Bio-invasions: the spread of exotic species.

Human mobility has radically increased the rate at which large numbers of living things are moving from one ecosystem to another. As more and more of these mobile "exotic" species invade natural communities that cannot cope with them, more and more native species lose out. Exotics are undermining global biological diversity - and becoming a growing economic burden as well.

Earlier generations of Americans thought the Florida Everglades required a cure. The immense marsh at the tip of the peninsula - a hot, unwholesome expanse of mosquito-infested sawgrass - was an obstacle to the advancing fronts of civilization and industry. So around the turn of the century, officials of the U.S. Department of Agriculture (USDA) began distributing seeds of the melaleuca tree (Melaleuca quinquenervia), a thirsty, fast-growing native of Australia. The tree had already been planted to drink up "fever swamps" elsewhere - and there was the possibility of timber production. Melaleuca took root in the popular imagination; by the 1930s, nurserymen weren't able to grow enough to satisfy demand. One forester even took to broadcasting melaleuca seed over the Everglades from his airplane.

But the melaleuca's only product has been a spreading thicket of trouble. Its impenetrable stands displace virtually all other vegetation. Its dense root mat oozes substances poisonous to other plants. Its airborne secretions are poisonous to people: they can cause severe respiratory and skin irritation. And the melaleuca is "fire adapted" - it spreads by burning. Its inner bark is a wet, insulating sponge, while its outer bark is tinder-dry and its leaves are laced with a flammable oil. So even though it sucks up water four times as fast as the native sawgrass, it burns with explosive force. A few days afterwards, the tree sprouts new growth and rains millions of seeds onto the burnt-over land. Germination begins in three days and a seedling may reach six feet in its first year. The melaleuca may already have invaded as much as 600,000 hectares of Florida wetland and if it is not controlled, says Ronald Myers, an expert on the problem, "the Everglades will be no more."

The melaleuca's rampage fits a pattern typical of exotic species - species introduced into ecosystems in which they are not native. Freed from the diseases, predators, and other factors that keep them in check in their native habitats, exotics can wreak ecological havoc. And as they spread, they displace ever greater numbers of rarer species, whose ranges are more circumscribed. About 30 percent of the creatures on the official U.S. Endangered and Threatened List, for example, are there at least in part because of exotics.

Of course, the migration of species into new habitat has always been a part of nature, but human interference has so greatly amplified the process as to make it, taken globally, a phenomenon without precedent in the history of life. In Hawaii, where exotic invasions have reached epidemic proportions, an average of 18 new insects or other arthropods have established themselves every year over the past half century or so. That's more than a million times the natural rate of invasion for that group of organisms. Hawaii may be an extreme case, but all over the world, exotics are accounting for an ever larger share of the biota - the local assemblage of living things. Yet little is being done to stop this process, largely because it's bound up with so many economic activities - everything from intentional introductions of exotic grasses for grazing cattle, to accidental releases of shrimp viruses from aquaculture shipments.

Like other forms of environmental degradation, exotic invasions exact a price. In 1957, for example, the Nile perch (Lates niloticus) was released into Africa's Lake Victoria to improve the fishing. But the perch, a voracious predator, eliminated nearly half the lake's 400 native fish species. And it proved an inferior food fish. Its oily flesh must be smoked, so nearby forests were logged. Now the perch itself appears to be in decline, due to lack of prey, overfishing, and the deoxygenation of algae-choked waters provoked by the loss of the herbivorous fish. Uganda, Kenya, and Tanzania - the countries surrounding the lake - must endure the loss of both fisheries and forests, even as their populations continue to grow.

The full costs of a disruptive invasion are beyond reckoning, but the immediate costs may be clear. In the United States, the USDA estimated losses and control expenses during the 1981 outbreak of the gypsy moth (Lymantria dispar), a forest pest native to Europe, at $764 million. (About 40 percent of all serious insect pests in the United States are exotic; so are at least half the weeds.) The zebra mussel (Dreissena polymorpha), a shellfish from the Caspian region that is spreading throughout eastern North America, could increase the cost of U.S. power production. Since it breeds prolifically and encrusts almost any available surface, including powerplant water pipes, the mussel could force the power industry to spend $800 million redesigning its plants - plus $60 million annually on maintenance. Exotics can pose serious public health threats as well. Over the last decade, the Asian tiger mosquito (Aedes albopictus), already widespread in Asia, established itself in Brazil, southern Europe, South Africa, and the continental United States. This mosquito is known to carry dengue fever, yellow fever, and encephalitis.

