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Don't judge species on their origins

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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 Eltons 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
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
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.
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.
Dont 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.
The book that began
invasion ecology:
9 JUNE 2011 | VOL 474 | NATURE | 153
© 2011 Macmillan Publishers Limited. All rights reserved
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 peoples 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.
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.
1. Carroll, S. P. Evol. Appl. 4, 184–199 (2011).
2. Fleishman, E. et al. Bioscience 61, 290–300 (2011).
3. Chew, M. K. & Hamilton, A. L. in Fifty Years of
Invasion Ecology (ed Richardson, D. M.) 35–47
(Wiley-Blackwell, 2011).
4. Wilcove, D. S., Rothstein, D., Dubow, J., Phillips, A.
& Losos, E. BioScience 48, 607–615 (1998).
5. Davis, M. A. Invasion Biology (Oxford Univ. Press,
6. Gardener, M. R., Cordell, S., Anderson, M. &
Tunnicliffe, R. D. Rangeland J. 32, 407–417 (2010).
7. Chew, M. K. J. Hist. Biol. 42, 231–266 (2009).
8. Stromberg, J. C., Chew, M. K., Nagler, P. L. &
Glenn, E. P. Rest. Ecol. 17, 177–186 (2009).
9. Aukema, J. E. et al. Bioscience 60, 886–897 (2010).
10. Gleditsch, J. M. & Carlo, T. J. Diversity Distrib. 17,
244–253 (2010).
Full author affiliations accompany this article online
Management of introduced species such as (left to right) tamarisks, pheasants, honeysuckle and zebra mussels should be based on rational, not emotive reasons.
154 | NATURE | VOL 474 | 9 JUNE 2011
© 2011 Macmillan Publishers Limited. All rights reserved
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... The implementation of transitional measures may be hindered by a reluctance to accept change or to further intervene in changing systems. For example, proposals to translocate species outside of their historical range to assist the species' survival in light of climate change (Hoegh-Guldberg et al., 2008), to value the contributions of populations outside of their "native range" toward preventing extinction (Wallach et al., 2020), or to value all species for their intrinsic and instrumental values regardless of their origin (Davis et al., 2011;Schlaepfer, 2018) are routinely opposed by researchers with static, nativist world-views (Cuthbert et al., 2020;Pauchard et al., 2018;Ricciardi & Simberloff, 2009). ...
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Usable space for northern bobwhite (Colinus virginianus) has been reduced across South Texas rangelands because of the spread of non-native, invasive grasses. A native grass, tanglehead (Heteropogon contortus), has also rapidly expanded its dominance in South Texas within the past 25 yr. It has formed high-density monocultures, similar to non-native grasses, which are associated with losses of herbaceous diversity and bare ground, both key components of bobwhite habitat. The objectives of our research were to 1) determine selection-avoidance of invasive grass cover by bobwhites and 2) determine effects of invasive grass cover on vegetation characteristics. We hypothesized the effects of tanglehead on bobwhite would be negative and similar to the effect of non-native grasses in the region. We recorded covey locations via helicopter surveys in December 2014 and 2015, with field measurements of bobwhite habitat at covey and random locations taken each spring. We measured grass and forb species richness, herbaceous vegetation height, woody-plant cover, tanglehead canopy cover, and non-native grass canopy cover at all covey detections and an equal number of random locations. We developed continuous selection ratios using Simple Saddlepoint Approximations to determine habitat selection by bobwhites. We also used quantile regression, at the 10th, 50th, and 90th quantiles, to determine relationships between invasive grass cover and vegetation factors. Bobwhites avoided areas of high canopy cover (> 20%) of all invasive grasses measured. We found negative relationships between native and non-native invasive grass cover and forb and grass species richness, as well as bare ground, and a positive relationship with herbaceous vegetation height at all quantiles modeled. Our results demonstrate the negative effects of increased tanglehead cover on native rangeland habitats, which aligns with similar non-native invasive grasses. Further expansion by tanglehead has the potential to significantly reduce usable space for bobwhites in South Texas.
