FIGHTING BACK:
Stemming the Tide of Invading Species
Jocelyn Kaiser
Researchers agree that
prevention is the best medicine, but they are also battling established
exotic species with everything from chemicals to traps
Last Easter weekend, four divers from Australia's
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
research agency splashed into a marina in Darwin on the north coast in a
routine inspection. They got an unpleasant surprise: Hundreds of millions
of fingernail-sized mussels were clumped in balls stuck to boats, lines,
and piers--where no mussels had been 6 months before. Taxonomists
identified the critter as a Central American cousin of the zebra mussel,
notorious for clogging up North America's Great Lakes. Within 5 days,
Australia's Northern Territory government had braved objections from boat
owners and quarantined all boats in the marina, closing off the entire
1.5-kilometer-area, and two other marinas where the black-striped mussel had
been spotted. Then they poisoned the marinas with chlorine and copper,
killing every living thing in the water.
It sounds like drastic medicine, but no black-striped mussels have been
seen since, the natural biota is bouncing back, and CSIRO is counting the
$1.5 million strike as perhaps one of the most dramatic defeats ever of a
marine invader. "Nobody's questioning it at all," says Ron
Thresher, head of the CSIRO Centre for Research on Introduced Marine Pests
in Hobart, Tasmania. "If it shows up again, we'll do it again."
Australia's quick victory over the mussel, which probably arrived stuck
to a yacht's hull, shows that it is possible to battle exotic species and
win. The world's ecosystems will never revert to the pristine state they
enjoyed before humans began to routinely crisscross the globe, and the pet
and nursery industries still import many alien species. But people are
fighting back against invasive species as never before, with weapons
ranging from ballast-water exchanges that keep species out of harbors, to
parasites that attack exotic plants and insects (see p. 1841).
On heavily invaded territory, such as parts of Hawaii, fenced
"exclosures" claim at least some patches of territory for the
natives (see sidebar on p. 1837).
"There's more interest in invasives now than there has been in the
last 25 years," says Jim Carlton, a marine biologist at Williams
College-Mystic Seaport in Mystic, Connecticut.
Researchers agree that preventing an invasive species from getting in is
far and away the best and cheapest approach. But they are having increased
success at managing exotics that have already landed. Although conventional
wisdom once held that removing an already established exotic is all but
impossible, some scientists are becoming more optimistic that local invasions
can sometimes be stopped--if they're caught in time. Invaders such as
parasitic worms in California abalone and a South American rodent ravaging
British estates (see sidebar on p. 1838)
have succumbed to aggressive counterattacks. "The practical approach
is to have a diverse portfolio: Prevent as many things as you can and
control the things you can control," says Liz Chornesky, a senior
scientist with The Nature Conservancy.
Still, stopping ongoing invasions is a daunting task, and even
preventing them is not easy. Because of the scope of the problem, and
because exotic plants and animals are transported as part of international
trade, control measures "potentially step on a lot of toes," says
Daniel Simberloff, an ecologist at the University of Tennessee, Knoxville.
Tackling invasives requires not only beefing up the budget--more than $500
million in the United States, with the bulk of the money going for customs
inspections--but also politically sensitive steps such as cracking down on
what timber companies can import and plant nurseries can sell. "It's
very easy to get people aware of the problem. ... But there are some parts
of this that are really intractable," says Alan Holt, a senior scientist
with The Nature Conservancy.
Closing the borders
One strategy is simply to ban traffic in exotic species known to pose a
threat. Today, in addition to a hodgepodge of federal and state laws
restricting transport of various plants, animals, and insects, the United
States bans imports of more than 100 troublemaker taxa on
"blacklists" established by various agencies. But blacklists will
always be incomplete, leaving the door open for many other noxious
organisms.
One solution comes from Australia. In mid-1997, in a policy also
followed by New Zealand, it adopted a "white list" model for
plants: All plant species are barred unless they have been determined to be
safe. For new plants, officials developed a 49-question form handed out at
airports and seaports that asks, for example, whether a species has been
invasive elsewhere or reproduces by windblown seeds. The weed questionnaire
has slowed influx of new species "by about 30%," says CSIRO
ecologist Mark Lonsdale.
As U.S. exotics from the zebra mussel to a Western weed called leafy
spurge make a huge dent in the nation's bottom line, researchers argue that
the time is right for such strict measures. "I don't see why we
couldn't do something as stringent as [what] Australia [has done],
personally," says Bill Brown, science adviser to Interior Secretary
Bruce Babbitt. Many scientists also favor this approach. "The burden
of proof would be on the person bringing it in. That changes things quite a
bit," says San Francisco Estuary Institute marine biologist Andrew
Cohen.
Back in the 1970s, however, the U.S. Fish and Wildlife Service proposed
such a white list policy--and got nowhere. "They had their heads
handed back to them" after the pet and nursery industries--and even
some scientific groups--skewered the idea, says Don Schmitz, a biologist
with the Florida Department of Environmental Protection.
The prospect of tighter regulations on exotic species still riles
powerful industries, including nurseries and pet stores. Marshall Meyers,
general counsel for the Pet Industry Joint Advisory Council, fears that
officials may regulate "easy targets" like animals sold in pet
stores, even if accidental introductions or biocontrol agents gone awry are
a greater threat.
