- What are some of the reasons invasive plants have been deliberately introduced into the U.S.? Discuss at least 3 reasons along with some of the specific plant species used.
- Why are there less invasive species in dry lands and why is that trend changing?
- What is xeriscaping? Discuss the positive and negative sides to xeriscaping.
- What was the research the authors conducted to quantify the potential of xeriscaping as a source of new invasive plants and what were the results?
- What recommendations do the authors offer based on their research findings?
Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Frontiers in Ecology
and
the Environment.
http://www.jstor.org
Global change, global trade, and the next wave of plant
invasions
Author(s): Bethany A Bradley, Dana M Blumenthal, Regan Early, Edwin D Grosholz, Joshua J
Lawler, Luke P Miller, Cascade JB Sorte, Carla M D’Antonio, Jeffrey M Diez, Jeffrey S Dukes,
Ines Ibanez and Julian D Olden
Source: Frontiers in Ecology and the Environment, Vol. 10, No. 1 (February 2012), pp. 20-28
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/41479982
Accessed: 02-06-2015 21:33 UTC
REFERENCES
Linked references are available on JSTOR for this article:
http://www.jstor.org/stable/41479982?seq=1&cid=pdf-reference#references_tab_contents
You may need to log in to JSTOR to access the linked references.
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://www.jstor.org/page/
info/about/policies/terms.jsp
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content
in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship.
For more information about JSTOR, please contact support@jstor.org.
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org
http://www.jstor.org/action/showPublisher?publisherCode=esa
http://www.jstor.org/stable/41479982
http://www.jstor.org/stable/41479982?seq=1&cid=pdf-reference#references_tab_contents
http://www.jstor.org/page/info/about/policies/terms.jsp
http://www.jstor.org/page/info/about/policies/terms.jsp
http://www.jstor.org/page/info/about/policies/terms.jsp
REVIEWS REVIEWS REVIEWS
Global
change, global trade,
and the next
wave
of
plant
invasions
Bethany A Bradley1*, Dana M Blumenthal2, Regan Early3, Edwin D Grosholz4, Joshua J Lawler5, Luke P Miller6,
Cascade JB Sorte7, Carla M D’Antonio8, Jeffrey M Diez9, Jeffrey S Dukes10, Ines Ibanez9, and Julian D Olden11
I
Many
new
water
horticultural
invasion
semiarid
fying
the
confluence
time
generation
and
restrictions
non-
native
to
Africa
patterns
preventing
of
implement
trade
forces
as
of
plants
are
well
of
invasive
partners
could
previously
increasing
invasions,
as
horticulture
in
the
expose
the
species
are
Middle
US
likely
introduced
demand
although
the
have
may
import
East.
US
to
be
become
for
rapidly
to
Risk
at
some
screening
species
a
new
our
range
assessment
problematic
modifications
doorstep.
increase
types
and
of
measures
new
of
show
species
invasion
strategies
Here,
invasive
invaders
that
are
to
we
adapted
prevent
needed
risk.
novel
review
have
species,
of
At
native
proven
species
to
th
is
trends
to
the
including
war
m
address
new
same
and
successful
introductions
in
and
wave
managed
the
time,
emerging
many
dry
horticultural
of
climate
elsewhere
environments.
plant
from
ecosystems,
from
threats.
invasions.
tropical
change
trade
emerging
at
Now
identi-
but
This
and
and
and
is
a
new generation of invasive species may be at our doorstep. Here, we review trends in the horticultural trade and
invasion patterns of previously introduced species and show that novel species introductions from emerging
horticultural trade partners are likely to rapidly increase invasion risk. At the same time, climate change and
water restrictions are increasing demand for new types of species adapted to warm and dry environments. This
confluence of forces could expose the US to a range of new invasive species, including many from tropical and
semiarid Africa as well as the Middle East. Risk assessment strategies have proven successful elsewhere at identi-
fying and preventing invasions, although some modifications are needed to address emerging threats. Now is
the time to implement horticulture import screening measures to prevent this new wave of plant invasions.
Front Ecol Environ 2012; 10(1): 20-28, doi:10.1890/l 10145 (published online 2 Dec 201 1)
One gardening
need only
center
peruse
to
a
realize
nursery
the
catalog
enormous
or visit
array
a local
of gardening center to realize the enormous array of
plant choices available to the everyday American gar-
dener. Unfortunately, this wealth of consumer choices
comes at a steep cost. Non-native plants introduced
through the horticulture trade often become invasive
(Mack and Lonsdale 2001; Reichard and White 2001),
which we define here as introduced species whose popu-
lations are surviving and reproducing beyond the loca-
tion of introduction (sensu Blackburn et al 2011).
