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Assisted migration: Helping nature to relocate
03 October 2007
NewScientist.com news service
Bob Holmes

Enlarge If they can't stand the heat

The Venus flytrap is one of the plant kingdom's most notorious predators. Now, however, the trapper has become the trapped. Its home in the coastal swamps of North Carolina is changing so quickly that it cannot disperse fast enough to find suitable new places to live. It is not alone. In the coming years, many other plants and animals will face a race to keep up as global warming shifts suitable habitats to higher latitudes. Those that fall behind - perhaps a million species or more - may end up marooned and facing extinction in increasingly unsuitable environments.

It doesn't take a specialist to see one possible solution to this impending crisis, says ecologist Jason McLachlan from Notre Dame University in South Bend, Indiana, who studies the dispersal rates of trees. In fact, he hears it all the time. "Whenever I present my research, there is always somebody in the audience who says, 'Well, this isn't really a problem, because we can just move the species'." Give nature a hand and FedEx struggling species to comfortable new locations, if you will. "At first, I just dismissed the idea because it seemed sort of glib," says McLachlan, "but when you start thinking about it, you realise it will probably end up being one of the tools that we use."

As simple as it sounds, the practicalities of this so-called "assisted migration" are highly complex and could involve enormous costs, huge ecological risks and even a challenge to the very foundation of conservation biology. That is why earlier this year McLachlan, together with his Notre Dame colleague Jessica Hellmann and Mark Schwartz of the University of California, Davis, called on conservation biologists to start thinking carefully - and urgently - about the subject (Conservation Biology, vol 21, p 297). "It's going to be a train wreck if we wait for species to start going extinct, then panic and start moving them," he says. "We need to have the conversation now, because it will probably take a decade to reach some consensus."

The case in favour of assisted migration is straightforward: without a helping hand, many species are likely to perish as climates change. Three years ago, a team of researchers led by Chris Thomas of the University of Leeds tried to assess the number of species at risk. Using published estimates of the amount of warming that might occur by 2050, they calculated that between 38 and 52 per cent of the world's 5 to 10 million species would need to shift their geographic ranges to survive (Nature, vol 427, p 145). No one knows what proportion of these would be able to move quickly enough (see "Survival of the fleetest"), but even optimistic estimates predict a major conservation threat.

"Anything that isn't very good at dispersing on its own is going to be in big trouble," says biologist Dov Sax from Brown University in Rhode Island. "If I had to be conservative, I would guess that would be at least 10 per cent [of all species] - which is a lot." Species living in temperate climates are likely to be at greatest risk, since climate models predict that mid-latitude habitats will shift most dramatically. "We're going to lose a lot of species if we don't move them. I think it's going to become a major strategy," Sax says.

However, knowing we may need to move some species is not the same as knowing which species to move, where to move them, and how. For one thing, the species most likely to need help are not the widespread, successful ones but the rare and marginal ones. They are often already struggling for other reasons, such as habitat loss or highly specialised ecological needs, so they may not thrive even if moved. Besides, conservationists may have a hard time convincing authorities to let them experiment with moving even small numbers of individuals from plant or animal species that are already endangered. "The US Fish and Wildlife Service is rarely thrilled about the idea of taking organisms from an endangered population," says Hellmann.

How many should you move?

What's more, moving a species is harder than it sounds - as previous efforts simply to reintroduce species into parts of their former ranges testify. "It's much more complicated than figuring out the average temperatures where a species occurs and finding another place with the same temperatures," says David Wilcove, a conservation biologist at Princeton University in New Jersey. "You have to ask what other species it is dependent on. Will those other species be at the site you want to move it to? How readily can the species be moved? How many do you want to move?" Even if the Arctic icecap vanishes altogether, for example, conservationists are unlikely to succeed in - or even attempt - to relocate polar bears to Antarctica. The task becomes harder still when populations are genetically adapted to local conditions, because if the new habitat is even slightly different they may not have the flexibility to cope with it. As a rule of thumb, ecologists figure that only 1 in 10 introductions actually result in the successful establishment of a population.

To complicate things further, climate change will not simply push existing ecosystems towards the poles. Instead, conditions are likely to change in unpredictable ways, creating climate combinations and ecosystems never seen before. For example, much of the south-eastern US is likely to end up with an entirely novel climate during the next 100 years, according to a recent analysis by John Williams and his colleagues at the University of Wisconsin (Proceedings of the National Academy of Sciences, vol 104, p 5738). They do not specify the form this climate might take, but other teams suggest that summers will be drier and hotter, more like the south-western US, and winters will be drier and more like those in the Caribbean. "We're moving into a novel situation. We need to ask if there is any place these species can move to in the future," says ecologist James Clark of Duke University in Durham, North Carolina.

All these uncertainties mean that anyone wanting to move a species will probably need to try repeatedly and be prepared to nurture the nascent populations for several years. "I think it's deceptive to think there are any easy answers here. The amount of monitoring and management involved in doing this right will make it expensive," says Lee Hannah, a climate change biologist at the University of California in Santa Barbara. Existing reintroduction programmes give an indication of just how expensive it will be: efforts to re-establish California condors have already cost millions of dollars, and even reintroductions of small plants can run up bills of tens of thousands of dollars.

Costs like these mean assisted migration will never be possible for more than a select few species. "I definitely see this as a secondary effort," says Malcolm Hunter, a wildlife ecologist at the University of Maine in Orono. "You could call it boutique conservation but, frankly, much of what we do at a species level is that already."