THE DOMINO EFFECT

Not every exotic is a monster, of course - the ginkgo tree on the lawn will probably never take over the countryside. It's likely that only a small percentage of exotics even manage to survive in their new homes. And according to many ecologists, an exotic that does establish itself will not necessarily do measurable harm. But at present, there is simply no way to identify the serious troublemakers - the invasive exotics - until the damage is done.

Part of the reason for this is that the invaders do their damage in so many different ways. Sometimes they cause a kind of ecological domino effect. The zebra mussel, for instance, is stripping the plankton out of more and more North American lakes and rivers, forcing an abrupt shift in the basic community food source - from plankton to bottom sediment. Over the long term, that shift may suppress plankton-feeding fish species, and increase bottom-feeding organisms like aquatic worms and crawfish. Such effects can extend far beyond the immediate ecosystem. In the 1960s and 1970s, for example, the U.S. state of Montana introduced the exotic opposum shrimp (Mysis relicta) into the state's Flathead River. It was hoped that the shrimp would serve as an additional food for a species of salmon, also exotic, that had been introduced as a game fish. But the shrimp proved a formidable predator of the local zooplankton - the main food of juvenile salmon. The salmon population collapsed, taking with it a wide spectrum of terrestrial species that had come to depend on the fish. Among the wildlife the shrimp has displaced are eagles, gulls, otters, coyotes, and bears.

The "food web" - the network of predator-prey relationships - is not the only ecological process that's vulnerable. Fire-adapted exotic grasses are changing the role that fire plays in natural areas of the American west, Hawaii, and Australia. Other effects are more complex. Why, for example, should the arrival of the Argentine ant (Iridomyrmex humilis) threaten the Cape Floral Kingdom, an extremely diverse and unique South African plant community? Because some 1,300 plant species depend on native ants for seed burial, and the native ants are being displaced by the invader.

But the damage can also be direct. Sometimes, for instance, the intruders simply eat the natives. Rats are the most important exotic predators of island birds, which have often evolved in the absence of any predators. This lack of evolutionary defenses explains what happened in 1964, when rats arrived on New Zealand's Big South Cape Island: five bird and one bat species disappeared. (More than 80 percent of the world's major islands now have exotic rat populations.)

Sometimes exotics out-compete natives for an essential resource. On the prairies of the western United States, Eurasian cheat grass (Bromus tectorum) has proven a stronger competitor for water than the once dominant native wheat grass (Agropyron spicatum); cheat grass now carpets more than 40 million hectares.

Sometimes an invader brings along its diseases or parasites. The Chinese grass carp (Ctenopharyngodon idella), widely used for aquatic weed control, has infected freshwater fish in Europe and North America with an Asian tapeworm (Bothriocephalus opsarichthydis).

But perhaps the most insidious form of damage occurs when exotic and native merge. Interbreeding can swamp a native gene pool in foreign genes, thereby eliminating its distinctiveness and eroding the species' genetic diversity. In the U.S. Pacific Northwest, massive releases of hatchery-bred salmon are believed to be diluting endangered salmon populations in this way. Interbreeding may even create new, hybrid species, as when crosses between crop and wild potatoes produced the Bolivian weed potato (Solanum sucrense).

Although the damage that invasive exotics do is well documented, the key to their success is still a mystery. For decades, ecologists have been compiling lists of invasive characteristics, then rejecting them as having no predictive value. But one useful approach to the problem is a concept called "weediness." A text book case of weediness can be found in the red fire ant (Solenopsis invicta), a viciously aggressive stinging ant from Brazil that is now widespread in the U.S. southeast. The fire ant thrives in disturbed areas, like plowed fields. It disburses widely, by mating swarms or by clumping together to form floating mats during floods. Its reproductive abilities are amazing even by insect standards - no other ant species produces as high a proportion of sexually active individuals. Its colonies also grow very dense: they may contain hundreds of queens (most ant species form only single-queen colonies) and command over an acre (four-tenths of a hectare) of territory. And fire ants will eat just about anything. In the United States, they have killed off as much as 40 percent of all native insect species in some areas. Preference for disturbed habitat, efficient dispersal, rapid population growth, and opportunistic feeding - these hallmarks of weediness appear to some degree in a huge assortment of other widespread exotics. Ragweed, rats, and starlings are all weeds.

