Don't judge species on their origins

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O
ver the past few decades, ‘non-native’
species have been vilified for driving
beloved ‘native’ species to extinction
and generally polluting ‘natural’ environ-
ments. Intentionally or not, such characteri-
zations have helped to create a pervasive bias
against alien species that has been embraced
by the public, conservationists, land manag-
ers and policy-makers, as well by as many
scientists, throughout the world.
Increasingly, the practical value of the
native-versus-alien species dichotomy in
conservation is declining, and even becoming
counterproductive1. Yet many conser-
vationists still consider the distinction a core
guiding principle2.
Today’s management approaches must
recognize that the natural systems of the past
are changing forever thanks to drivers such
as climate change, nitrogen eutrophication,
increased urbanization and other land-use
changes. It is time for scientists, land man-
agers and policy-makers to ditch this preoc-
cupation with the native–alien dichotomy
and embrace more dynamic and pragmatic
approaches to the conservation and manage-
ment of species — approaches better suited
to our fast-changing planet.
The concept of nativeness was first outlined
by the English botanist John Henslow in 1835.
By the late 1840s, botanists had adapted the
terms native and alien from common law to
help 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 others
were doing the same. By the time the British
ecologist Charles Elton wrote his famous 1958
book The Ecology of Invasions by Animals
and Plants, some 40 scientists had published
descriptions of non-natives, but no consensus
had been reached on the desirability of inter-
vening when alien species were introduced.
It wasn’t until the 1990s that ‘invasion
biology’ became a disci-
pline in its own right. By
this point, partly fuelled
by Elton’s book, propo-
nents of biodiversity
preservation and ecological restoration
commonly used military metaphors and
exaggerated claims of impending harm to
help convey the message that introduced
species are the enemies of man and nature.
Certainly, some species introduced by
humans 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 European
settlers brought in song and game birds —
has killed off more than half of the islands’
native bird species. Zebra mussels (Dreissena
polymorpha), originally native to the lakes of
southeast Russia and accidentally introduced
to North America in the late 1980s, have cost
the US power industry and water utilities
hundreds of millions (some say billions) of
dollars in damage by clogging water pipes.
But many of the claims driving people’s
perception that introduced species pose an
apocalyptic threat to biodiversity are not
backed by data. Take the conclusion made in
a 1998 paper
4
that invaders are the second-
greatest threat to the survival of threatened
or endangered species after habitat destruc-
tion. Little of the information used to support
this claim involved data, as the original
authors were careful to point out. Indeed,
recent analyses suggest that invaders do not
represent a major extinction threat to most
species in most environments — predators
and pathogens on islands and in lakes being
the main exception5. In fact, the introduc-
tion of non-native species has almost always
increased the number of species in a region
5
.
The effects of non-native species may vary
with time, and species that are not causing
harm now might do so in the future. But the
same is true of natives, particularly in rapidly
changing environments.
BIOLOGICAL BIAS
Nativeness is not a sign of evolutionary
fitness or of a species having positive effects.
The insect currently suspected to be killing
more trees than any other in North America
is the native mountain pine beetle Dendroc-
tonus ponderosae. Classifying biota accord-
ing to their adherence to cultural standards
of belonging, citizenship, fair play and
morality does not advance our understand-
ing of ecology. Over the past few decades,
this perspective has led many conservation
and restoration efforts down paths that make
little ecological or economic sense.
Take the effort to eradicate the devil’s
claw plant (Martynia annua), introduced
from Mexico to Australia in the nineteenth
century, probably as a horticultural oddity.
For the past 20years, the Northern Terri-
tory Parks and Wildlife Service, along with
hundreds of volunteers, have been manually
digging up the plants along 60kilometres
of creek bed in Gregory National Park.
Don’t judge species
on their origins
Conservationists should assess organisms on
environmental impact rather than on whether they are
natives, argue Mark Davis and 18 other ecologists.
A forester engages in efforts to eradicate the velvet tree Miconia calvescens in Hawaii.
