FEATURE

(p. 28-34)

When Worlds Collide

Climate change will shuffle the deck of plants, animals, and ecosystems in ways we'veonly begun to imagine

By Douglas FoxJanuary-March 2007 (Vol. 8, No. 1)

A DYING RACE IS MAKING ITS LAST STAND in the drippy forests of Florida. Its name isTorreya taxifolia, a species of yew tree. Only a few hundred of its kind remain--in somecases, nothing but a few green sprouts pleading for life on a rotting stump. But the tree'squiet 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 anaturalist. And so a loose band of enthusiasts calling themselves the Torreya Guardians isnow 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 spotsfrom North Carolina to New York State--up to 1,000 kilometers north of its currentgeographic range. The Torreya Guardians hope to stem their tree's decline--which theyblame 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--butit's also a sign of the times.

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

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

That kind of movement in today's fragmented habitats spells trouble. "Many species thatshifted habitat thousands of years ago may not be able to do it now," says Parmesan. Upto 70 species of harlequin frogs have already disappeared, as well as hundreds of coralspecies. Plenty more species, stranded on mountaintops or other shreds of habitat, couldalso see their ecological niches evaporate before they escape.

It's leading some people to consider the unspeakable--picking up threatened species andmoving them to cooler places.

Human-assisted migration is a solution that some people find scary. It pits our desire toprevent extinctions against deep-rooted values of preserving ecosystems in their nativestate. 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 climatechange over the next 200 years will make anything that a few wayward naturalists dolook tame. It will melt away biological communities that we know today, shuffling thedeck of surviving species into new ecosystems.

When we contemplate the impacts of climate change, we tend to think of extinctions anddie-offs. But would we be better prepared if we thought more in terms of new arrivalsand turnover? If we've learned anything from how ecosystems have responded to massiveclimate 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 againstone another. "When you go to meetings, people are talking about this," says RichardPrimack, a plant ecologist at Boston University. "Some people think it's a good idea, andother people become quite angry when I mention this."

The naysayers offer plenty of objections. For one, transplantation can turn the meekestbunny into a marauding invader. Consider the Monterey pine: it inhabits just a fewnarrow strips along California's coastal waterfront, and many biologists consider itthreatened in its native range. In fact, it's just the kind of species that you would expect tosuffer from climate change: it requires a specific microclimate of ocean fog andseasonally 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 andsteamrolled its way up and down the coasts.

And then there's the black locust tree, native to the Appalachian Mountains of NorthAmerica, 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'dwant to move the tree--several hundred kilometers north of its current distribution, whereclimate models predict it will thrive in 100 years.

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

Invasion is but one threat, yet maybe not even the worst. "Really the other danger ismuch 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, anddespite rare examples like the Monterey pine, kick-starting new populations is usuallychallenging.

Primack points to the example of Minot Pratt, a close friend of Henry David Thoreauwho once prowled the woods around Concord, Massachusetts. Pratt liked to garden withnature. Throughout the 1850s, he worked to introduce new plants into the Concord areathat he felt would grow there, based on his own observations. Although he planted over60 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. GuyMidgley, 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 petspecies.

Midgley studies the Cape Floristic Province, an area of low coastal mountains that hosts6,000 endemic species, including 300 species of protea, a flowering plant. Many proteaseeds are dispersed by ants--meaning they move just a few meters per generation. Aslittle as one kilometer of rolling sandstone hills separate communities of entirely differentspecies. And there, within this hotbed of biodiversity, lies the problem.

Given their recent evolutionary separation, protea species readily interbreed--whichworries 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 youwere trying to save?" But even without human meddling, climate change itself willrearrange our ecosystems.

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

If that were true, then it might bolster the naysayers' claims that assisted migration woulddo more harm than good by disrupting ecosystems that might otherwise have shiftedwithout mixing. But Darwin could not have been more wrong.

Studies of pollen, seeds, and other fossils from the last glacial retreat 10,000 to 17,000years ago regularly show species living in odd combinations. Fourteen thousand yearsago, mixed forests of spruce, oak, ash, and hophornbeam dominated much of the landfrom 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. "Theranges of all of those taxa overlap today, but we don't see areas where they're thedominant combination."

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

"There's a good deal of evidence that points to our going into a no-analog world withinthe next 100 to 200 years," says Jackson. "The communities of the future will look verydifferent from the communities of today."

A team lead by A. Townsend Peterson at the University of Kansas in Lawrence hasmodeled the likely movements of 1,870 species of birds, mammals, and insects acrossMexico, based on projected climate change for the year 2055. Their model, published inNature, predicts that species turnover due to local die-offs and new arrivals will exceed40 percent in many places. (2)

Of course, predictions are just that. But in some places, the tilt toward an age of noanalogs may have already begun.