Another approach to the problem seeks common features among the areas invaded. Here too, every theory invites a host of exceptions, but many ecologists argue that small ecosystems - islands and lakes, for example - are more readily invaded than large ones. Their vulnerability may be due to evolutionary isolation, which tends to foster small assemblages of rather narrowly adapted species. And previous disturbance is also commonly seen as a factor: unhealthy ecosystems may be more hospitable to exotics.

To date, the study of exotics could be summed up in three negative statements: it's impossible to predict where an exotic will establish itself, or what it will do afterwards, or when it will do it. The collared dove (Streptopelia decaocto), for example, was brought into southern Europe more than 200 years ago by the Ottoman Turks. Why did it wait until around 1930 to begin an invasion that has now nearly covered the continent? Climate change and a preference for urban settings have been suggested as causes, but no one really knows if they are. In the U.S. midwest, several previously well-mannered garden shrubs have recently become invasive, apparently because they have adapted to their new surroundings. Invasive exotics often adapt readily to new environments. Since its arrival in North America in the mid-1980s, for instance, the Asian tiger mosquito has already fine-tuned its daylength sensitivity to suit various locations on the continent.

A CULTURAL CONSTANT

Moving other creatures around is a deeply ingrained human habit. Exotics are an ancient and constant cultural effect - a fact of life for the neolithic farmers who watched the spread of Europe's first weeds nearly 10 millenia ago; a fact of life for the people who brought the dingo (Canis familiaris dingo) to Australia perhaps 3,000 years later. Some ancient introductions were made over great distances, but the general pattern was a fairly gradual intermingling within regional biotas. That began to change about 500 years ago, with the spreading ascendancy of European cultures, beginning in the Americas. Sometimes the New World proved better ground for European plants and animals than for Europeans themselves. When French colonists arrived in Florida in 1560, the native peoples supplied them with pork from well-established herds of feral pigs a legacy of earlier explorers. By the 17th century, an extensive Old World agricultural assemblage was available throughout the colonies of eastern North America.

With the intentional introductions came the unintentional ones, some of them disastrous. The arrival of European and African pathogens in the New World was one of the greatest calamities in human history; it is estimated that during the course of the 16th century, 30 million native Americans, perhaps two-thirds of the hemisphere's population, died from Old World diseases. Other invasions were less catastrophic but no less permanent. In the same century, Spanish agriculture in what is now Chile released a massive invasion of Mediterranean weeds.

In later centuries, the coalescence of a global trading network, with Europe at its hub, drew much of the world into the biotic mixing bowl. In seaports all over the globe, exotic flora sprouted in ballast heaps unloaded from ships. Rats made their way throughout the Pacific and into Australia. And more and more organisms became objects of trade themselves. In the 1830s, the "Wardian case," a kind of traveling terrarium that looked like a miniature greenhouse, revolutionized the introduction of American and Asian plants into European gardens. Throughout the 19th century, "acclimatization societies" in North America and Australia dedicated themselves to haphazard releases of exotics for various reasons - usually on the assumption that the local fauna was inferior to that of Europe. The starling (Sturnus vulgaris), for example, owes its conquest of North America to a society bent on bringing to the New World every bird mentioned in the plays of William Shakespeare. Railroad "fish cars" were trundled all over the American landscape to introduce exotic fish, with the choice of stream often left to the whim of the crew. In Australia, the rabbit, which still plagues that continent, was released to stock an unfamiliar land with familiar game.

Today the pattern has shifted again, into a kaleidoscopic network of movement in which practically any point on the globe can send or receive material. The complexity of movement defies full description, but a look at a few major "pathways" (the mechanisms through which exotics are introduced) offers a perspective on it.