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With the exception of climate change, biological invasions have probably received more attention during the past ten years than any other ecological topic. Written fifty years after the publication of Elton's pioneering monograph on the subject, Invasion Biology provides a comprehensive and up-to-date review of the science of biological invasions while also offering new insights and perspectives relating to the processes of introduction, establishment, and spread. The book connects science with application by describing the health, economic, and ecological impacts of invasive species as well as the variety of management strategies developed to mitigate harmful impacts. The author critically evaluates the approaches, findings, and controversies that have characterized invasion biology in recent years, and suggests a variety of future research directions. Carefully balanced to avoid distinct taxonomic, ecosystem, and geographic biases, the book addresses a wide range of invasive species (including proti
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Nonindigenous forest insects and pathogens affect a range of ecosystems, industries, and property owners in the United States. Evaluating temporal patterns in the accumulation of these nonindigenous forest pests can inform regulatory and policy decisions. We compiled a comprehensive species list to assess the accumulation rates of nonindigenous forest insects and pathogens established in the United States. More than 450 nonindigenous insects and at least 16 pathogens have colonized forest and urban trees since European settlement. Approximately 2.5 established nonindigenous forest insects per year were detected in the United States between 1860 and 2006. At least 14% of these insects and all 16 pathogens have caused notable damage to trees. Although sap feeders and foliage feeders dominated the comprehensive list, phloem-and wood-boring insects and foliage feeders were often more damaging than expected. Detections of insects that feed on phloem or wood have increased markedly in recent years.
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Biotic invaders and similar anthropogenic novelties such as domesticates, transgenics, and cancers can alter ecology and evolution in environmental, agricultural, natural resource, public health, and medical systems. The resulting biological changes may either hinder or serve management objectives. For example, biological control and eradication programs are often defeated by unanticipated resistance evolution and by irreversibility of invader impacts. Moreover, eradication may be ill-advised when nonnatives introduce beneficial functions. Thus, contexts that appear to call for eradication may instead demand managed coexistence of natives with nonnatives, and yet applied biologists have not generally considered the need to manage the eco-evolutionary dynamics that commonly result from interactions of natives with nonnatives. Here, I advocate a conciliatory approach to managing systems where novel organisms cannot or should not be eradicated. Conciliatory strategies incorporate benefits of nonnatives to address many practical needs including slowing rates of resistance evolution, promoting evolution of indigenous biological control, cultivating replacement services and novel functions, and managing native–nonnative coevolution. Evolutionary links across disciplines foster cohesion essential for managing the broad impacts of novel biotic systems. Rather than signaling defeat, conciliation biology thus utilizes the predictive power of evolutionary theory to offer diverse and flexible pathways to more sustainable outcomes.
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Dispersal of biota by humans is a hallmark of civilization, but the results are often unforeseen and sometimes costly. Like kudzu vine in the American South, some examples become the stuff of regional folklore. In recent decades, "invasion biology," conservation-motivated scientists and their allies have focused largely on the most negative outcomes and often promoted the perception that introduced species are monsters. However, cases of monstering by scientists preceded the rise of popular environmentalism. The story of tamarisk (Tamarix spp.), flowering trees and shrubs imported to New England sometime before 1818, provides an example of scientific "monstering" and shows how slaying the monster, rather than allaying its impacts, became a goal in itself. Tamarisks' drought and salt tolerance suggested usefulness for both coastal and inland erosion control, and politicians as well as academic and agency scientists promoted planting them in the southern Great Plains and Southwest. But when erosion control efforts in Arizona, New Mexico and Texas became entangled with water shortages, economic development during the Depression and copper mining for national defense during World War Two, federal hydrologists moved quickly to recast tamarisks as water-wasting foreign monsters. Demonstrating significant water salvage was difficult and became subsidiary to focusing on ways to eradicate the plants, and a federal interagency effort devoted specifically to the latter purpose was organized and continued until it, in turn, conflicted with regional environmental concerns in the late 1960s.
Eradication of an invasive species is a holy grail sought by land managers, scientists and policy makers alike. This prize is particularly attractive to funding bodies that foresee a one-off investment to solve a problem. We evaluate a 20-year eradication project on the annual weed Martynia annua L. from remote Gregory (Jutburra) National Park in northern Australia. M. annua was regionally introduced in the 1860s and has since become naturalised and locally abundant on some pastoral properties. When land use changed from grazing to national park in the mid 1980s, M. annua was thought to be a serious problem. An eradication project was started in the late 1980s. Eradication of all individuals from within the National Park has not been successful but there have been other benefits of the project. We analysed operational, biological, social and economic criteria to find that the principal barriers to eradication were: occasional inaccessibility during the crucial seed production window; many widely dispersed small infestations; a perennial seed bank; and long-distance dispersal mechanisms. The two successes of the project were control of the weed to a level where ecological impact was negligible; and extensive community engagement. A novel approach adopted by the National Park, a biannual event called the Devil’s Claw Festival, has trained and educated hundreds of local, national and international people about biological invasions and conservation issues in remote northern Australia. Long-term institutional leadership and investment have been crucial for this project. We offer recommendations to policy makers embarking on eradication projects of widespread rangeland weeds.