Take, for example, the die-offs that have withered juniper-piñon woodlands acrossthe American Southwest. The region suffered from drought in the 1950s and againstarting several years ago. During the most recent drought, 100 percent of piñon treesdied 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 waterbudgets, for 50 to 70 years at best," says David Breshears, a University of Arizonaecologist who has helped document the die-off. But that's a retrospective view--trying toget 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 predictincreasingly frequent droughts in the Southwest, and that has some people wonderingwhether these die-offs aren't simple one-shot events but rather the tipping point for morepermanent change. "It's one of the most disturbing questions from a conservation point ofview," says Jackson. "Will these dead trees be replaced by piñons, or will we beratcheted 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. Noone can say which species those might be.

The no-analog scenario also invites a different view of the black locust invasions in NewYork. 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 locustenriches the nutrient-poor soils in those communities--setting the stage for otheroutsiders, such as honeysuckle, to move in even after the trees are gone. "You could endup with very a different soil substrate in time," says Steven Rice of Union College inSchenectady, New York. "The possibility that it might alter successional trajectories isreal." Add tomorrow's no-analog climates into the picture, and those bending trajectoriescould become permanent detours, leading to late-successional ecosystems a century fromnow that nurture some of the same pine barren species of today--but blended with aradically different mix of other species.

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

THE FACT THAT ASSEMBLAGES OF SPECIES will migrate more like cats than a herd ofbuffalo 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 biodiversitysuperstars. 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). HisNGO and others are working to design nature reserves that will accommodate the naturalmovement of species as they respond to climate.

In China's Yunnan Province, TNC and partners are working not only to protect temperatemountain 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 WorldWildlife Foundation has pursued a similar strategy with its coastal marine reserves inEast Africa, working to establish a series of stepping-stone reserves that will help speciesto hop poleward in small increments. And in the Cape Floristic Province of South Africa,several migration corridors (including the Cederberg Mega-Reserve) are beingestablished to connect mountains with coastlines.

The challenge is to predict where and how quickly species will actually move. In theCape Floristic Province, Midgley and Lee Hannah (of Conservation International) havemodeled the future movements of 280 species of protea. Their models incorporate notonly changes in climate but also modes of seed dispersal--ant versus wind--for eachspecies. Their results could help prioritize what other land should be set aside to protectthe largest number of species.

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

In the foothills of the Sierra Nevada Mountains of California, the endemic blue oak isexpected to migrate upslope by about 300 meters in the next century. But those upslopeareas are already covered in conifer forests--blocking blue oaks from moving in. "If youhave 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 toestablish seedlings up there." In other words, the swiftness of climate change couldnecessitate actively burning some areas in order to speed the movement of species.

Facilitating the movement of species could also involve genetic manipulation. Peopleusually assume that as isotherms on the weatherman's map move poleward, theindividuals at the warmer end of a species' geographic range will experience problemsfirst. But some species could decline simultaneously across their entire range.

The devil in the details is local adaptation, says Julie Etterson, an evolutionary biologistat 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 localconditions unrelated to climate. "A plant smack-dab in the center of the range mayexperience as much challenge with climate change as something on the southern margin,"says Etterson. She suggests overcoming this problem by promoting the migration ofgenes 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 formigration.

ONE THING IS CERTAIN: species--tens of thousands of them--will move whether or nothumans do the moving. And many currently protected habitats will become populatedwith exotic species. The question is whether newly populated habitats will function atleast on a basic level.

"You may have the same hydrology or ecosystem services," says Etterson. "But whetheror not that's what we want, that's a question of our values." The single comfort may bethat people working at cash-strapped NGOs have looked that question in the eye plenty oftimes before.

"The closest analogy that I can think of is Hawaii," says Stanley. "You have a largepercentage of the community, large parts of Hawaii, that are not native. And they havevalue because they are a distinct and unique community." Invasive strawberry guavatrees, 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 areliable watershed and provide havens for the endangered Hawaiian bat and numerousspecies of endangered tree snails. So despite the invasive scourge, a large swath of thisforest 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] couldvery 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 byone--assisted migration à la Torreya Guardians. On the surface, it's impractical andexpensive.

"That said, there may be some incredibly iconic species that people decide this isworthwhile for," says Lara Hansen, senior climate change scientist for the WWF inWashington, 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 doso--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. Whateversurprises climate change might bring, that long history seems likely to continue.

1. The Origin of Species, chapter 11, 1859.

2. Peterson, A.T. et al. 2002. Future projectionsfor Mexican faunas under global climate changescenarios. Nature 416(6881):626-629.

3. Norris, S. 2006. Evolutionary Tinkering,Conservation In Practice, 7(3):28-34.

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