PREDICTING INVASIONS:
Biological Invaders Sweep In
Martin Enserink
As a tidal wave of exotic
species transforms environments worldwide, ecologists are scrambling to
predict where and when new invaders may strike
One spring morning in 1995, ecologist Jayne Belnap
walked into a dry grassland in Canyonlands National Park, Utah, an area
that she had been studying for more than 15 years. "I literally
stopped and went, 'Oh my God!' " she recalls. The natural
grassland--with needle grass, Indian rice grass, saltbush, and the
occasional pinyon-juniper tree--that Belnap had seen the year before no
longer existed; it had become overgrown with 2-foot-high Eurasian
cheatgrass. "I was stunned. It was like the aliens had landed,"
says Belnap, a researcher with the U.S. Geological Survey (USGS) in Moab.
"Now, we've lost this ecosystem forever."
A few years earlier and a continent away, as Yugoslavia fell apart in a
series of wars, Serbian scientists discovered a new enemy in a field near
Belgrade airport: the western corn rootworm, apparently flown in from the
United States. Vigorous international action might have curbed this pest's
first known venture outside North America, says entomologist József Kiss of
the University of Agricultural Sciences in Gödöllô , Hungary, but the
turmoil of war prevented such a collaboration. Now it's too late. By 1995,
the rootworm--which is actually a beetle, Diabrotica vigifera,
whose wormlike larvae feed on corn roots--had spread into Croatia and
Hungary. It has now been spotted in Romania, Bosnia-Herzegovina, Bulgaria,
and Italy. There's little doubt that Diabrotica will eventually
gnaw its way into every corn-producing country in Europe and perhaps
beyond, reducing crops and forcing farmers to use chemicals, says Kiss:
"There are no limits. It's a big disaster."
Meanwhile, in South Africa, ecologists are bracing for the rise of Varroa,
a mite that parasitizes honeybees. After sweeping through Europe and North
America for decades, Varroa was found near the Cape Town harbor in
1997; now it's all over South Africa, and the first colonies have died,
says Mike Allsopp, who heads the honeybee section of the Plant Protection
Research Institute in Stellenbosch. What worries Allsopp most is the fact
that between 50% and 80% of South Africa's native flower species are
pollinated by bees--a much higher percentage than in Europe or the United
States. "Commercial keepers can keep their colonies alive with
chemical treatments," says Allsopp. "But if Varroa wipes
out 99% of the natural colonies, as it has done elsewhere, what effect is
that going to have on the indigenous flora? No one really knows."
These are just three examples on a list that could be extended almost
endlessly. As the world shrinks and travel and trade boom, plant and animal
species have become globetrotters too, sometimes because humans decide to
take them along, sometimes by accident. And whereas globalization may be
the mantra of the new economy, for the environment it may spell disaster.
The innocent-looking zebra mussel, a Eurasian invader that entered U.S.
waters in the late 1980s and clings by the thousands to every hard surface
it finds, does tens of millions of dollars worth of damage each year by
clogging U.S. water pipes. Even worse, exotic species can devour or
outcompete species that have called an ecosystem home for tens of thousands
of years. Biological invasions are the second biggest cause of biodiversity
loss in the United States, after habitat destruction, according to a 1998
study; they could soon become the first.
Ecologists are paying more and more attention, if only because they
increasingly find themselves studying not primordial ecosystems but
collections of microbes, plants, and animals from around the world, flung
together in an ecological melting pot. "It's the fate of all
ecology," says marine ecologist Jeb Byers of the University of
California, Santa Barbara. Some ecologists have suggested, only
half-jokingly, that the field should start calling itself "mixoecology"
or "recombination ecology." Many fear that another century or so
of frenetic international traffic will lead to an "ecological
homogenization" of the world, with a small number of immensely
successful species, like the zebra mussel, cheatgrass, the European house
sparrow, and the Argentine ant dominating nature everywhere--a global
McEcosystem.
Hopes of arresting this process are spurring new studies. Policy-makers
trying to restrict traffic in exotic species and prevent invaders from
running rampant (see p. 1836)
are hampered by not knowing exactly where the danger will spring up next.
