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Over the past few decades, non-nativespecies have been vilified for drivingbeloved native species to extinction

and generally polluting natural environ-ments. Intentionally or not, such characteri-zations have helped to create a pervasive biasagainst alien species that has been embracedby the public, conservationists, land manag-ers and policy-makers, as well by as manyscientists, throughout the world.

Increasingly, the practical value of thenative-versus-alien species dichotomy inconservation is declining, and even becomingcounterproductive1. Yet many conser-

vationists still consider the distinction a coreguiding principle2.

Todays management approaches mustrecognize that the natural systems of the pastare changing forever thanks to drivers suchas climate change, nitrogen eutrophication,increased urbanization and other land-usechanges. It is time for scientists, land man-agers and policy-makers to ditch this preoc-cupation with the nativealien dichotomy

and embrace more dynamic and pragmatic

approaches to the conservation and manage-ment of species approaches better suitedto our fast-changing planet.

The concept of nativeness was first outlinedby the English botanist John Henslow in 1835.By the late 1840s, botanists had adapted theterms native and alien from common law tohelp them distinguish those plants that com-posed a true British flora from artefacts3.

Over the next century, many botanists and

a few zoologists described and studied intro-duced species without being aware that otherswere doing the same. By the time the Britishecologist Charles Elton wrote his famous 1958book The Ecology of Invasions by Animalsand Plants, some 40 scientists had publisheddescriptions of non-natives, but no consensushad been reached on the desirability of inter-vening when alien species were introduced.

It wasnt until the 1990s that invasionbiology became a disci-pline in its own right. Bythis point, partly fuelledby Eltons book, propo-

nents of biodiversity

preservation and ecological restorationcommonly used military metaphors andexaggerated claims of impending harm tohelp convey the message that introducedspecies are the enemies of man and nature.

Certainly, some species introduced byhumans have driven extinctions and under-mined important ecological services such as

clean water and timber resources. In Hawaii,for instance, avian malaria probably intro-duced in the early 1900s when Europeansettlers brought in song and game birds has killed off more than half of the islandsnative bird species. Zebra mussels (Dreissenapolymorpha), originally native to the lakes ofsoutheast Russia and accidentally introducedto North America in the late 1980s, have costthe US power industry and water utilitieshundreds of millions (some say billions) ofdollars in damage by clogging water pipes.

But many of the claims driving peoplesperception that introduced species pose an

apocalyptic threat to biodiversity are notbacked by data. Take the conclusion made ina 1998 paper4that invaders are the second-greatest threat to the survival of threatenedor endangered speciesafter habitat destruc-tion. Little of the information used to supportthis claim involved data, as the originalauthors were careful to point out. Indeed,recent analyses suggest that invaders do notrepresent a major extinction threat to mostspecies in most environments predatorsand pathogens on islands and in lakes beingthe main exception5. In fact, the introduc-tion of non-native species has almost alwaysincreased the number of species in a region5.

The effects of non-native species may varywith time, and species that are not causingharm now might do so in the future. But thesame is true of natives, particularly in rapidlychanging environments.

BIOLOGICAL BIAS

Nativeness is not a sign of evolutionaryfitness or of a species having positive effects.The insect currently suspected to be killingmore trees than any other in North Americais the native mountain pine beetle Dendroc-tonus ponderosae. Classifying biota accord-ing to their adherence to cultural standards

of belonging, citizenship, fair play andmorality does not advance our understand-ing of ecology. Over the past few decades,this perspective has led many conservationand restoration efforts down paths that makelittle ecological or economic sense.

Take the effort to eradicate the devi lsclaw plant (Martynia annua), introducedfrom Mexico to Australia in the nineteenthcentury, probably as a horticultural oddity.For the past 20

years, the Northern Terri-tory Parks and Wildlife Service, along withhundreds of volunteers, have been manuallydigging up the plants along 60kilometres

of creek bed in Gregory National Park.

Dont judge specieson their originsConservationists should assess organisms on

environmental impact rather than on whether they arenatives, argue Mark Davisand 18 other ecologists.

A forester engages in efforts to eradicate the velvet tree Miconia calvescensin Hawaii.

F.LANTING/NAT.

GEOGR.

NATURE.COM

The book that beganinvasion ecology:

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Today, devils claw is still found in the

park and is abundant in adjacent cattle sta-tions. Is the effort worth it? There is littleevidence that the species ever warrantedsuch intensive management it does notsubstantially change the fundamental char-acter of its environment by, say, reducingbiodiversity or altering nutrient cycling6.

Another example is the US attempt toeradicate tamarisk shrubs (Tamarixspp)introduced from Eurasia and Africa intothe countrys arid lands in the nineteenthcentury. These drought-, salt- and erosion-resistant plants were initially welcomed intothe United States, first as ornamental speciesfor peoples gardens and later as shade trees fordesert farmers. Then in the 1930s, when watersupplies in eastern Arizona, central NewMexico and western Texas ran short, theywere indicted as water thieves, and later, dur-ing the Second World War, as alien invaders.Beginning in 1942, they became the objectof a 70-year suppression project involvingherbicides, bulldozers and the picturesquelynamed LeTourneau Tree Crusher7.