Although only a portion of species that become invasive
cause ecological damage (Williamson and Fitter 1996;
Sax et al 2002), and some have benefited biodiversity
(Davis et al 201 1 ; Schlaepfer et al 201 1 ), invasive plants
as a whole substantially reduce native species abundance
and diversity (Vilà et al 2011) and alter ecosystem func-
1 Department of Environmental Conservation, University of
Massachusetts , Amherst , MA *( bbradley@eco.umass.edu); 2Range –
land Resources Research Unit , US Department of Agriculture
(USD A) Agricultural Research Service , Fort Collins, CO; 3 Cátedra
Rui N abeiro, Universidade de Évora, Évora, Portugal; (continued
on p28)
tion (Ehrenfeld 2010; Vilà et al 2011). Several well-
known invasive plants in the US were deliberately intro-
duced, including kudzu (Puer aria lobata; planted to stabi-
lize soil), oriental bittersweet (Celastrus orbiculatus ;
planted for aesthetics), purple loosestrife (Ly thrum sali-
caria; planted for aesthetics), and tamarisk (Tamarix spp;
first planted for aesthetics and later to act as wind
breaks). Indeed, Mack and Erneberg (2002) estimated
that over 60% of established, non-native species in the
US were deliberately introduced. Moreover, the intro-
duction process can select for species more likely to
become invasive, because traits useful in horticulture –
such as rapid establishment, broad climatic tolerance,
and high resource allocation to flowers – can also
increase invasiveness (Mack 2005). Global change is
already aiding the spread of invasive species and increas-
ing their ecological impacts (Dukes and Mooney 1999;
Bradley et al 2010a). As global change proceeds, how-
ever, it will influence not just the success of introduced
plants but the introduction process itself (Hellmann et
al 2008). Gardeners are poised to plant new species from
warmer regions, as earlier onset of spring (Schwartz et al
2006) and warmer temperatures decrease the require-
ment for winter-hardiness in ornamental plants (Arbor
Day Foundation 2006). Similarly, as human populations
increase in the arid and semiarid regions of the world,
such as the American Southwest (Mackun and Wilson
2011), demand for drought- tolerant plants is expanding
– a trend likely to accelerate in areas where climate
change exacerbates drought (eg Seager and Vecchi
2010). At the same time, economic globalization offers
opportunities to import new types of plants from previ-
ously untapped parts of the world. Here, we review how
global changes in trade and climate could influence sup-
ply and demand for introduced ornamental plants. We
predict the consequences for future plant invasions in
www.frontiersinecology.org © The Ecological Society of America
In a nutshell:
• New horticultural trading partners supply novel species that
may become invasive
• Increasing demand for drought-tolerant species promotes the
introduction of invasive species in dryland regions
• Gardeners are likely to plant new species as soon as rising
temperatures allow, favoring the assisted migration of intro-
duced species relative to non-horticultural natives
• Emerging trade supply coupled with shifting demand due to
climate change makes the US susceptible to a new array of
invasive plants
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
BA Bradley et al. Invasion risk from horticulture and climate change
the conterminous US and conclude with a review of pol-
icy changes that could mitigate this new generation of
invasions.
■ Supply: identifying emerging trading partners
There is a clear link between increasing amounts of trade
and abundance of invasive plant species (Westphal et al
2008; Hulme 2009). In Europe and North Africa, mone-
tary values of imports were one of the best predictors of
invasive plant abundance (Vilà and Pujadas 2001), and
in the UK, plants more widely available in 20th-century
nurseries were more likely to be invasive today (Dehnen-
Schmutz et al 2007). As the number of horticultural trad-
ing partners with the US (“source” countries) continues
to rise, so too will the number of introductions of non-
natives that will later become invasive and potentially
problematic (Hulme 2009). Moreover, the rate of intro-
duction of future invasive species is steepest in the early
stages of new trade partnerships, due to the sheer volume
of novel introductions (Figure 1; Levine and D’Antonio
2003), and may contribute to an “invasion debt” that w
ill
be realized decades in the future (Essi et al 2011). As
trade partners become established and novel introduc-
tions slow, the number of invasive plants emerging from
the trading partnership may still rise, but largely as a
result of the persistent increase in propagule pressure
(Lockwood et al 2005) as more and more individuals of a
given species are planted throughout the landscape. The
greatest risk of new invasive plants arriving in the US is
therefore likely to come from emerging trade partners.
We identified emerging and established sources of nurs-
ery plant imports to the US using the US Department of
Agriculture’s (USDAs) global agricultural trade system
online database (USDA 2011). Nursery product cate-
gories assessed included bulbs and roots, trees and shrubs,
herbaceous perennials, unrooted vines, and mosses and
lichens, but excluded any plant identified by name (eg
azaleas). We compiled import dollar value by country from
1989 (the first year of record) through 2010 (all dollar
amounts hereafter are in US$). We defined “emerging”
trade partners as countries with an increasing trend during
the period of record, but an average import value of less
than $100000 per year from 2000-2010. “Established”
trade partners had an average import value of greater than
$100 000 per year from 2000-2010. This threshold
divided the pool of nursery plant trade partners roughly in
half. We then determined whether invasive plants have
arrived from predominantly emerging or established trade
partners by assessing the countries comprising the native
ranges for 2608 invasive plants in the US as documented
by the USDA (USDA ARS 2011; USDA NRCS 2011).
We identified a total of 42 emerging trade partners, clus-
tered mainly across tropical regions, the Middle East, and
Eastern Europe (Figure 2a). Global imports of unnamed
(ie non- varie tal) nursery plants were valued at more than
$250 million in 2010; only $4 million of those imports
Figure 1 ♦ New influxes of non-native invasive species are most
prominent in the early stages of new trade partnerships . With
established trade partners , invasions continue to rise with
increase in trade, but at a slower rate. Adapted from Levine and
D’Antonio (2003).
were from emerging trade partners. However, imports
from emerging trade partners are on the rise as compared
with those from established partners: trade value from
emerging sources rose by 69% from 2000-2010, while
imports from established sources declined by 9% over the
same period.
The average invasive plant in the US had only 29% of
its native range in emerging countries (Figure 2b) versus
54% in established countries (Figure 2c), supporting pre-
vious findings that more trade leads to more invasive
species (Vilà and Pujadas 2001; Levine and D’Antonio
2003; Westphal et al 2008). In addition, more than a
quarter of invasive plants in the US have native ranges
that do not include any emerging trade partners (Figure
2b). The low contribution of emerging trade partners to
the current complement of invasive plants in the US
implies that there is considerable scope for further inva-
sive species introductions from these countries.