If ecologists could identify likely invaders, governments could simply
restrict imports of those treacherous species, and managers could
mercilessly weed them out or trap them. But making such predictions has
been devilishly difficult; the few predictive models are still hotly
debated, and they apply to only a narrow range of organisms at best. Some
past invasions seem to fit no pattern at all. A true theory of
prediction--what several researchers call the "Holy Grail of invasion
biology"--still proves elusive. "It will always be very difficult
to predict," says ecologist Ted Case of the University of California,
San Diego (UCSD).
Portrait of an invader
For most of human history, shipping animals and plants around has been
considered a good thing. New World colonists brought in seeds, plants, and
livestock and took other species back to Europe; 19th century
"acclimatization societies" strived to populate America and
Australia with European plants, birds, and mammals, including every bird
mentioned in Shakespeare's work. Most such imports quickly die, but
others--perhaps one in 10--settle into their new home. Of those, perhaps another
10% spread unchecked. As early as the late 19th century, when imported
rabbits started ravaging Australian vegetation (see p. 1842),
it became clear that newcomers could be dangerous. Now, the U.S. Department
of Agriculture (USDA) intercepts about 3000 potential pests at the border
every year, but many others make it through, and thousands of exotics are
firmly established. In a lush state like Florida, one in every three or
four plant species is non-native; in parts of San Francisco Bay, a
staggering 99% of all biomass is thought to belong to non-native species.
After a slow start, the field of invasion biology is at last taking off.
The journal Biological Invasions was launched just last month, and
invasion biologists suddenly find themselves attracting more and more
grants, students, and postdocs. Hundreds of scientists are staging plant
takeovers in the lab or fencing off patches of sea floor to watch
competition between marine critters in action. "People who worked on
invasions used to feel like bastard children," says ecologist Sarah
Reichard of the University of Washington, Seattle. "Like we had said
something dirty. Now, all of a sudden everybody is interested. It's
great!"
For scientists seeking the ecological principles behind invasions, one
place to start is with common traits of invaders. Many researchers have
noted, for example, that invaders often grow fast and have short reproductive
cycles; plants typically have small seeds that spread easily, and most
invaders are generalists that aren't too picky about their environment.
Some researchers are now trying to use these commonalities to understand
why certain alien species overrun natives while others don't.
Take one of South Africa's biggest pests, pine trees from Europe and the
Americas. Introduced for forestry as early as 1680, pines have spread out
of plantations and into the Cape's fynbos biome, a richly diverse belt of shrublands
north and east of Cape Town, says David Richardson of the University of
Cape Town. To understand why only a handful of the 100 or so introduced
pine species have encroached on native territory, Richardson, together with
Marcel Rejmánek of the University of California, Davis, compared the life
histories of invasive and noninvasive pine species. They found that
"invasiveness" depends largely on three characteristics that help
a species reproduce fast and spread widely: a short interval between successive
large seed crops, small seed size, and a short juvenile period. Richardson
used these factors to create a "discriminant function" that is
"pretty accurate in distinguishing invasive from noninvasive
species," he says. And although built on pine studies, it works for
other plants as well--the model gave the right answer for 38 of 40 known
invasive woody plants.
But such tidy results are rare in other systems. In a recent review in Ecography,
Mark Williamson of the University of York pointed out serious disagreements
among three studies since 1995 that sought common traits among Britain's
invasive plants. One found that large seeds favored invasions, another
found that it was small seeds, while a third said size didn't matter. Back
in the 1980s, the Scientific Committee on Problems of the Environment had
raised similar doubts after trying to tease out the factors that make for
successful invaders or vulnerable habitats, says Williamson. "We just
didn't find anything," he says. "I don't think there is an answer
to come up with."
Vulnerable territory
Even if the stereotypical invader's signature is still uncertain, is there
a typical ecosystem that easily gets invaded? Many researchers have found
that exotic species move in more easily amid other types of ecological
disruption. That was the common denominator Case identified in a set of
invasions by ants, birds, and geckoes. Argentine ants are abundant in
Californian towns and suburbs, says Case, and although they sometimes
spread into the surrounding coastal sage scrub, they are never farther than
50 or 100 meters from the humanmade landscape.
Case also studied why native, asexually reproducing geckoes were driven
out of Pacific islands by a sexual species from Southeast Asia. Turning
Hawaiian aircraft hangars into makeshift laboratories, he watched how well
the two species do under different circumstances and found that the
newcomers are good at snapping insects on smooth surfaces with abundant
light--in other words, on the walls of buildings. The invaders don't do
nearly as well in forests, and without urbanization, Case says, they
wouldn't have made it.
