Title: Will climate change hasten the spread of invasive plants?
[grist.com 4 Jan, 2011 by Seth Shulman] -- When Bethany
Bradley describes her research as probing the link between global warming and
"alien invaders," as she did in a recent journal article, the reader may be
understandably disconcerted, especially since Bradley's early graduate work
involved mapping the surface of Mars. But rest assured: The alien invaders she
studies today as a climate scientist and biogeographer at the University of
Massachusetts, Amherst, are nonnative plant species such as kudzu (Pueraria
lobata), endemic through the southern U.S., and purple loosestrife
(Lythrum salicaria), which now clogs many of the nation's waterways and
canals.
While invasive plants are surely not as unsettling as the arrival of invaders
from outer space, many such species have disrupted vast tracts of rangelands as
well as farms and critical natural habitats. Bradley examines how global warming
threatens to accelerate the spread of invasive plants, addressing a largely
ignored risk and offering specific projections that can help ranchers, farmers,
and conservation biologists anticipate what to expect.
As Bradley explains, "Invasive plant species are well suited to thriving in
novel environments because of their ability to beat out competitors for
resources. So it stands to reason that the more we disrupt the climate, the more
these plants might be able to expand their reach."
Bradley cautions, however, that invasive plant species are a diverse lot;
accurately predicting how they will adapt to a warming climate means
understanding a lot of complex interactions. Take, for instance, her recent work
on cheatgrass (Bromus tectorum), a widespread scourge in North
America's Great Basin, bounded on the east by the Rocky Mountains and on the
west by the Sierra Nevada range. Cheatgrass, Bradley says, was the species that
drew her to study invasive plants in the first place. Early in her career as a
graduate student in planetary geology at Brown University, Bradley recalls, one
of her advisors gave her work on a project incorporating "remote sensing"
satellite data of the earth. It seemed to her that she was seeing biology writ
large. "I took to it immediately," she says. "I had always been interested in
biology but found it so much more fascinating on this kind of broad landscape
scale." Plus, compared with her studies of Mars, "it felt good to work on
something closer to home that could be more directly useful to people."
Cheatgrass was probably introduced into North America as seed contaminants in
hay imported from Europe in the late 1800s. Farmers gave it that name because it
pushes out wheat crops. In early spring, it looks like wheat but soon turns dry
and tough, thereby "cheating" farmers of their expected yields. Worse yet,
Bradley says, cheatgrass is 10 times more prone to fire than the sagebrush and
grass habitat it most often displaces, and the resulting wildfires leave a
burnt-out, barren landscape that facilitates the species' further expansion.
Bradley says that cheatgrass, more than some kinds of vegetation, stands out
particularly clearly in satellite data, "with a strong and distinctive green
signal." When she first observed it, she had no idea what it was, but the vast
swath of land in the Great Basin that it covered was unmistakable. As she
recalls, "Having grown up on the East Coast [in Connecticut], I was awed by the
scale of the territory cheatgrass covered; I soon learned that it had come to
dominate some 40,000 square kilometers (15,444 square miles) in this region --
an expanse 10 times larger than Rhode Island, where I was in graduate school at
the time."
A decade ago, when she began graduate school, Bradley says, the field of
climate change was not nearly as well established as it is today, and the
effects of human-driven carbon emissions were less fully understood.
Nonetheless, she was drawn to learning more about how urbanization and other
human disruptions of the environment have helped foster the spread of invasive
plant species. She adds that her current research into the effects of
human-caused disruption of the climate is a natural extension of this earlier
work.
"Climate change threatens to profoundly alter entire global ecosystems,"
Bradley says. "Invasive species have been exciting to study as a window into
such potential changes. Ideally, this kind of work can help us more
realistically assess the risks and opportunities we face."
To this end, Bradley uses a technique called "bioclimatic envelope modeling,"
which analyzes the climatic constraints within which a particular plant species
thrives. Such an approach, she says, is well suited to studying how global
warming might make a region increasingly vulnerable to invasions of nonnative
species. In particular, Bradley says she tries to correlate the specific
climatic factors most closely associated with a given species' propagation. To
accomplish this, Bradley first uses detailed satellite data and field
observations to track the spread of an invasive species over time. Then, by
comparing the species' geographical spread to localized climate data, she teases
out the climatic variables most closely associated with its successful spread.
"One way of thinking about it," Bradley says, "is that you test the changing map
of the plant species' whereabouts against every climate variable you can think
of and see what factors float to the top."
In the case of cheatgrass, Bradley determined that the variable most closely
correlated with its propagation was spring rain or snow. "Precipitation later in
the summer has a strong effect on cheatgrass's competitors," she says. "But
precipitation in the spring, even more than temperature variables, correlates
most closely with cheatgrass's spread." Presumably this spring water "gives
cheatgrass its most potent advantage over competitors in quickly establishing
itself."
With that information in hand, Bradley consults each of the world's 10 top
climate models to see how much spring precipitation in a given geographical
region it projects under different warming scenarios in the years to come. Then
she can make detailed projections that link the projected climate conditions
(resulting from our heat-trapping carbon emissions) to the geographical spread
of this particular species.
Bradley has to contend with many challenges, including variations in the
climate models' projections and the confounding effects of normal year-to-year
variability in weather patterns. Still, she is able to project with a good
degree of confidence that cheatgrass is likely to move its range significantly
northward invading new areas of Montana, Wyoming, Utah, and Colorado. Her
research also indicates that significant portions of southern Nevada and
southern Utah are likely to become climatically unsuitable for cheatgrass in
coming decades, a finding that may allow land-use managers to help restore
other, more desirable native species to these regions.
As Bradley puts it, "One of the great things about this work is that, unlike
other areas of climate-change research where different constituencies have
legitimate competing interests, in this case, all the interested parties are on
the same side: just about everyone hates invasive species like cheatgrass and
would like to contain their spread. It's an aspect that makes this research feel
more like I am working to understand and maybe even help solve a problem rather
than doing battle."
This is the 15th
installment of America's Climate Scientists: A series from the Union of
Concerned Scientists. Click here
to read all the climate scientist profiles.
The Union of
Concerned Scientists is currently leading a campaign to elevate the voices of
climate scientists and educate the public about the overwhelming scientific
evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.