F. LANTING/NAT. GEOGR.
NATURE.COM
The book that began
invasion ecology:
go.nature.com/5aiwqt
9 JUNE 2011 | VOL 474 | NATURE | 153
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Today, devil’s claw is still found in the
park and is abundant in adjacent cattle sta-
tions. Is the effort worth it? There is little
evidence that the species ever warranted
such intensive management — it does not
substantially change the fundamental char-
acter of its environment by, say, reducing
biodiversity or altering nutrient cycling6.
Another example is the US attempt to
eradicate tamarisk shrubs (Tamarix spp)
introduced from Eurasia and Africa into
the country’s arid lands in the nineteenth
century. These drought-, salt- and erosion-
resistant plants were initially welcomed into
the United States, first as ornamental species
for people’s gardens and later as shade trees for
desert farmers. Then in the 1930s, when water
supplies in eastern Arizona, central New
Mexico and western Texas ran short, they
were indicted as ‘water thieves’, and later, dur-
ing the Second World War, as ‘alien invaders’.
Beginning in 1942, they became the object
of a 70-year suppression project involving
herbicides, bulldozers and the picturesquely
named LeTourneau Tree Crusher7.
NEW GUIDING PRINCIPLES
Ecologists have since discovered that tama-
risks use water at a rate comparable to that
of their native counterparts8. And the plants
are now the preferred nesting habitat of the
endangered southwestern willow flycatcher
Empidonax traillii extimus.
Tamarisks, which survive under common
water-management regimes that destroy
native trees and shrubs, arguably have a
crucial role in the functioning of the human-
modified river-bank environment9. Yet
between 2005 and 2009 alone, the US Con-
gress authorized US$80 million to support
ongoing tamarisk control and eradication.
What, then, should replace the native
versus non-native species distinction as
a guiding principle in conservation and
restoration management?
Most human and natural communities
now consist both of long-term residents
and of new arrivals, and ecosystems are
emerging that never existed before. It is
impractical to try to restore ecosystems to
some ‘rightful’ historical state. For example,
of the 30planned plant eradication efforts
undertaken in the Galapagos Islands since
1996, only 4 have been successful. We must
embrace the fact of ‘novel ecosystems’ and
incorporate many alien species into man-
agement plans, rather than try to achieve the
often impossible goal of eradicating them or
drastically reducing their abundance. Indeed,
many of the species that people think of as
native are actually alien. For instance, in the
United States, the ring-necked pheasant, the
state bird of South Dakota, is not native to
the great plains of North America but was
introduced from Asia as a game bird in the
latter half of the nineteenth century.
Specifically, policy and management
decisions must take into account the positive
effects of many invaders. During the 1990s,
the US Department of Agriculture (USDA)
declared several species of introduced hon-
eysuckles to be alien (harmful), and banned
their sale in more than 25 states. Ironically,
from the 1960s to the 1980s, the USDA had
introduced many of these same species in land
reclamation projects, and to improve bird
habitats. Recent data suggest that the agency’s
initial instincts may have been appropriate. In
Pennsylvania, more non-native honeysuckles
mean more native bird species. Also the seed
dispersal of native berry-producing plants
is higher in places where non-native honey-
suckles are most abundant10.
Clearly, natural-resource agencies and
organizations should base their manage-
ment plans on sound empirical evidence
and not on unfounded claims of harm
caused by non-natives. Another valuable
step 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 introduced
species, or that governments should stop
trying to prevent potentially harmful spe-
cies from entering their countries. But we
urge conservationists and land managers to
organize priorities around whether species
are producing benefits or harm to biodiver-
sity, human health, ecological services and
economies. Nearly two centuries on from
the introduction of the concept of native-
ness, it is time for conservationists to focus
much more on the functions of species, and
much less on where they originated. ■
Mark A. Davis is DeWitt Wallace professor
of 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
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& Losos, E. BioScience 48, 607–615 (1998).
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6. Gardener, M. R., Cordell, S., Anderson, M. &
Tunnicliffe, R. D. Rangeland J. 32, 407–417 (2010).
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Full author affiliations accompany this article online
at go.nature.com/cgbm1y.
K. MOLONEY/THE NEW YORK TIMES/REDUX/EYEVINE; T. & P. LEESON/ARDEA.COM; P. DEL TREDICI; J. WEST/PHOTOLIBRARY
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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