Rapidly multiplying invasive species
pose host of dangers to U.S.
By Rachel Ehrenberg
Knight Ridder Newpapers
Even in this age of heightened security, dangerous invaders
continue to infiltrate the United States.
While they may appear harmless, they've got a rap sheet
a mile long. Their infiltration threatens human health;
it lays waste to fields, forests and waterways; and it
costs the nation billions of dollars each year.
These foreign critters are what scientists call "invasive
species," and they're the No. 2 cause of death to
endangered species - beating out pollution, overharvesting,
disease and global warming. They cause grave medical problems,
damage crops, and can restrict trade.
"The situation is getting worse," said Daniel
Simberloff of the University of Tennessee, Knoxville.
"Invasive species have a plethora of effects on the
environment. Some are so idiosyncratic there's no way
to predict the things that could happen."
New research is untangling the convoluted interactions
between invasive species and native plants and animals.
And scientists are learning more about how to manage and
eradicate non-native pests, whose numbers will continue
to grow with increased international travel and trade.
An estimated 50,000 "exotic" plants, animals
and microbes are already here, so the odds are good that
some of these critters will be very, very bad, even though
the risk that a single species will become invasive may
be low, said John Chapman of Oregon State University.
"It's just like roulette, you spin the chambers
and pull the trigger," Chapman said. "There's
only one bullet in there, but who wants to play?"
Recent estimates suggest that a menagerie of foreign
animals, plants and microbes costs the United States about
$137 billion in damage each year. The figure takes into
account the myriad ways invasive species hit the wallet,
from the cost of herbicides used against foreign weeds
to the cleanup of damage caused by feral hogs, said ecologist
David Pimentel of Cornell University. And the number of
invaders is likely to rise, he said.
"Globalization, the shipping of goods, more people,
more people traveling and traveling rapidly, all of these
factors increase the problem," said Pimentel, co-author
and editor of a recent book on biological invasions.
A lack of public awareness also exacerbates the problem,
Pimentel said. Without realizing the repercussions, people
may take a liking to an exotic pest, inviting the organism
to make itself comfortable in its new home. And many of
these non-natives can be purchased easily. In the case
of purple loosestrife, although more than 20 states have
prohibited the sale of the invasive garden plant, it still
can be bought at many nurseries.
While some nurseries encourage gardeners to use native
plants, there are still many people who can't resist a
flashy foreigner. "No one is ever happy with the
species that they have," Simberloff lamented recently
at a symposium in Knoxville sponsored by the Council for
the Advancement of Science Writing.
Of course not all "alien" species are harmful.
In fact, most cultivated plants and many important domesticated
animals - wheat, honeybees, cattle - aren't natives and
they provide substantial benefits to the U.S. economy.
What prompts concern is the occasional out-of-town critter
that escapes human control and ends up decimating crops,
or native plants, or people.
"West Nile has been extremely costly to the public,
both in terms of illness and lives lost and in terms of
mosquito control," said Dr. Norman Gratz, consultant
to the World Health Organization and former head of its
division of vector biology and control.
So how does a lowly worm or ratty thistle gain the ecological
upper hand? When a pathogen's host is already here, as
mosquitoes were with West Nile virus, it can tip the odds
in favor of the invader, Simberloff said. But no two invasions
An organism may thrive because its natural enemies are
absent in the new environment. It may have a biological
edge over its native counterpart - many plants are adept
at colonizing disturbed areas and quickly take over the
fresh dirt at construction sites or roadsides. Other species
may sit around innocuously for years and then something
shifts, unleashing the Jekyll within.
In Florida for example, several species of fig tree are
planted as ornamentals. Introduced in the 19th century,
the figs were without their pollinating wasps and couldn't
reproduce. But recently some of the wasps arrived and
now at least two of the fig species have become invasive,
and are gaining a foothold in Everglades National Park.
The impact of a successful invader can ripple through
an ecosystem in unpredictable ways, Simberloff said, and
sometimes "invasional meltdown" occurs. Different
non-native species can interact with each other, promulgating
the spread of each.
For example, the zebra mussel, a poster child for invasive
pests, is superior at filtering water. Clear water makes
the Great Lakes a more inviting habitat for the aquatic
weed Eurasian water milfoil. The milfoil changes the firmness
of the lake bottom, helping the zebra mussel spread. Thus
both species have become even bigger problems due to their
interaction with each other.