■ Demand: xeriscaping and invasion of dryland
regions
Although drylands can be heavily impacted by invasive
species, the number of species that have invaded such sys-
tems is low when compared with that in more mesic
regions (Figure 3a). Furthermore, within dryland regions,
many invasive species are restricted to relatively wet areas
(Stohlgren et al 1998). The limited number of invasive
species established in dryland regions is probably due to a
combination of historically low human population den-
sity – and therefore lower propagule pressure (Lockwood
et al 2005) – and historical preferences for species from
© The Ecological Society of America www.frontiersinecology.org
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
Invasion risk from horticulture and climate change BA Bradley et al.
Figure 2. Currently emerging and established horticultural trade partners , and contributions of those trade partners to US invasive
plants, (a) Emerging partners (shown in red) currently average less than $100000 yr~l in trade value , but imports are increasing.
Established partners (shown in blue) currently average more than $100000 yr
~l in trade value, (b) Relatively few of the current
invasive plant species in the US are native to emerging trade regions, (c) Many of the current US invasive plants are primarily native
to established trade regions.
mesic environments (Hilaire et al 2008). Both of those
trends are changing. Rapidly expanding human popula-
tions in dryland areas (Mackun and Wilson 2011) have
not only created new gardens and planted more non-
native species, but in the process have also had to con-
tend with limited water supplies (Palmer et al 2008). In
many areas, particularly those at lower latitudes, climate
change is expected to further reduce water available for
human use (Palmer et ai 2008; Seager and Vecchi 2010).
Water limitations have already led to restrictions on
water use for gardening and greater use of drought-toler-
ant species for landscaping in the US (Figure 3b; Hilaire
et ai 2008).
A large portion of residential water use in dryland
regions is directed toward lawn and garden maintenance
(eg 30-40% in California; Gleick 1996). Xeriscaping –
the use of drought-tolerant plants in landscaping – can
reduce that water use by as much as 76% (Sovocool et al
2006). Furthermore, because many of the aesthetic con-
siderations determining the appeal of xeriscapes depend
www.frontiersinecology.org © The Ecological Society of America
№ Sources of nursery plant imports to the US
Emerging sources Established sources
Annual import value 2000-2010 (x $1000) Annual import value 2000-2010 (x $100000) i – i Not a source
<0.5
J5gj4
1~2 ‘ of plant imports
(b) Sources of current US invasive plants (°) Sources of current US invasive plants
800 1 1 800 1 1
I
eoo I _ § 600
щ
8- Ш
Ш»
« ■ m
I
8- Ш
I
ill
Ш»
г
«
ill
■
Ц
m
I I llll
I I
Г liliu i
о I, – I I I о 1,1, 1,1,1,1, и.-.Д
0 <10 <20 <30 <40 <50 <60 <70 <80 <90<100 100 0 <10 <20 <30 <40 <50 <60 <70 <80 <90<100 100 Percent of countries in the species' native range Percent of countries in the species' native range
that are emerging that are established
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
BA Bradley et al. Invasion risk from horticulture and climate change
Figure 3 ♦ Expansion of xeriscaping demand could generate new
species introductions, (a) The floras of the West and Southwest
deserts – the areas most likely to receive new , water-tolerant
ornamental species – currently have the smallest percentages of
introduced plant species, (b) A xeriscaped yard in the Sonoran
Desert with predominantly introduced species native to the
Chihuahuan Desert in Mexico, (c) A survey of nine nursery
catalogs revealed that most species marketed as “drought- tolerant”
are introduced. One nursery specializing in drought-tolerant
species increased its selection of species between 2000 and 2011;
encouragingly, most of the new species were natives.
on the types of plants used (Hilaire et al 2008), accep-
tance of xeriscaping could increase as the availability and
variety of drought-tolerant species grows. Unfortunately,
increasing the availability and variety of non-native,
drought- tolerant species could also increase the probabil-
ity of introducing species capable of invading dryland
regions.
To quantify the potential of xeriscaping as a source of
new invasive plants, we tabulated the number of plants
being marketed for this practice. First, we identified nine
nurseries in the US that advertise more than ten
“xeriscape”, “drought-tolerant”, or “water-wise” plant
species (henceforth collectively referred to as “drought-
tolerant” species). Using the USDA PLANTS Database
(USDA NRCS 2011) and other online sources, we then
identified the origin (native or introduced) of 731
drought-tolerant species (Figure 3c). We found that more
than half (401) of the drought-tolerant species were non-
native, suggesting that many species that, in the future,
will become invasive in dryland regions of the US may
already have arrived and are increasingly being planted.
To explore how the xeriscaping industry may be chang-
ing, we requested catalogs from previous years from our
nine nursery sources, but received catalogs from only one,
a nursery that specializes in drought-tolerant plants. For
this source, we tabulated the number of native and intro-
duced drought-tolerant species by year. We found that
while the nursery expanded its drought-tolerant species
offerings by 37% between 2005 and 201 1, almost all of the
newly offered species were native to the US (Figure 3c).
This is a somewhat encouraging trend. Together with the
wide availability of native drought-tolerant plants (330)
available across sources, it suggests considerable potential
to supply the xeriscaping demand with US native species.
Although these species may not be native to the particular
US deserts in which they are planted, species introduced
from one North American region to another are seldom
reported as becoming problematic invaders (Mueller and
Hellmann 2008). Thus, the focus on US native plants by
the xeriscaping industry may help prevent detrimental
intercontinental invasions.
