When Worlds Collide

By Douglas Fox

Illustration by ©Guy Billout


A DYING RACE IS MAKING ITS LAST STAND in the drippy forests of Florida. Its name is Torreya taxifolia, a species of yew tree. Only a few hundred of its kind remain—in some cases, nothing but a few green sprouts pleading for life on a rotting stump. But the tree’s quiet demise far away in the woods is causing lots of hubbub.

Torreya is a charismatic tree. Its needled branches have touched the heart of many a naturalist. And so a loose band of enthusiasts calling themselves the Torreya Guardians is now doing exactly what we’re scolded not to do in this post-kudzu, exotic-wary age. They’re spreading Torreya around.

Like a church smuggling refugees to safe houses, they’re planting Torreya seeds in spots from North Carolina to New York State—up to 1,000 kilometers north of its current geographic range. The Torreya Guardians hope to stem their tree’s decline—which they blame on global warming—by moving it north to cooler climes.

It may sound like a case of eco-vigilantism—a charismatic tree with a cult following—but it’s also a sign of the times.

Species around the world are shifting their range toward higher ground and higher latitude to keep pace with global warming. A spate of recent studies documents the stampede: hundreds of species—from butterflies to birds, plants, bats, and rats—moving poleward by up to 300 kilometers. These surprising numbers are but a prelude of things to come, triggered by a minuscule temperature rise of just 0.6 degrees Celsius over the last century.

The next 100 years will be worse. The most recent Intergovernmental Panel on Climate Change (IPCC) report, published in 2001, predicts 2 to 6 degrees Celsius of warming for the twenty-first century—a rate of heating far faster than our only historical frame of reference, the last glacial retreat 12,000 years ago. Some species will shift thousands of kilometers closer to the poles, says Camille Parmesan, an ecologist at the University of Texas in Austin who is monitoring the movement.

That kind of movement in today’s fragmented habitats spells trouble. “Many species that shifted habitat thousands of years ago may not be able to do it now,” says Parmesan. Up to 70 species of harlequin frogs have already disappeared, as well as hundreds of coral species. Plenty more species, stranded on mountaintops or other shreds of habitat, could also see their ecological niches evaporate before they escape.

It’s leading some people to consider the unspeakable—picking up threatened species and moving them to cooler places.

Human-assisted migration is a solution that some people find scary. It pits our desire to prevent extinctions against deep-rooted values of preserving ecosystems in their native state. And it involves a level of human meddling that some have called hubris.

But the big picture is more complicated than that. If predictions are right, then climate change over the next 200 years will make anything that a few wayward naturalists do look tame. It will melt away biological communities that we know today, shuffling the deck of surviving species into new ecosystems.

When we contemplate the impacts of climate change, we tend to think of extinctions and die-offs. But would we be better prepared if we thought more in terms of new arrivals and turnover? If we’ve learned anything from how ecosystems have responded to massive climate changes in the past, it is that biological communities are ephemeral things.

THE IDEA OF MOVING SPECIES to save them is already starting to pit ecologists against one another. “When you go to meetings, people are talking about this,” says Richard Primack, a plant ecologist at Boston University. “Some people think it’s a good idea, and other people become quite angry when I mention this.”

The naysayers offer plenty of objections. For one, transplantation can turn the meekest bunny into a marauding invader. Consider the Monterey pine: it inhabits just a few narrow strips along California’s coastal waterfront, and many biologists consider it threatened in its native range. In fact, it’s just the kind of species that you would expect to suffer from climate change: it requires a specific microclimate of ocean fog and seasonally arid soils and cannot disperse due to commercial development that hems it in.

But move it, and it spreads. Across the southern hemisphere—in Chile, New Zealand, Australia, and South Africa—the Monterey pine has escaped lumber plantations and steamrolled its way up and down the coasts.

And then there’s the black locust tree, native to the Appalachian Mountains of North America, which has handily escaped groves that were planted on farmland further north. From New York to Wisconsin, colonies of black locust are pushing aside native plants—in some cases, rare endemic communities. Worse yet, this is happening right where you’d want to move the tree—several hundred kilometers north of its current distribution, where climate models predict it will thrive in 100 years.