Another long-standing theory is that ecosystems rich in species, with
their dense, interconnected webs of ecological relationships, can resist
invasions, while those with fewer species succumb. For example,
islands--which usually have fewer species than comparable areas of
mainland--are often also the most heavily invaded. Models and lab
experiments seem to support the idea; in an as-yet-unpublished study, for
instance, a team led by John Stachowicz of the University of Connecticut,
Groton, created artificial marine ecosystems with anywhere from zero to
three North Atlantic species and then seeded each with a known invader, a
Pacific tunicate called Botrylloides diegensis. The more species
there were, the smaller the tunicate's chance of survival.
But a growing number of researchers think it's exactly the other way
around. "To those small-scale experimenters and modelers I say: Go
take a hike," says ecologist Tom Stohlgren of Colorado State
University in Fort Collins--and he means it literally. His team recently
sampled 100 plots in nine natural grasslands, national parks, and wildlands
throughout the central United States. The number of exotic species in each,
he reported at an ecological meeting last month, was positively correlated
with the number of native species. The very circumstances that favor a
wealth of native species, says Stohlgren, such as light, water, and
nitrogen, also make a place attractive to newcomers. And experiments with
just a few species don't remotely resemble real life, he adds.
There's yet another shadow on the prospects of prediction. Scientists
have repeatedly witnessed exotic species living inconspicuously in their
new habitat for decades--until the population suddenly explodes like
Teletubbies in a toy store. In some cases, the reasons were obvious: Three
species of exotic fig trees grown in Florida gardens for a century started
spreading only 20 years or so ago--after the arrival of the fig wasp
species that pollinate them. But often, such lag times are "quite
mysterious," says ecologist Daniel Simberloff of the University of
Tennessee, Knoxville. Take Brazilian pepper, "an incredibly
awful" invader in south Florida, Simberloff says. "It sat around
in people's yards as a harmless ornamental for many years, doing nothing.
And suddenly in the late '40s, early '50s, it exploded"--and nobody
knows why.
The zebra mussel is another case in point. Scientists have predicted its
arrival into the Great Lakes from Europe via ballast water since the 1920s,
says aquatic invasion expert James Carlton of Williams College-Mystic
Seaport in Mystic, Connecticut. Yet the invasion just didn't happen.
"By 1988, it would have been a worthwhile academic exercise to figure
out why zebra mussels could not successfully establish themselves in
America," says Carlton. It's still not clear why the animal finally
invaded when it did. One possibility is that some change in the environment
makes it more suitable to a particular exotic species, says Simberloff,
although it's often unclear what that is. (In the case of the zebra mussel,
ironically, improving water quality in the Great Lakes has been blamed.)
Another likely boost to an invader's chances is simply repeated and
widespread introduction. Robert Pemberton, a weed scientist with the USDA
Agricultural Research Service in Fort Lauderdale, Florida, recently leafed
through old catalogs from the Royal Palm Nurseries, a famous, trend-setting
company that bred and sold plants in Manatee County, Florida, from 1881 to
1937. He found that plants sold for just 1 year had only a 1.9% chance of
establishing in the wild, while favorites that were in the catalog for over
3 decades had a 68.8% chance of taking hold.
And some species may simply need a stroke of good luck to get started.
One reason that cheatgrass exploded in Canyonlands in 1995 and not before,
says the USGS's Belnap, is unusually frequent rainfall in late 1994, which
spurred germination of hundreds of thousands of dormant seeds. In other
parts of the West, fires sometimes wipe out the existing perennials and
give annuals like cheatgrass, yellow star thistle, and medusahead rye their
lucky break.
Given all this uncertainty, many ecologists are quite modest about their
power to predict. For now, just forecasting the advance of a limited group
of species in a number of habitats is difficult enough. Belnap, for
instance, discovered that at least in Utah soils, cheatgrass often strikes
where the potassiummagnesium ratio in the soil is high, suggesting that
potassium uptake may be limiting for this species. She's now looking to see
if the same holds true for other annual weeds and for other soils. Such
studies are arduous, but they may be the only way to go. Says Case of UCSD:
"The best approach is case by case." It's scant comfort that
there will be many more cases to study.