■ Demand: shifting hardiness zones and the
American gardener
In addition to the specific demands for drought-tolerant
species, climate change could stimulate an overall
increase in the demand for new horticultural species that
thrive in new, warmer climates. Rising temperatures are
already causing US “hardiness zones” (temperature iso-
clines defined by the USDA) to shift (Arbor Day Founda-
© The Ecological Society of America www.frontiersinecology.org
2 CD w
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
Invasion risk from horticulture and climate change В A Bradley et al.
Figure 4 ♦ Climate change is likely to shift hardiness zones northward and upward in elevation , increasing the land area in warm zones
and altering demand for horticulture species, (a) Hardiness zones in the US, based on average climate from 1961-1990 and based on
projected average climate from 2040-206 9 (color scheme after that of Arbor Day Foundation 2006). (b) Change in land area of
hardiness zones, (c) Total land area in each hardiness zone by 2040-206 9.
tion 2006) and the growing season to lengthen (Schwartz
et ai 2006). The horticulture industry is likely to expand
the array of available species in expectation of, and in
response to, this new consumer demand (Figure 3c).
Building on the analysis of updated hardiness zones
(Arbor Day Foundation 2006), we explored how projected
changes in climate will affect the distribution of plant har-
diness zones over the next 40 years and how shifts in the
land area of hardiness zones might influence demand.
Hardiness zones are regions delineated by average mini-
mum annual temperatures that provide gardeners and
landscape designers with guidance on which plants can be
grown in which parts of the US (Cathey 1990). We
mapped current hardiness zones using the PRISM
(Parameter-elevation Regressions on Independent Slopes
Model) 4-km resolution average temperature dataset from
1961-1990 (Daly et al 2004) classified into 5.6°C bins
defined by the USDA (Cathey 1990). To match the spa-
tial resolution of projected future climate, we aggregated
the PRISM data to 0.125° spatial resolution (approxi-
mately 12 km2 in the US). We mapped potential future
hardiness zones using averaged projections from 16 differ-
ent general circulation models run for a mid-high (Special
Report on Emissions Scenarios [SRES] A2) greenhouse-
gas emissions scenario for the period from 2040-2069.
Statistically downscaled climate projections at 0.125° res-
olution were provided by the Climate Wizard project
(Girvetz et al 2009). For mapping both the current and
projected future hardiness zones, we calculated annual
minimum temperatures using average temperature of the
coldest month based on an empirical relationship between
the two (Prentice et al 1992).
Hardiness zones in the US are likely to shift substan-
tially northward over the next 40 years (Figure 4a). This
will result in an expansion of the warmer zones (particu-
larly zones 6 and 8-11), a contraction of some of the
www.frontiersinecology.org © The Ecological Society of America
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
BA Bradley et al. Invasion risk from horticulture and climate change
cooler zones (zones 3-5), and the near-complete loss of
zone 2, currently the coldest zone in the conterminous US
(Figure 4b). With concerted planting efforts, gardeners are
already helping garden plants to shift their geographic dis-
tributions into newly suitable climatic regions ahead of
non-propagated species (Van der Veken et al 2008). Many
invasive species are unintentionally taken along for the
ride (Maki and Galatowitsch 2004), whereas others are
actively planted in regions forecast to become suitable for
invasion with climate change (eg Bradley et al 2010b).
Unless assisted migration is also used for native species
(Richardson et al 2009), these trends raise the prospect
that both existing and new invasive species may be better
able to shift their ranges to align with new climatic condi-
tions, pre-empting and possibly precluding the establish-
ment of native species. Of further concern for invasive
species biologists and managers are the novel sets of
species that could be introduced in response to increasing
demand for heat-tolerant species: warm hardiness zones 8
and 9 are projected to expand by 45% and 120%, respec-
tively, and are likely to cover a substantial portion of US
land area by 2050 (Figure 4c). In regions such as these, the
performance of many native species will be compromised
as the climate warms beyond species’ current tolerance
(Walther 2004). Introducing non-native species that are
pre-adapted to the new climatic conditions in these
regions, and can thus outperform native species, further
increases the odds of invasion.
■ The intersection of supply and demand
The intersection of emerging supply and demand forces
creates considerable motivation for novel species intro-
ductions and poses the greatest risk for a new wave of
plant invasions into the US (Figure 1). Emerging trade
partners (and sources of novel species) included clusters
of nations in warm tropical regions and several arid
regions of the Middle East and Africa (Figure 2). At the
same time, demand for new species is increasing in dry-
land regions and is also likely to expand in warmer US
hardiness zones (Figure 4).
To test how emerging supply and demand might over-
lap, we created a map of global hardiness zones at 10-arc-
minute resolution using the same criteria as for the US
hardiness zones. Species are generally expected to become
naturalized in areas with similar climatic conditions to
that of their native range (eg Thuiller et al 2005). Indeed,
the hardiness zones where non-native species have
become naturalized in the US have historically tended to
match their native hardiness zones (see WebPanel 1 and
WebFigure 1), so it is reasonable to expect this trend to
continue in the future. We therefore explored the degree
of matching between future climatic conditions in the US
(2050, represented by mean conditions between 2040 and
2069) and historical conditions in established and emerg-
ing trade partners (represented by mean conditions
between 1961 and 1990). We used historical conditions in
source countries based on the assumption that plants
selected for the horticulture trade are adapted to historical
climatic conditions in their source countries. The hardi-
ness zones of emerging trade partners are strongly skewed
toward warmer climates (higher hardiness zone numbers;
Figure 5a), even more so than those of established trade
partners. The land area of source countries in zones 6-10,
which collectively represent the greatest increase in future
US land area, increases by 31% with the addition of
emerging trade partners. These emerging partners also
tend to be in drier parts of the world (Figure 5b).