And the booming gardening industry is probably an even bigger importer
of pests, researchers say. Richard Mack, an ecologist at Washington State
University in Pullman, says that he "keeps a collection of seed
packets that I bought in U.S. garden stores and nurseries that shouldn't be
sold anywhere in the country," like the Brazilian pepper and even
seemingly innocent European baby's breath. Both species are already problem
weeds in some states, yet are sold legally.
Nurseries are still opposed to legal restrictions, but they are
beginning to take voluntary measures. For example, the Florida Nurserymen
& Growers Association recently identified 24 marketed species on a
blacklist drawn up by Florida's Exotic Pest Plant Council--and decided to
discourage trade in 11 of them. True, those were the least promising
sellers, says exotic plant expert Ken Langeland, a researcher at the
University of Florida, Gainesville, but stopping such sales can help slow
invasions: "It's a major step. Before, they were just pushing the
issue aside and ignoring it."
Of course, even if intentional traffic in exotic species is slowed,
there remains the tougher problem of those organisms that sneak in with the
inadvertent help of humans, such as the Asian clams that hitchhiked into
San Francisco Bay in ballast water, or voracious brown tree snakes that arrived
in Texas hidden in a cargo shipment. "Accidental introductions will be
much more difficult to address," says ecologist Jennifer Ruesink of
the University of Washington, Seattle. Global traffic is so heavy that
"there's excellent evidence now that whatever can get in, will get
in."
Still, there are ways to help prevent such stealth invasions, too. The
United States is focusing on one vulnerable spot--ballast water, often a
veritable aquarium of exotic species, which ships generally discharge in
port. As of July, a Coast Guard regulation requests that the 30,000 to
60,000 ships that steam into U.S. waters each year voluntarily exchange
ballast water at sea; that way only marine species will be brought into
brackish water harbors. Each ship must report whether it has done so, and
after 2 years the Coast Guard will decide whether to make exchanges
mandatory. But exchanging ballast risks unbalancing the ship in rough seas,
and spot checks in the United States and Australia suggest that many
captains lie about the exchange. A better long-term fix may be to sterilize
the water with ultraviolet light or biocides, says Smithsonian Institution
marine biologist Greg Ruiz.
Other pathways require even more creative solutions. The Asian longhorn
beetle, for example, a pest whose larvae are now devouring hardwoods in New
York City and Chicago, likely hitchhiked in on wooden packing crates and
pallets from China around 1996. Experts are still struggling with their
options, Brown says: Short of banning wood packing materials altogether,
the government might require that the wood be fumigated or heat-treated at
its port of origin, or even allow only plastic pallets. Other pest
conduits, such as personal mail, can't be addressed without trampling on
people's civil rights. "If you want to send me a brown tree snake,
just put it in first-class mail," says Holt.
Contain and control measures
For those invaders that inevitably get across borders, the other half of
the challenge is to craft better management strategies, including a
monitoring network for spotting invaders early, researchers say.
Australia's victory over the mussels, for example, was triggered by a new
monitoring effort that sent divers down to inspect boats and piers. But
most existing monitoring programs don't track species in sufficient
taxonomic detail.
Once the alarm is raised, wiping out recently established invaders can
be done if there's enough political will to do it, insists Simberloff. He
notes that the medfly has twice been eradicated from Florida, and over the
past decade North Carolina has almost conquered witchweed--a parasitic
plant from Africa that strangles corn and sorghum crops--with a combination
of hand pulling, chemicals, and quarantines. California scientists last
month declared victory over a South African parasitic worm that infects a
wild abalone species, after having plucked from shorelines 1.5 million
black turban snails, one of the worm's main hosts. "By far and away,
the most effective and cheapest way is to destroy it soon after you've discovered
it," says Mack.
For those invaders already too entrenched to remove, coordinated effort
can keep them in check--but such coordination is often lacking. Indeed,
sometimes federal or state agencies actually help spread exotic species.
For example, the Natural Resources Conservation Service, a U.S. Department
of Agriculture agency that has focused on tasks such as helping farmers
reduce soil erosion, has a history of planting non-native, weedy species to
reduce the threat of forest fires or stabilize road embankments. Or one
agency will spray weeds with pesticides, thwarting another agency's
biocontrol insects released on the weeds in an adjacent field.
To eliminate such problems, President Bill Clinton in February signed an
executive order calling on federal agencies to stop activities that spread
invasive species; the order also created a high-level federal council
charged with devising a "management plan" for invasive species by
August 2000. With this new high-level directive, "I'm hopeful that cross-purposes
will disappear," says Mack. For even more coordination, Simberloff,
Schmitz, and some others are lobbying for a government-sponsored North
American Center for Biological Invasions to keep a directory of experts and
maintain a sort of 911 emergency number that anyone could call to report an
invasion.
Ultimately, it will take action on the part of millions of individuals
to stop the tide of invaders. Perhaps one model is Australia, where
"the average taxi driver" is well aware of the devastation wrought
by invading species, says CSIRO's Thresher, an American expatriate. Such a
culture supports strong measures, such as insecticide spraying on arriving
overseas flights, and airport "amnesty boxes" where passengers
can hand over fruits or wood.
Right now such tactics are hard to envision elsewhere, but even so, some
scientists are increasingly optimistic. "I'm amazed at the attention
that's coalescing around this, the disparate factions," says Nature
Conservancy senior scientist Bruce Stein. Adds Simberloff: "This has
taken so long to get under way. I'm hoping for the moon."