Even if the challenges inherent to assisted migration are overcome, such programmes may create conservation nightmares. There is a risk that transplanted species will become invaders that overrun their new habitats, just as rabbits and prickly pear cacti have in Australia and the kudzu vine has in the south-eastern US. This risk is relatively small, however, because invasive species tend to be robust, weed-like types characterised by nimble dispersal and rapid reproduction - the very traits that would help such species adapt to climate change without human assistance. Even so, bitter experience has shown that even ostensibly weak species can become tough customers in a new environment. Two of the world's most aggressive invaders, the Monterey pine and the Java sparrow, for example, are both highly restricted, marginal species within their native range. Biologists contemplating assisted migration will need to consider each case carefully, perhaps beginning with test introductions in areas of low ecological importance such as restored mine sites, Hunter suggests. This will inevitably boost the cost and time commitments involved.

Then there are the legal implications. So far, US courts have ruled that conservationists cannot be held liable for damage caused by species such as wolves that are reintroduced into their native ranges. This is on the grounds that the species belong there, says Holly Doremus, an environmental law professor at the University of California, Davis. However, if species were moved beyond their normal range, Doremus warns, conservationists might find themselves stuck with a bill for any damage incurred.

The final hurdle facing assisted migration is a different kind of obstacle altogether - the ecological scruples of conservationists themselves. McLachlan recounts a telling chat with a fellow conservation biologist working in the Netherlands to preserve rare plants threatened by climate change. "I asked her whether she thought it would be a good thing to move those species to, say, Scandinavia, where they don't currently exist. She thought that was a good idea," he recalls. "But when I asked her if she would be equally comfortable with a Spanish ecologist moving some of his species to the Netherlands, she baulked."

McLachlan's interpretation of that exchange is that conservation ethics are strongly rooted in a sense of place, and a feeling for what belongs where. Assisted migration asks biologists to violate that sense of place, and so is sure to encounter resistance. After all, "conservation" literally means "keeping things the same". Until now, the idea of actively creating new assemblages of species, rather than merely restoring what once was, has been anathema in conservation circles. Assisted migration has the potential to rewrite the rules in a big way.

Yet despite all these issues, most of the ecologists thinking about assisted migration believe it deserves a place in the conservationist's tool kit - if only as a last resort. "You try to conserve things in the wild first, then you work down your range of options. Assisted migration is on the list of things you need to consider," says Hannah. "To me it falls into the category of intensively managing species at risk." For Richard Primack, a plant ecologist at Boston University, the case is even clearer. "The alternative to moving species is just to watch them go extinct," he says.

In the conservation trenches, things have yet to get so desperate. "Assisted migration is not a major part of our plans right now," says Patrick Gonzalez from The Nature Conservancy (TNC), a conservation group based in Arlington, Virginia. "We are developing and testing a suite of adaptation strategies to help ecosystems adapt to warmer temperatures." These include protecting habitat corridors to give species a better chance to shift to higher latitudes on their own - a much less labour-intensive option, if it works.

Gonzalez and his colleagues are also exploring ways to manage existing habitats to minimise the effects of climate change. In areas that are becoming drier, for example, TNC is encouraging the growth of more heat-resistant varieties of native plants and performing controlled burns more frequently to reduce the risk of catastrophic bush fires. They are also trying to identify sites that may be naturally less vulnerable to climate change - northern slopes of mountain ranges, for example, and habitats near large lakes that can buffer temperature changes. Such refuges may offer species a place to survive climate change without needing to migrate too far.

The most effective tool in helping species adapt to climate change, however, is one that is already on everyone's agenda: slow down the rate of global warming. After all, if species have a century instead of 50 years to move a given distance, more of them are likely to survive the journey. "Every bit we can do to slow down climate change really helps," says McLachlan.

From issue 2624 of New Scientist magazine, 03 October 2007, page 46-49

Survival of the fleetest The current global warming episode is not the first race that species have had to run to keep up with a changing climate. When the ice sheets receded at the close of the last ice age, the plants and animals of North America, Europe and Asia faced the similar challenge of tracking their habitats as they shifted northward. The species alive today proved they could do it. Those that failed - such as the spruce tree Picea critchfieldii once widespread in eastern North America - are long gone. In fact, analyses of fossil pollen suggest that at least some North American tree species moved northward at an average rate of about 1 kilometre per year at the end of the last glacial period. "Dispersal mechanisms are surprisingly effective," says Keith Bennett, a palaeoecologist at Queen's University Belfast, UK. Since this is similar to the rate of habitat shift projected for present-day warming, Bennett's gut feeling is that today's species should be up to the challenge. Not everyone is so sanguine. A kilometre per year for a tree with a lifespan of a century means that by the time it dies, it must have successfully established offspring fully 100 kilometres north of itself, which stretches credulity. "Most ecologists have been extremely sceptical of those rates of spread," says Jessica Hellmann of Notre Dame University in South Bend, Indiana. Indeed, Hellmann's colleague Jason McLachlan has found genetic evidence that tree species may have ridden out the ice age in more northerly glacial refuges, giving them far less distance to travel once the climate started warming (Ecology, vol 86, p 2088). Bennett counters that species are clearly capable of dispersing vast distances in a single leap. European plants and beetles quickly colonised Iceland when the glaciers had left, a feat that required them to jump hundreds of kilometres over the sea. In addition, tree species crossed the North American Great Lakes at a single bound, testifying to the effectiveness of various methods of seed dispersal, including being carried by birds, wind and water. Even so, if climate change proceeds as rapidly as scientists fear, today's species may not have the luxury of even a few hundred years to make their poleward leaps. What's more, they are likely to find the going a lot tougher than their ancestors did 15,000 years ago. Cities and cornfields now dominate the landscape in many regions, leaving migrating species far fewer safe havens in which to land as they disperse. The race against climate change may have more losers this time than last.

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