Although established trading partners already include
large areas with relatively low precipitation, source-coun-
try land area receiving less than 20 cm of precipitation per
year – equivalent to climates such as those of Las Vegas,
Nevada and Phoenix, Arizona – increases by 44% with
the addition of emerging trade partners. These patterns
suggest that emerging plant trade partners are well poised
© The Ecological Society of America www.frontiersinecology,org
Figure 5» Supplies of novel species from emerging trade partners
could meet increasing demand for species adapted to warm and
dry environments . (a) Current hardiness zones of emerging
source countries are skewed toward warmer climates, (b)
Current mean annual precipitation of emerging source countries
includes some of the driest areas on Earth.
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
Invasion risk from horticulture and climate change BA Bradley et al.
to supply the very drought- and heat-tolerant species that
nurseries in the US are, or will soon be, looking to sell. We
predict that the strong overlap of emerging supply and
emerging demand could lead to a sharp rise in introduc-
tions of new invasive species (Figure 1).
■ Management, policy, and stakeholder solutions
Identifying invasive plants before they arrive is crucial for
preventing damage to native biodiversity. Post- introduc-
tion control of established species may not repair the
desired ecosystem functions because invasive species,
combined with other elements of global change, may
have already altered biotic interactions (Schlaepfer et ai
2011). Currently, under the US Plant Protection Act
(Public Law 106-224), plant importation conforms to a
“Black List”, which labels plants as “prohibited” or
“restricted” after they are proven harmful. This policy is
akin to closing the barn door after the horse has bolted,
however, and will prove particularly ineffective if, as we
predict, changing supply and demand forces facilitate a
new generation of introductions. In contrast, “Green
List” (or “White List”) approaches – based on weed risk
assessments (WRAs) – have been used in countries such
as Australia and New Zealand (Perrings et al 2005).
Predictors of invasive plants include whether the species
has a history of invasion elsewhere, climate matching,
and reproduction and dispersal strategies (Pheloung et al
1999). WRA scores have repeatedly been shown to have
good predictive power for separating out invasives from
the array of horticultural import species (see examples in
McClay et ai 2010) and provide a clear economic benefit
(Keller et ai 2007). The Animal and Plant Health
Inspection Service (APHIS) branch of the USDA has
proposed a new rule for plant imports that would add the
category “NAPPRA” (“Not authorized pending pest risk
analysis”) to the Plant Protection Act (USDA APHIS
2009). The NAPPRA rule would require APHIS to per-
form a WRA at the request of plant importers on taxa
that have previously not been imported, and then deter-
mine whether the plant should be accepted or prohibited
(USDA APHIS 2009). If implemented, this rule would
represent a major positive step in the long-term preven-
tion of new invasive species.
One concern with WRA efficacy – in light of emerging,
novel invasives – arises because a major component of
risk assessment considers whether the species has a his-
tory of invasion elsewhere (Pheloung et al 1999).
Emerging US trade partners are unlikely to have long-
established trade relations with other parts of the world,
so the invasiveness of species supplied by these partners
will be unknown. This lack of information should not be
mistaken for a lack of invasiveness, and WRAs will have
to be adjusted accordingly for emerging trade partners. In
addition, the climate-matching criteria of WRAs should
be applied to both current and future climate conditions
of the recipient region.
The US horticulture industry is among the most impor-
tant players in the prevention of future invasions, and
increasing awareness of invasive species among nursery
professionals is a critical step. The St Louis Declaration
(Baskin 2002) made an important contribution toward
identifying ways the horticulture industry can help, but
the information has reportedly not reached industry pro-
fessionals (Burt et al 2007). The voluntary codes of con-
duct in the Declaration would reduce invasions by
encouraging nursery professionals to assess invasive
potential of new plants before selling them and to pro-
mote native species in breeding programs. Many nursery
professionals will discourage customers from planting
known invasive species and will phase out plants known
to be invasive but often lack information on which
species are problematic (Burt et ai 2007). Links between
invasive plant managers and the horticulture industry
need to be strengthened to promote better dissemination
of that information (D’Antonio et ai 2004; Peters et al
2006). The trade in drought-tolerant species – which is
ultimately driven by environmental concerns (Hilaire et
al 2008) but presents novel environmental risks for dry-
land regions (Figure 3) – may afford a particularly good
opportunity for education. Preferential use of species for
xeriscaping that are native to the US, and especially to
the region of planting, would greatly reduce propagule
pressure from non-native species. This move might be
attractive to environmentally conscious gardeners and
horticultural companies.
Finally, the horticulture industry can take proactive steps
to aid risk assessment and prevention, such as providing
needed information to WRAs based on field trials and
identifying and ceasing production of plants that escape
cultivation easily (Mack 2005). Furthermore, the horticul-
ture industry can help fight range expansion of invasive
plants by stopping the sale of plants known to be invasive
elsewhere in the US. Collaborative groups involving repre-
sentatives from the horticultural industry, nursery and
landscape organizations, regulatory agencies, and non-gov-
ernmental organizations are now working toward limiting
the sales of invasive plants, in part by developing lists of
non- invasive alternatives (eg Cal-HIP 2004).
■ Conclusions
Many lines of evidence suggest that global change will,
on average, increase risk of plant invasion (Dukes and
Mooney 1999; Bradley et ai 2010a). Here, we identify
another risk – one that policy can effectively address.
Climate change is likely to increase demand for drought-
and heat-tolerant landscaping plants in the US.