It’s one thing to decide to move a species, concludes Mark Schwartz, an ecologist at the University of California in Davis. “It is much harder to find people who want to bring these outside species into their communities.” Schwartz acknowledges that people will inevitably resort to assisted migration to rescue some species, but he hopes that guidelines can be developed to regulate the practice. An upcoming paper by Schwartz and two colleagues will address these issues in Conservation Biology.

Invasion is but one threat, yet maybe not even the worst. “Really the other danger is much more serious,” says Primack, “that our efforts to introduce species will fail.” Species with specialized niches are generally the ones we’ll need to move, he says, and despite rare examples like the Monterey pine, kick-starting new populations is usually challenging.

Primack points to the example of Minot Pratt, a close friend of Henry David Thoreau who once prowled the woods around Concord, Massachusetts. Pratt liked to garden with nature. Throughout the 1850s, he worked to introduce new plants into the Concord area that he felt would grow there, based on his own observations. Although he planted over 60 species, the newcomers never took hold.

“One hundred and forty years later, only two of these species remain,” says Primack. “They exist as just a few individuals.”

But human-assisted migration could turn haywire in plenty of other ways, too. Guy Midgley, an ecologist at the South African National Biodiversity Institute in Cape Town, personally shrinks from the idea, even though it could possibly save some of his pet species.

Midgley studies the Cape Floristic Province, an area of low coastal mountains that hosts 6,000 endemic species, including 300 species of protea, a flowering plant. Many protea seeds are dispersed by ants—meaning they move just a few meters per generation. As little as one kilometer of rolling sandstone hills separate communities of entirely different species. And there, within this hotbed of biodiversity, lies the problem.

Given their recent evolutionary separation, protea species readily interbreed—which worries Midgley. “If you start moving these around,” he says, “you’ll get hybrid zones. What happens if you evolve a new hybrid species that eliminates the parent species you were trying to save?” But even without human meddling, climate change itself will rearrange our ecosystems.

ONE HUNDRED AND FIFTY YEARS AGO, Charles Darwin imagined that the movement of species in response to climate change was an orderly affair. He assumed that as glaciers expanded and retreated over the eons, species had shuffled north and south en masse, as intact communities. “During their long southern migration and re-migration northward,” wrote Darwin, “they will have kept in a body together; consequently their mutual relations will not have been much disturbed.” (1)

If that were true, then it might bolster the naysayers’ claims that assisted migration would do more harm than good by disrupting ecosystems that might otherwise have shifted without mixing. But Darwin could not have been more wrong.

Studies of pollen, seeds, and other fossils from the last glacial retreat 10,000 to 17,000 years ago regularly show species living in odd combinations. Fourteen thousand years ago, mixed forests of spruce, oak, ash, and hophornbeam dominated much of the land from Minnesota to Ohio to Tennessee. “That’s a combination we just don’t see today,” says Stephen Jackson, a paleoecologist at the University of Wyoming in Laramie. “The ranges of all of those taxa overlap today, but we don’t see areas where they’re the dominant combination.”

Paleoecologists call these odd combinations no-analog communities; nothing like them exists in the present world. They arose from odd combinations of climate variables—temperature, rain, seasonality, fire, floods, soil type, and topography—that don’t exist today. Two species that lived side by side in the same ecosystem for eons often reacted differently to climate change. One species might creep uphill or up-globe, while the other persisted in the same place, even as other tree species in the same forest canopy faded to brown.

“There’s a good deal of evidence that points to our going into a no-analog world within the next 100 to 200 years,” says Jackson. “The communities of the future will look very different from the communities of today.”

A team lead by A. Townsend Peterson at the University of Kansas in Lawrence has modeled the likely movements of 1,870 species of birds, mammals, and insects across Mexico, based on projected climate change for the year 2055. Their model, published in Nature, predicts that species turnover due to local die-offs and new arrivals will exceed 40 percent in many places. (2)

Of course, predictions are just that. But in some places, the tilt toward an age of no analogs may have already begun.

Take, for example, the die-offs that have withered juniper-piñon woodlands across the American Southwest. The region suffered from drought in the 1950s and again starting several years ago. During the most recent drought, 100 percent of piñon trees died in some parts of Arizona and New Mexico. Grasses—already reduced by grazing—also died. The impact has been profound.