The fallout from an invader establishing itself can be
great - damage incurred by the zebra mussel is estimated
to be $310 million - and many researchers urge aggressive
eradication campaigns as soon as a potential pest is noticed.
Eradication has a bad name," Simberloff said, but
this is mostly because of the notoriety of the failures.
Like the $200 million spent in the 1980s to eliminate
the fire ant from the southern United States. Without
predators, these aggressive pests quickly spread and the
pesticides and bait traps used against them often just
wiped out their competition. After 11 years there were
more fire ants than at the outset, leading some to call
the campaign the "Vietnam of entomology."
Certainly, the best-case scenario is acting on good information
about an organism's biology. But a lack of knowledge should
not be used as an excuse to delay action, Simberloff wrote
in Conservation Biology earlier this year.
And a quick and dirty response may be crucial to preventing
an invader from staking a claim. Nine days after the Caribbean
black stripe mussel's presence was noted, the Australian
government poured gallons of bleach and copper sulfate
into the bay where the mussel had taken up residence.
Everything was killed. But juveniles of various native
marine species gradually came in from the open sea and
repopulated the bay. The black stripe mussel hasn't been
"The bottom line on eradication is, it is important
not to be too pessimistic about it, but to think carefully
and act quickly," Simberloff said.
Bringing in a pest's natural enemies, or "biocontrol,"
is often touted as a greener alternative to massive chemical
eradication campaigns. And in some instances biocontrol
agents do the job. But often they do the job too well,
and not necessarily on the right species, Simberloff said.
The rosy wolf snail, a central Florida native, was introduced
to several Pacific islands to control the invasive giant
African snail. But the rosy wolf snail was too small to
take on its intended prey, and proceeded to make its meals
elsewhere. It is now blamed for more than 50 extinctions
of native snails in Hawaii, Tahiti and other Pacific islands.
The possibility that a biocontrol agent would switch
hosts, as the rosy wolf snail did, has been recognized
for some time, said Dean Pearson, a research ecologist
at the University of Montana. So scientists usually test
an organism's palate before the creature is brought in
as a weapon. Now scientists are looking beyond a creature's
dietary preferences, toward the indirect ways biocontrol
agents can affect an ecosystem.
"We are having a second revolution in biocontrol
research," said Pearson, whose work focuses on the
less obvious impacts of a mangy thistle relative, spotted
knapweed. Two species of fly, introduced to eat the knapweed,
may inadvertently increase the prevalence of the sometimes
deadly Sin Nombre hantavirus, Pearson and a colleague
reported recently in Trends in Ecology and Evolution.
Although predicting how a species will affect a community
is difficult, "We can't afford to get rid of biocontrol,"
"Invasives are such a huge and growing problem,
we need all the tools we can get," he said. "But
we have to be really careful, this is Mother Nature we're
The public is often oblivious to the damage that invasive
species can cause, said Cornell University's Pimentel.
A soft-hearted person might release a foreign fish into
a pond or dump unused bait into a waterway without killing
it. In fact, many species believed to have arrived in
the ballast water of ships may actually have come into
the country as live seafood, said Oregon State's Chapman.
Chapman, an invertebrate zoologist, just completed a
survey of the clams, mussels and oysters that are available
for sale in fishing markets in the western United States.
Twenty-four of the 37 bivalves are non-natives, and 11
of these have already established self-sustaining populations
in the Pacific Northwest, Chapman and his colleagues reported
recently in the journal Conservation Biology.
"There should be warnings on packaging, `Live seafood,
do not throw into nature,' " Chapman said.
So why aren't there ads to educate the public about the
effects of invasive species? A campaign like "your
brain on drugs" but aimed toward the dangers of invasives
could be very effective, Pimentel said. (Imagine: "This
is your pond. This is your pond under water hyacinth.")
But the agencies charged with managing non-native pests
are poorly funded and organized, and without adequate
resources for tackling invaders, education won't do much
"We are dealing with such a complex problem and
even the federal agencies who are supposed to know really
lack the information," Pimentel said.
Increasingly, scientists are championing "ecosystem
management" as a way of handling non-native pests.
The approach focuses on maintaining the health of an entire
community, and while more research is needed, it is a
promising strategy, Simberloff said.
But the success of any approach will depend on funding
and public support. The western regional panel for invasive
species had a budget of $40,000 last year, Chapman said,
who is a member of the panel. "That's for every state
west of the Mississippi," Chapman said. "What
can we do with that? It's really just a drop in the bucket."