Trade remains the most important factor overall, and among its many pathways, one is of special concern. Shipping containers, the big metal boxes that can be stacked on ships, then offloaded onto trucks or trains, have revolutionized shipping - and may do the same thing for the movement of exotics. World container traffic now stands at around 100 million 20-foot units a year, and it's growing. In 1980, container ships accounted for 1.6 percent of world shipping tonnage; by 1993, the most recent year for which figures were available, that share had more than tripled to 4.9 percent. (A good two-thirds of the world's shipping consists of such noncontainer commodities as minerals, grain, and oil.)

That may not sound like a lot, but it's more than enough to make the shipping container the Wardian case of our era - except that some of its contents are accidental. The Asian tiger mosquito arrived in the United States in containers of used tires imported from Japan. "The safe, protected environment of the sealed container," writes George Craig, Jr., one of the mosquito's observers, "provides a splendid mobile pram between continents." Craig notes that container traffic has also broken the old link between invasion and seaport: containers may not be unloaded until they reach their final destination - and that could be anywhere a road or railroad leads.

Ballast water has proven a more effective pathway than the solid ballast it replaced - though for different classes of organisms. Ships carry immense quantities of ballast water from sea to sea, pumping it in or out as cargo is loaded or unloaded. A single bulk carrier may have a ballast capacity of nearly 50 million U.S. gallons; the total capacity of the 1,625 bulk carriers in the world's major fleets might therefore approach 80 billion gallons - about 300,000 cubic meters. And in terms of tonnage, bulk carriers make up only about a third of the world's shipping.

Ballast water exchange has become a manmade overlay to the globe's natural currents - a network of artificial rivers running through the oceans. Myriad small creatures - fish, worms, plankton, crabs, and clams, to name a few - ride these new currents all over the world. In Coos Bay, Oregon, a study cataloged the ballast water release of some 400 exotic species from 1988 to 1991. At least 14 of the more than 100 exotics known to have been released into Australian ports have become established. In North America, the zebra mussel arrived in ballast water. And that's how the comb jelly Mnemiopsis leidyi found its way into the Black Sea. The jelly has devastated local fisheries and sometimes accounts for as much as 95 percent of the sea's wet weight biomass.

From time to time, a brown tree snake (Boiga irregularis) is found on the runway of a Hawaiian airport, where it has fallen from a wheel well or cargo bay of an airplane arriving from the Pacific island of Guam. The snake was inadvertently introduced into Guam from the Papua New Guinea region; it has overrun the island and eliminated nine of Guam's 18 native bird species. Thus far, the snake is not known to have established itself in Hawaii, but every new find leaves Hawaiian ecologists wondering whether any stowaways survived a flight undetected. Air traffic is another rapidly broadening global pathway. In 1989, only three airports received more than a million metric tons of cargo; by 1993 that number had risen to seven. The speed of air transport may help spread pathogens and other organisms that would probably be detected in the course of ship passage.

But the pathways themselves don't tell the full story; entire economic sectors depend on or actively promote the spread of exotics. Agriculture, for instance, molds landscapes to suit exotic crops and livestock, which must then be defended against exotic pests. Forestry razes native forests to grow exotic timber which, when cut, can transport exotic forest pests. Aquaculture dumps exotic fish into rivers and streams. Just one aquaculture species, the Mozambique tilapia (Oreochromis mossambicus), is now established in nearly every tropical and subtropical country. And dozens of smaller industries - horticulture and the aquarium trade, for example - are moving all sorts of creatures from place to place. The aquarium trade is the source for 65 percent of the exotic fish species known to have become established in the United States.

A young industry, biotechnology, may force the process into yet another quantum leap. An entirely new dimension of pathways has opened up with the development of transgenic organisms - creatures whose genetic complement contains DNA from other species with which they could not possibly have interbred. Many of these organisms are already in the field testing stage - insect viruses, crop plants, and fish among them. Critics of the industry point out that a transgenic organism's "exotic" genes could move into wild populations through ordinary interbreeding - and that such an event might have serious consequences. Crop plants, for example, often have wild relatives with which they interbreed. Crops engineered to be especially tolerant of herbicides or salt might confer their new traits on their wild relatives, allowing them to invade a salt marsh - or a heavily sprayed wheat field.