Emerging trade partners have warm, dry climates that are
well matched to this future demand and could supply
many new and potentially invasive species. This emerg-
ing threat intensifies the need for preemptive screening of
nursery stock species prior to import. Although the num-
bers and abundance of invasive species already in the US
www.frontiersinecology.org © The Ecological Society of America
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
BA Bradley et al. Invasion risk from horticulture and climate change
might engender complacency, we suggest that active
management of new invasion risks will remain important
well into the future.
■ Acknowledgements
Many thanks to M McCutchen for help compiling data
on drought- tolerant species and to S Jones for early dis-
cussions. E Brusati and M Smith provided helpful com-
ments on earlier versions of this manuscript. This review
was conducted as part of the Climate & Invasions
Working Group with support provided by the National
Center for Ecological Analysis and Synthesis, a Center
funded by the National Science Foundation (grant #EF-
0553768), the University of California, Santa Barbara,
and the State of California. We gratefully acknowledge
the modeling groups, the Program for Climate Model
Diagnosis and Intercomparison (PCMDI), and the World
Climate Research Programme’s (WCRP’s) Working
Group on Coupled Modelling (WGCM) for their roles in
making available the WCRP Coupled Model
Intercomparison Project 3 (CMIP3) multi-model dataset.
Support of this dataset is provided by the Office of
Science, US Department of Energy.
■ References
Arbor Day Foundation. 2006. New arborday.org hardiness zone map
reflects warmer climate, www.arborday.org/media/zones.cfm.
Viewed 30 Sep 201 1.
Baskin Y. 2002. The greening of horticulture: new codes of conduct
aim to curb plant invasions. BioScience 52: 464-71.
Blackburn TM, Pysek P, Bacher S, et al. 2011. A proposed unified
framework for biological invasions. Trends Ecol Evol 26:
333-39.
Bradley BA, Blumenthal DM, Wilcove DS, et al. 2010a. Predicting
plant invasions in an era of global change. Trends Ecol Evol 25 :
310-18.
Bradley BA, Wilcove DS, and Oppenheimer M. 2010b. Climate
change increases risk of plant invasion in the eastern United
States. Biol Invasions 12: 1855-72.
Burt JW, Muir AA, Piovia-Scott J, et al. 2007. Preventing horticul-
tural introductions of invasive plants: potential efficacy of vol-
untary initiatives. Biol Invasions 9: 909-23.
Cal-HIP (California Horticultural Invasive Prevention). 2004.
California horticultural invasive prevention partnership.
www.plantright.org. Viewed 30 Sep 2011.
Cathey HM. 1990. USDA plant hardiness zone map. Washington,
DC: US National Arboretum. Miscellaneous publication 1475.
D Antonio CM, Jackson NE, Horvitz CC, et al. 2004. Invasive
plants in wildland ecosystems: merging the study of invasion
processes with management needs. Front Ecol Environ 2:
513-21.
Daly C, Gibson WP, Doggett M, et al. 2004. Up-to-date monthly
climate maps for the conterminous United States. 14th
American Meterological Society Conference on Applied
Climatology; 13-16 Jan 2004; Seattle, WA. Portland, OR:
PRISM Climate Group, Oregon State University.
Davis M, Chew MK, Hobbs RJ, et al. 2011. Don’t judge species on
their origins. Nature 474: 153-54.
Dehnen-Schmutz K, Touza J, Perrings С, et al. 2007. A century of
the ornamental plant trade and its impact on invasion success.
Divers Distrib 13: 527-34.
Dukes JS and Mooney HA. 1999. Does global change increase the
success of biological invaders? Trends Ecol Evol 14: 135-39.
Ehrenfeld JG. 2010. Ecosystem consequences of biological inva-
sions. In: Futuyma DJ, Shafer HB, and Simberloff D (Eds).
Annual review of ecology, evolution, and systematics, vol 41.
Pale Alto, CA: Annual Reviews.
Essi F, Dullinger S, Rabitsch W, et al. 2011. Socioeconomic legacy
yields an invasion debt. P Natl Acad Sci USA 108: 203-07.
Girvetz EH, Zganjar C, Raber GT, et al. 2009. Applied climate-
change analysis: the Climate Wizard tool. PLoS ONE 4: e8320;
doi: 10.1371 /journal.pone.00083 20.
Gleick PH. 1996. Basic water requirements for human activities:
meeting basic needs. Water Int 21: 83-92.
Hellmann JJ, Byers JE, Bierwagen BG, et al. 2008. Five potential
consequences of climate change for invasive species. Conserv
Biol 22: 534-43.
Hilaire RS, Arnold MA, Wilkerson DC, et al. 2008. Efficient water
use in residential urban landscapes. Hortscience 43: 2081-92.
Hulme PE. 2009. Trade, transport and trouble: managing invasive
species pathways in an era of globalization. J Appi Ecol 46:
10-18.
Keller RP, Lodge DM, and Finnoff DC. 2007. Risk assessment for
invasive species produces net bioeconomic benefits. P Natl
Acad Sci USA 104: 203-07.
Levine JM and D Antonio CM. 2003. Forecasting biological inva-
sions with increasing international trade. Conserv Biol 17:
322-26.
Lockwood JL, Cassey P, and Blackburn T. 2005. The role of propag-
ule pressure in explaining species invasions. Trends Ecol Evol
20: 223-28.
Mack RN. 2005. Predicting the identity of plant invaders: future
contributions from horticulture. Hortscience 40: 1168-74.
Mack RN and Erneberg M. 2002. The United States naturalized
flora: largely the product of deliberate introductions. Ann MO
Bot Gard 89: 176-89.
Mack RN and Lonsdale WM. 2001. Humans as global plant dis-
perses: getting more than we bargained for. BioScience 51:
95-102.