Biologists are nearly unanimous in their belief that humanity is in the process of extirpating a significant portion of the earth's spe­ cies. The ways in which we are doing so reflect the magnitude and scale of human enterprise. Everything from highway construction to cattle ranch­ ing to leaky bait buckets has been implicated in the demise or endan­ germent of particular species. Ac­ cording to Wilson (1992), most of these activities fall into four major categories, which he terms "the mind­ less horsemen of the environmental apocalypse": overexploitation, habi­ tat destruction, the introduction of non-native (alien) species, and the spread of diseases carried by alien species. To these categories may be added a fifth, pollution, although it can also be considered a form of habitat destruction. Surprisingly, there have been reIa­ tively few analyses of the extent to which each of these factors-much less the more specific deeds encomDavid S. Wilcove is a senior ecologist at the Environmental Defense Fund, Wash­ ington, DC 20009. David Rothstein re­ ceived his J.D. in 1997 from Northeastern
Initially introduced to western United States to provide ecosystem services such as erosion control, Tamarix by the mid-1900s had became vilified as a profligate waster of water. This large shrub continues, today, to be indicted for various presumed environmental and economic costs, and millions of dollars are expended on its eradication. In this review, we examine the role of scientists in driving changes in perceptions of Tamarix from valuable import to vilified invader and (in some instances) back to a productive member of riparian plant communities. Scientists over the years have sustained a negative perception of Tamarix by, among other things, (1) citing outmoded sources; (2) inferring causation from correlative studies; (3) applying conclusions beyond the scope (domain) of the studies; and (4) emphasizing findings that present the species as an extreme or unnatural agent of change. Recent research is challenging the prevailing dogma regarding Tamarix’s role in ecosystem function and habitat degradation and many scientists now recommend management shifts from “pest plant” eradication to systemic, process-based restoration. However, prejudice against this and other non-native species persists. To further close the gap between science and management, it is important for scientists to strive to (1) cite sources appropriately; (2) avoid reflexive antiexotic bias; (3) avoid war-based and pestilence-based terminology; (4) heed the levels of certainty and the environmental domain of studies; (5) maintain up-to-date information on educational Web sites; and (6) prior to undertaking restoration or management actions, conduct a thorough and critical review of the literature.
Aim Biological invasions pose one of the most severe threats to global biodiversity. Still, invasions can create positive ecological relationships and services, which can sometimes result in challenges for conservation efforts. A case in point is the invasion of alien plants that form mutualisms with native frugivorous birds. Here, we examined the correlation between honeysuckles (Lonicera spp.) and the bird communities in a landscape of central Pennsylvania during the fall months. Location State College area in central Pennsylvania, USA. Methods We conducted point counts to quantify the abundance of birds and fleshy-fruited plant species within a 187.5 km2 landscape that included forested, urban and agricultural lands. We also compared fruit-removal rates for a native fruiting plant under low and high Lonicera densities. Results The abundance of birds showed a strong positive association with Lonicera fruits, with the abundance of Turdus migratorius and Dumetella carolinensis showing the strongest correlations. We also found that fruit-removal rates were 30% larger for a native plant species in areas of high Lonicera density compared to a site with low density of Lonicera. Main conclusions Our results suggest that Lonicera may currently serve as a main axis for the organization of bird communities and the networks of frugivore–plant interactions in central Pennsylvania. Since populations of key bird frugivores may be currently depending on Lonicera resources, we argue that control measures should account for the negative effects that the loss of this fruit resource could have on populations of native consumers in highly invaded regions.
Davis is DeWitt Wallace professor of biology at Macalester College
  • A Mark
  • Usa Minnesota
  • K Matthew
  • Richard J Chew
  • Ariel E Hobbs
  • John J Lugo
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.