Of course, a transgenic organism could be extremely disruptive even without releasing its unusual genes. Last year, two Oregon State University scientists discovered in laboratory tests that a soil bacterium they had engineered to produce the fuel ethanol also cut soil populations of mycorrhizal fungi by more than half. These fungi are essential to nutrient uptake in higher plants. "So if the bacterium had been released," one of the scientists dryly observed, "it could have been a real problem. If the organism survived readily and spread widely, very likely we would be unable to grow crops without a control measure for this organism." Thus far, there have been at least 2,258 experimental releases of genetically engineered organisms in the United States alone.

Whether it's an intermingling of chromosomes or oceans, this blurring of ancient biological boundaries is frequently a matter of policy. Too often, exotics are a specious "easy way out" for natural resource managers. It's much easier to introduce a fish that will tolerate a dirty river than it is to clean the river up so that its native fish can thrive. It's easier to plant quick-growing exotic timber than it is to manage a natural forest for sustainable production. And it's not just a matter of rivers and forests - the local people may lose out as well. The introduction of exotics is often an ingredient in activities that undermine local cultures and economies. In agriculture, for instance, when mass-market crops replace traditional varieties, they encourage a dependence on international suppliers and buyers. In forestry, a preference for exotic timber can lead to situations like the one in Chile, where a subsidized reforestation program promotes the planting of exotic pines and Eucalyptus species. Major landowners, trying to bring as much land as possible into the program, are reported to have displaced large numbers of small farmers. The process affects fisheries too: Lake Victoria's Nile perch has fed an export-oriented fish processing industry much more effectively than it has fed the families of local fishers.

UNSCRAMBLING THE EGG

Coming to grips with the exotic menace will require a degree of ecological realism. Current technology does not generally permit the wholesale removal of an exotic once it has established itself in an ecosystem. As one ecologist puts it, "getting it out of that system is like trying to unscramble an egg." But there are some exceptions to the rule. It is often possible to eradicate an exotic during the initial phase of its invasion, while it is still just "settling in." And there are some instances of later success. Perhaps the most remarkable of these was the eradication of the malaria mosquito (Anopheles gambiae) from Brazil in the 1930s, a feat achieved by the liberal use of arsenic against a creature that, fortunately, proved poorly adapted to its new environment.

But more typical is the stalemated effort to rid the western hemisphere of the yellow fever mosquito (Aedes aegypti), beginning at the turn of the century and continuing sporadically today. Even the United States, which spent $100 million dosing this mosquito with DDT in the 1960s, has failed to dislodge it. Among the factors that usually doom such efforts are an inability to locate all habitat - every tree hole, empty beer can, or junked tire, in the case of the yellow fever mosquito. High reproduction rates are another obstacle; "weedy" species usually bounce back fast even after enormous losses. A policy that insists on total eradication may actually make things worse, as in the USDA's $200 million war on the fire ant in the 1950s and 1960s. The Department's pesticides hurt native ants more than the fire ant, giving the invader an edge in occupying the newly emptied habitat.

Luckily, complete eradication isn't usually necessary. Most exotics need only be "controlled" - reduced to the point of ecological insignificance. Sometimes this can be done by physical or chemical means. In the western United States, some small natural areas have been cleared of tamarisk trees (Tamarix species) simply by repeated cutting. But even where such techniques succeed, they must be continually renewed, and the use of chemicals often has serious ecological or political liabilities. During the 1980s, California's spraying programs against the mediterranean fruit fly (Ceratitis capitata), a serious agricultural pest, sparked considerable public opposition, despite a major publicity campaign. The program was perhaps a greater biological success than a political one: the fruit fly was eradicated, but left in its wake no clear public consensus on how to respond to future infestations. Damage claims against the state totaled more than $2 billion, of which some $3.7 million were actually paid.

Another strategy, biological control, pursues the offending exotic by introducing other exotics to attack it. In crude form, biocontrol has been around for centuries, but in early instances of its use, the cure often proved worse than the disease. In 1762, for instance, a Jamaican sugar planter introduced an aggressive Cuban ant (Formica omnivora) onto his plantation, because the ant attacked rats - already a well-established pest in the Caribbean. The ants flourished, but didn't much bother the rats. So an enormous South American toad (Bufo marinus) was introduced to control both. It flourished as well - it's now called the cane toad for its success in infesting sugar plantations. The rats remained, however, so the small Indian mongoose (Herpestes auropunctatus) was introduced; by 1898, 26 years after it arrived, the mongoose had decimated the island's bird and reptile fauna, along with much of the islanders' livestock.