NEW GUIDING PRINCIPLES

Ecologists have since discovered that tama-risks use water at a rate comparable to thatof their native counterparts8. And the plants

are now the preferred nesting habitat of theendangered southwestern willow flycatcherEmpidonax traillii extimus.

Tamarisks, which survive under commonwater-management regimes that destroynative trees and shrubs, arguably have acrucial role in the functioning of the human-modified river-bank environment9. Yetbetween 2005 and 2009 alone, the US Con-gress authorized US$80 million to supportongoing tamarisk control and eradication.

What, then, should replace the nativeversus non-native species distinction asa guiding principle in conservation and

restoration management?

Most human and natural communities

now consist both of long-term residentsand of new arrivals, and ecosystems areemerging that never existed before. It isimpractical to try to restore ecosystems tosome rightful historical state. For example,of the 30planned plant eradication effortsundertaken in the Galapagos Islands since1996, only 4 have been successful. We mustembrace the fact of novel ecosystems andincorporate many alien species into man-agement plans, rather than try to achieve theoften impossible goal of eradicating them ordrastically reducing their abundance. Indeed,many of the species that people think of asnative are actually alien. For instance, in theUnited States, the ring-necked pheasant, thestate bird of South Dakota, is not native tothe great plains of North America but wasintroduced from Asia as a game bird in thelatter half of the nineteenth century.

Specifically, policy and managementdecisions must take into account the positiveeffects of many invaders. During the 1990s,the US Department of Agriculture (USDA)declared several species of introduced hon-eysuckles to be alien (harmful), and bannedtheir sale in more than 25 states. Ironically,from the 1960s to the 1980s, the USDA hadintroduced many of these same species in land

reclamation projects, and to improve birdhabitats. Recent data suggest that the agencysinitial instincts may have been appropriate. InPennsylvania, more non-native honeysucklesmean more native bird species. Also the seeddispersal of native berry-producing plantsis higher in places where non-native honey-suckles are most abundant10.

Clearly, natural-resource agencies andorganizations should base their manage-ment plans on sound empirical evidenceand not on unfounded claims of harmcaused by non-natives. Another valuablestep would be for scientists and profes-

sionals in conservation to convey to the

public that many alien species are useful.

We are not suggesting that conservation-ists abandon their efforts to mitigate seri-ous problems caused by some introducedspecies, or that governments should stoptrying to prevent potentially harmful spe-cies from entering their countries. But weurge conservationists and land managers toorganize priorities around whether speciesare producing benefits or harm to biodiver-sity, human health, ecological services andeconomies. Nearly two centuries on fromthe introduction of the concept of native-ness, it is time for conservationists to focusmuch more on the functions of species, andmuch less on where they originated.

Mark A. Davisis DeWitt Wallace professorof biology at Macalester College, St Paul,Minnesota, USA.Matthew K. Chew,Richard J. Hobbs, Ariel E. Lugo, John J.Ewel, Geerat J. Vermeij, James H. Brown,Michael L. Rosenzweig, Mark R. Gardener,Scott P. Carroll, Ken Thompson,Steward T. A. Pickett, Juliet C. Stromberg,Peter Del Tredici, Katharine N. Suding,Joan G. Ehrenfeld, J. Philip Grime,Joseph Mascaro, John C. Briggs.e-mail: davis@macalester.edu

1. Carroll, S. P. Evol. Appl.4,184199 (2011).2. Fleishman, E. et al. Bioscience61,290300 (2011).3. Chew, M. K. & Hamilton, A. L. in Fifty Years of

Invasion Ecology(ed Richardson, D. M.) 3547(Wiley-Blackwell, 2011).

4. Wilcove, D. S., Rothstein, D., Dubow, J., Phillips, A.& Losos, E. BioScience48,607615 (1998).

5. Davis, M. A. Invasion Biology(Oxford Univ. Press,2009).

6. Gardener, M. R., Cordell, S., Anderson, M. &Tunnicliffe, R. D.Rangeland J.32,407417 (2010).

7. Chew, M. K.J. Hist. Biol. 42,231266 (2009).8. Stromberg, J. C., Chew, M. K., Nagler, P. L. &

Glenn, E. P.Rest. Ecol.17,177186 (2009).9. Aukema, J. E. et al. Bioscience60,886897 (2010).10. Gleditsch, J. M. & Carlo, T. J. Diversity Distrib.17,

244253 (2010).

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Management of introduced species such as (left to right) tamarisks, pheasants, honeysuckle and zebra mussels should be based on rational, not emotive reasons.

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