Mackun P and Wilson S. 2011. Population distribution and
change: 2000 to 2010. Washington, DC: US Census Bureau.
Maki К and Galatowitsch S. 2004. Movement of invasive aquatic
plants into Minnesota (USA) through horticultural trade. Biol
Conserv 118: 389-96.
McClay A, Sissons A, Wilson C, et al. 2010. Evaluation of the
Australian weed risk assessment system for the prediction of
plant invasiveness in Canada. Biol Invasions 12: 4085-98.
Mueller JM and Hellmann JJ. 2008. An assessment of invasion risk
from assisted migration. Conserv Biol 22: 562-67.
Palmer MA, Liermann CAR, Nilsson C, et al. 2008. Climate
change and the world’s river basins: anticipating management
options. Front Ecol Environ 6: 81-89.
Perrings С, Dehnen-Schmutz К, Touza J, et al. 2005. How to man-
age biological invasions under globalization. Trends Ecol Evol
20: 212-15.
Peters WL, Meyer MH, and Anderson NO. 2006. Minnesota horti-
cultural industry survey on invasive plants. Euphytica 148:
75-86.
Pheloung PC, Williams PA, and Halloy SR. 1999. A weed risk
assessment model for use as a biosecurity tool evaluating plant
introductions. J Environ Manase 57: 239-51.
Prentice 1С, Cramer W, Harrison SP, et al. 1992. A global biome
model based on plant physiology and dominance, soil proper-
ties and climate. J Biogeogr 19: 117-34.
Reichard SH and White P. 2001. Horticulture as a pathway of inva-
sive plant introductions in the United States. BioScience 51:
103-13.
Richardson DM, Hellmann JJ, McLachlan JS, et al. 2009.
Multidimensional evaluation of managed relocation. P Natl
Acad Sci USA 106: 9721-24.
Sax DF, Gaines SD, and Brown JH. 2002. Species invasions exceed
© The Ecological Society of America www.frontiersinecology.org
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
Invasion risk from horticulture and climate change BA Bradley et al.
extinctions on islands worldwide: a comparative study of plants
and birds. Am Nat 160: 766-83.
Schlaepfer MA, Sax DF, and Olden JD. 2011. The potential con-
servation value of non-native species. Conserv Biol 25: 428-37.
Schwartz MD, Ahas R, and Aasa A. 2006. Onset of spring starting ear-
lier across the northern hemisphere. Gbb Change Biol 12: 343-51.
Seager R and Vecchi GA. 2010. Greenhouse warming and the 21st
century hydroclimate of southwestern North America. P Natl
Acad Sci USA 107 : 21277-82.
Sovocool KA, Morgan M, and Bennett D. 2006. An in-depth
investigation of xeriscape as a water conservation measure. J
Am Water Works Ass 98: 82-93.
Stohlgren TJ, Bull К A, Otsuki Y, et al 1998. Riparian zones as
havens for exotic plant species in the central grasslands. Plant
Ecol 138: 113-25.
Thuiller W, Richardson DM, Pysek P, et al. 2005. Niche-based
modelling as a tool for predicting the risk of alien plant inva-
sions at a global scale. Glob Change Biol 11: 2234-50.
USDA (US Department of Agriculture). 2011. Global agricultural
trade system online. Washington, DC: US Department of
Agriculture, Foreign Agricultural Service, www.fas.usda.gov/
gats/default.aspx. Viewed 30 Sep 2011.
USDA APHIS (US Department of Agriculture Animal and Plant
Health Inspection Service). 2009. Importation of plants for
planting; establishing a category of plants for planting not
authorized for importation pending pest risk analysis.
Washington, DC: Federal Register, www.federalregister.gov/
articles/201 1/05/27/201 1- 13054/importation-of-plants-for-
planting-establishing-a-category-of-plants-for-planting-not-
authorized-for. Viewed 30 Sep 2011.
USDA ARS (US Department of Agriculture Agricultural Research
Service). 201 1. National genetic resources program. Germplasm
Resources Information Network (GRIN). Belts ville, MD:
National Germplasm Resources Laboratory, USDA ARS.
USDA NRCS (US Department of Agriculture Natural Resources
Conservation Service). 2011. The PLANTS Database. Baton
Rouge, LA: National Plant Data Center, http://plants.usda.gov.
Viewed 30 Sep 2011.
Van der Veken S, Hermy M, Vellend M, et al. 2008. Garden plants
get a head start on climate change. Front Ecol Environ 6:
212-16.
Vilà M, Espinar JL, Hejda M, et al. 2011. Ecological impacts of
invasive alien plants: a meta-analysis of their effects on species,
communities and ecosystems. Ecol Lett 14: 702-08.
Vilà M and Pujadas ]. 2001. Land-use and socio-economic corre-
lates of plant invasions in European and North African coun-
tries. Biol Conserv 100: 397-401.
Walther GR. 2004. Plants in a warmer world. Perspect Plant Ecol 6:
169-85.
Westphal MI, Browne M, MacKinnon K, et al. 2008. The link
between international trade and the global distribution of
invasive alien species. Biol Invasions 10: 391-98.
Williamson M and Fitter A. 1996. The varying success of invaders.