“You’re probably not going to have a similar system, with similar energy and water budgets, for 50 to 70 years at best,” says David Breshears, a University of Arizona ecologist who has helped document the die-off. But that’s a retrospective view—trying to get back to what we know. The prospective view could be a leap into the unknown.

What actually happens will depend on future dry spells; climate models predict increasingly frequent droughts in the Southwest, and that has some people wondering whether these die-offs aren’t simple one-shot events but rather the tipping point for more permanent change. “It’s one of the most disturbing questions from a conservation point of view,” says Jackson. “Will these dead trees be replaced by piñons, or will we be ratcheted to something else?” Those die-offs may well open the door to new arrivals—species from lower elevations or lower latitudes with greater tolerance to hot and dry. No one can say which species those might be.

The no-analog scenario also invites a different view of the black locust invasions in New York. Black locust generally invades pine barren areas populated by scrub oaks, herbs, and grasses—many of them rare, endemic species. As an avid nitrogen-fixer, black locust enriches the nutrient-poor soils in those communities—setting the stage for other outsiders, such as honeysuckle, to move in even after the trees are gone. “You could end up with very a different soil substrate in time,” says Steven Rice of Union College in Schenectady, New York. “The possibility that it might alter successional trajectories is real.” Add tomorrow’s no-analog climates into the picture, and those bending trajectories could become permanent detours, leading to late-successional ecosystems a century from now that nurture some of the same pine barren species of today—but blended with a radically different mix of other species.

It’s true that humans have set this ecological Rube Goldberg machine in motion by introducing the black locust, but it could also be seen as a natural process. Regardless of how they got there, those nitrogen-spewing trees are growing exactly where climate models predict they will thrive in 100 years. So even without human interference, they might have gotten there anyway.

THE FACT THAT ASSEMBLAGES OF SPECIES will migrate more like cats than a herd of buffalo poses problems for anyone trying to transplant them in an intelligent way. Another problem is that it’s one thing to move a few charismatic species such as leopards, pandas, or Torreya—but quite another to move every anonymous species, every low-down soil fungus or belly-crawling centipede that lives alongside those biodiversity superstars. That has ecologists thinking along different lines.

“What we’re trying to do is enable nature to migrate on its own,” says Bill Stanley, Director of the Global Climate Change Initiative at The Nature Conservancy (TNC). His NGO and others are working to design nature reserves that will accommodate the natural movement of species as they respond to climate.

In China’s Yunnan Province, TNC and partners are working not only to protect temperate mountain forests but also to secure grassland areas immediately upslope of those forests, where at least some of the forest species will migrate as temperatures rise. The World Wildlife Foundation has pursued a similar strategy with its coastal marine reserves in East Africa, working to establish a series of stepping-stone reserves that will help species to hop poleward in small increments. And in the Cape Floristic Province of South Africa, several migration corridors (including the Cederberg Mega-Reserve) are being established to connect mountains with coastlines.

The challenge is to predict where and how quickly species will actually move. In the Cape Floristic Province, Midgley and Lee Hannah (of Conservation International) have modeled the future movements of 280 species of protea. Their models incorporate not only changes in climate but also modes of seed dispersal—ant versus wind—for each species. Their results could help prioritize what other land should be set aside to protect the largest number of species.

But migration corridors aren’t all laissez-faire. Making them work could sometimes involve hands-on management, including the use of fire.

In the foothills of the Sierra Nevada Mountains of California, the endemic blue oak is expected to migrate upslope by about 300 meters in the next century. But those upslope areas are already covered in conifer forests—blocking blue oaks from moving in. “If you have fire in that forest above the blue oaks, that would open things up,” says Hannah, who is working with TNC to model likely scenarios. “Blue oaks might then be able to establish seedlings up there.” In other words, the swiftness of climate change could necessitate actively burning some areas in order to speed the movement of species.

Facilitating the movement of species could also involve genetic manipulation. People usually assume that as isotherms on the weatherman’s map move poleward, the individuals at the warmer end of a species’ geographic range will experience problems first. But some species could decline simultaneously across their entire range.

The devil in the details is local adaptation, says Julie Etterson, an evolutionary biologist at the University of Minnesota in Duluth. Even in species with wide north-south ranges, individuals in a given location are often genetically adapted to soil and other local conditions unrelated to climate. “A plant smack-dab in the center of the range may experience as much challenge with climate change as something on the southern margin,” says Etterson. She suggests overcoming this problem by promoting the migration of genes rather than individuals. (3) Hybridizing plants from warmer areas of a species’ distribution with those at the cooler edge might improve heat tolerance and buy time for migration.

