still there.

Species can also turn out to be resilient in ways scientists might not expect. In 2010 in southern Australia, 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 and inshore, 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 changes than expected—and there was more to eat.

Predicting that kind 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 moths have almost 155 different ‘opinions’ on how much the climate has changed and whether it has got better or worse,” observes Chris D. Thomas of the University of York.

So where does all this uncertainty leave a species like the tuatara? Tuatara males can breed every year, whereas the North Brother females can produce a clutch of eggs only once every nine years. That means the females suffer constant mating harassment, which quickly erodes their ability to stay healthy—a problem that is worsening as the sex ratio of the species skews toward males. Because North Brother Island offers no shade and hardly any nooks and crannies to reduce the gender-skewing effect of warmer temperatures, 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 or so 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,” says 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.” Relocating the tuatara to a cooler site might actually be a simpler solution, but it is expensive: 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 other, nearby islands. The most pragmatic solution, then, may be to regard the North Brother Island tuatara as a sacrifice population. That is, scientists might just want to wait and watch how things work out on their own, and if the North Brother group crashes, so be it.

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 die out. “When does it feel like you're working with natural processes, and when does it feel like gardening?” University of Minnesota's Hellman muses. “You can't garden all of biodiversity.”