Ecology 77: 1661-66.
department of Environmental Science and Policy , University of
California , Davis, Davis, CA; 5 School of Forest Resources, University
of Washington, Seattle, WA; 6M arine Science Center, Northeastern
University, Nahant, MA; 7 Department of Environmental, Earth and
Ocean Sciences, University of Massachusetts, Boston, MA;
8Department of Ecology, Evolution and Marine Biology, University of
California, Santa Barbara, Santa Barbara, CA; 9 School of Natural
Resources, University of Michigan, Ann Arbor, MI; l0Department of
Forestry and Natural Resources and Department of Biological
Sciences, Purdue University, West Lafayette, IN; 11 School of Aquatic
and Fishery Sciences, University of Washington, Seattle, WA
www.frontiersinecology.org © The Ecological Society of America
Assistant Professor of Plant Pathology
Microbial Invasive Species, University of California, Riverside
The Department of Plant Pathology and Microbiology invites applications for a 9-month tenure-track faculty position (research and teaching in
the Agricultural Experiment Station), emphasizing the invasion and impacts of microbial (such as bacteria, fungi, viruses) pathogen or symbiont
species into agricultural or wildland ecosystems.
Applicants studying microbes that regulate invasive plants will also be considered. Approaches could include genetics, genomics, population
ecology/evolution, biochemical, bioinformatics, ecoinformatics and/or modeling. The successful candidate will join a vibrant community of
researchers studying microbe-host and microbe-environment interactions, have opportunities to collaborate with researchers in UC’s Division of
Agriculture and Natural Resources, the Center for Conservation Biology, the Center for Invasive Species Research, the Institute for Integrative
Genome Biology, and have access to modern campus facilities in genomics, proteomics, microscopy, ecological sensing technologies and field
stations and facilities.
Consult WWW.plantpath.ucr.edu for details about the department.
Applicants will be expected to pursue vigorous, extramurally funded research and contribute to undergraduate and graduate teaching in Programs in
Microbiology, Plant Pathology, or Genetics, Genomics and Bioinformatics. A Ph.D. and demonstrated excellence in research are required.
Email curriculum vitae, statements of research and teaching interests, selected reprints, and three letters of reference to:
Dr. James Borneman, c/o Tiffany Lindsey, Department of Plant Pathology and Microbiology,
University of California, Riverside, California 9252 1-04 15.
Email: PLPAJobs@ucr.edu
Evaluation of applications will begin February 17, 2012, but the position will remain open until filled.
Position will be available July 1, 2012.
UNIVERSITY OF CALIFORNIA
The University of California is an Affirmative I I Щ Ш I V Я ■
1 m I
Action/Equal Opportunity Employer. Ill I ШЯ ■”* III I®
This content downloaded from 209.129.85.4 on Tue, 02 Jun 2015 21:33:57 UTC
All use subject to JSTOR Terms and Conditions
http://www.jstor.org/page/info/about/policies/terms.jsp
- Article Contents
- Issue Table of Contents
p. 20
p. 21
p. 22
p. 23
p. 24
p. 25
p. 26
p. 27
p. 28
Frontiers in Ecology and the Environment, Vol. 10, No. 1 (February 2012) pp. 1-56
Front Matter
GUEST EDITORIAL
Managing water, harvesting results [pp. 3-3]
Dispatches
Tiger forest felling greenwashed? [pp. 4-4]
A glimpse of future ocean acidification [pp. 4-4]
Ocean radioactivity after Japan nuclear crisis [pp. 5-5]
ABC calls in legal eagles [pp. 5-5]
Restoring the source of the roses [pp. 6-6]
US trees to power UK plant [pp. 6-6]
Norway’s new method of assessing non-native species [pp. 7-7]
Breaking bad oil-change habits [pp. 7-7]
Just add water [pp. 8-8]
New threats to India’s snow leopards [pp. 8-8]
WRITE BACK
Seafood genetic identification: aiming our pipettes at the right targets [pp. 10-10]
Seafood mislabeling: a response to Mariani [pp. 10-11]
Multi-dimensional space use: the final frontier [pp. 11-12]
Is assisted colonization feasible? Lessons from past introductions [pp. 12-13]
RESEARCH COMMUNICATIONS
Linking climate variability to mushroom productivity and phenology [pp. 14-19]
REVIEWS
Global change, global trade, and the next wave of plant invasions [pp. 20-28]
Does community forest management provide global environmental benefits and improve local welfare? [pp. 29-36]
CONCEPTS AND QUESTIONS
Nitrogen fluxes from the landscape are controlled by net anthropogenic nitrogen inputs and by climate [pp. 37-43]
Artificial modifications of the coast in response to the “Deepwater Horizon” oil spill: quick solutions or long-term liabilities? [pp. 44-49]
TRAILS AND TRIBULATIONS
Light dawns in a Congolese forest [pp. 50-51]
LIFE LINES
Lost livestock of the Nile [pp. 56-56]
Back Matter
Discussion 4: Invasive Species Lecture
Prior to reading the article “Global Change, Global Trade, and the Next Wave of
Plant Invasions” it is important that we understand the difference between plants
that are native, non-native, and invasive.
In simple terms, native plants live in a region naturally and before settlers began
moving in. A non-native plant grows in an area because it was imported or
introduced. You will see in the article that humans import and introduce plants
for many different reasons.
An invasive plant species is a non-native plant that has spread beyond its
boundaries and does ecosystem damage and or economic harm.
“When plants that evolved in one region of the globe are moved by humans
to another region, a few of them flourish, crowding out native vegetation
and the wildlife that feeds on it. Some invasives can even change
ecosystem processes such as hydrology, fire regimes, and soil chemistry.
These invasive plants have a competitive advantage because they are no
longer controlled by their natural predators, and can quickly spread out of
control” (CIPC 2016, 1).
Bibliography
CIPC. California Invasive Plant Council 2016. Invasive Plants: Definitions and
Impacts. Available from http://www.cal-ipc.org/ip/definitions/index.php;
Internet; (accessed 14 January 2016).
http://www.cal-ipc.org/ip/definitions/index.php