ONE THING IS CERTAIN: species—tens of thousands of them—will move whether or not humans do the moving. And many currently protected habitats will become populated with exotic species. The question is whether newly populated habitats will function at least on a basic level.

“You may have the same hydrology or ecosystem services,” says Etterson. “But whether or not that’s what we want, that’s a question of our values.” The single comfort may be that people working at cash-strapped NGOs have looked that question in the eye plenty of times before.

“The closest analogy that I can think of is Hawaii,” says Stanley. “You have a large percentage of the community, large parts of Hawaii, that are not native. And they have value because they are a distinct and unique community.” Invasive strawberry guava trees, for example, dominate lowland forests across the island of Oahu. In some places, the species accounts for 50 percent of all trees. Yet these forests supply the island with a reliable watershed and provide havens for the endangered Hawaiian bat and numerous species of endangered tree snails. So despite the invasive scourge, a large swath of this forest is now protected through the Ko’olau Mountains Watershed Partnership.

“I think that with climate change you’re going to be looking at that [kind of situation],” says Stanley. “Those new communities that are moving in [with climate change] could very well be of interest to The Nature Conservancy.”

And then there’s the starry-eyed notion of moving species in order to save them one by one—assisted migration à la Torreya Guardians. On the surface, it’s impractical and expensive.

“That said, there may be some incredibly iconic species that people decide this is worthwhile for,” says Lara Hansen, senior climate change scientist for the WWF in Washington, D.C. By that, she means the pandas, tigers, and Torreyas of the world, which humans have a history of manipulating in and out of captivity. The decision to do so—or not—will be an emotional one.

For better or worse, we humans have played favorites with species for thousands of years, from our first days of burning forests and scattering the seeds of useful plants. Whatever surprises climate change might bring, that long history seems likely to continue.

For more information on the latest ideas and research about assisted migration:

1. McLachlan, J.S., J.J. Hellmann and M.W. Schwartz. In press. A framework for debate of assisted migration in an era of climate change. Conservation Biology.

2. Zimmer, C. January 23, 2007. A Radical Step to Preserve a Species: Assisted Migration. The New York Times.

3. Contact Jason McLachlan.

Literature Cited

1. The Origin of Species, chapter 11, 1859. 2. Peterson, A.T. et al. 2002. Future projections
for Mexican faunas under global climate change scenarios. Nature 416(6881):626-629. 3. Norris, S. 2006. Evolutionary Tinkering,
Conservation In Practice, 7(3):28-34.

About the Author

Douglas Fox is a freelance writer based in San Francisco. He has written for New Scientist, Natural History, and Discover.



  • Steven Schaaf March 22, 2009 at 12:54 pm

    I have been looking into planting some California endemic trees in British Columbia. It is a complex issue, and must be carefully applied to avoid catastrophe. On the other hand, doing nothing has proven ineffective to solve the extinction issue which we’ve created.


  • Charly July 14, 2009 at 8:10 pm

    I would guess that 50 percent of the plant mass in the U.S. is non-endemic to its area. Grasses and “weeds” are spread around the world with cattle. Beggars tick, native to tropical America, now grows in temperate and tropical climates nearly every where. The genie is out of the bottle. Unless gardeners and farmers are limited to endemic species, and food transport is eliminated we will have the uncontrolled introduction of species. Imagine that I put potato peelings in my compost pile. What bacteria and fungi have I introduced into the ecosystem? What’s the big deal about planting some Torreya trees that seems poorly adapted to spread. I would be much more worried about the rotting boniato and malanga from Costa Rica I see in the market, or the stuff in the Chinese market that might end up in the soil. It is the little tiny things that spread in the billions that are the risk.


  • Marco February 15, 2010 at 8:10 am

    maybe some europeans will take care about the Torreya taxifolia.Hope they will give us some trees to plant, so we can save them there, if the american population is extincted….


  • Marc September 22, 2010 at 1:27 pm

    Hi, where i can get those seeds.I want to sow them in Germany to install a exil for them and a refuge.
    Who can help me?



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