Modern biocontrol seeks more decisive results, generally by using the culprit organism's native predators. Solid successes have been achieved this way, as with the control of the water fern Salvinia molesta, in Papua New Guinea's Sepik River. By 1980, a decade after it first appeared, the fern had made the Sepik unfishable and impassable, starving out local villages in the process. By 1990, the infestation had been largely cleared by the introduction of a weevil, one of a group of insects that regulates the fern's growth in its native Brazil. And so far, at least, its new setting hasn't tempted the weevil away from its exclusive diet of water fern.

But the technique has important limitations. Sound biocontrol requires "host specificity" - the ideal biocontrol agent preys exclusively on the target organism and pursues it to the edge of oblivion. Neither target nor agent is expected to die out entirely, but any increase in the target's population is met by an increase in the agent's. It's possible to find candidate agents among the insects that approach this ideal, but even with insects, careful testing is necessary. Several wasp species introduced into Hawaii for crop pest control apparently caused the extinction of native moths. Many insects - and many other types of invertebrates - have diets too broad for biocontrol. For the same reason, the use of any vertebrate invites serious trouble. One of the field's biggest debacles has been the introduction of mosquitofishes (Gambusia species) all over the world for mosquito control. The effect on the mosquitoes is generally negligible, but the mosquitofish have extirpated native fish by eating their larva and fry. Of course, many pathogens are host specific, and some diseases have been successfully employed as biocontrols, notably myxomatosis virus, a rabbit disease. But concerns that microbes could mutate and attack nontarget organisms have greatly limited their use thus far.

The principles of biocontrol have often led resource managers to assume that for every pest, there's an ideal predator that can reduce it to insignificance. Unfortunately, this does not appear to be true. For 35 years, researchers have been seeking biocontrols for the balsam woolly adelgid (Adelges piceae), a European insect that has infested North American fir trees (Abies species). The adelgid has nearly eliminated a variety of balsam fir (A. balsamea var. phanerolepsis) and is on its way to destroying the Fraser fir (A. fraseri) but so far, no effective biocontrol agents have been found.

Even though it's no panacea, biocontrol may be the best remedy available in many cases. Certain Australian insects may, for instance, be Florida's best bet against the melaleuca. In any case, there are few other options. Some promising new technologies could prove useful against particular classes of exotics - artificial pheromones may confuse mating reflexes in certain insects, for example, and it's possible to engineer oral vaccines that cause infertility in mammals. But at present, no emerging technology works against as broad a range of exotics, in as many different environments, as biocontrol.

EXOTIC POLITICS

The greatest challenges in dealing with exotics are not biological, however; they are political. On an international level, exotics usually get inadequate coverage in treaties that regulate important pathways. They do figure in the International Plant Protection Convention, which requires cooperation on agricultural pest control. But the Convention on Biological Diversity contains only a vague resolution to control dangerous exotics "as far as possible and appropriate." Marine exotics get more thorough treatment in the Convention on the Law of the Sea, but this treaty only went into effect last year, so it's too soon to gauge the results. Within their own jurisdictions, most countries seem to give serious attention to exotics only when they disturb a major industry or become an important public health threat.

It's not hard to find weak points in this legal framework. In response to the arrival of the Asian tiger mosquito, for instance, the United States restricted its Asian tire imports - but not its tire exports. Gordon Craig, the mosquito authority, argues that if U.S. tire shipments bring the mosquito to the Caribbean, the region's disease burden could be greatly aggravated. Dengue fever is already well established in the region, but it lacks an effective rural vector. The Asian tiger mosquito could readily perform that service.

Biological pollution, as invasions are sometimes called, is a policy nightmare because it results from so many disparate - and important - economic activities. Yet the same can be said of chemical pollution, and there is widespread acceptance of complicated regulatory shields against dangerous chemicals. One of the reasons for this is the publicity surrounding some major instances of chemical pollution, like the Bhopal disaster in India, in which a gas leak from a Union Carbide plant killed thousands of people. Few people probably view exotics with similar alarm, but an education campaign publicizing some worst cases - actual or potential - could change that. Widespread American interest in forest conservation, for instance, usually focuses on logging as the primary threat. But in much of the country, exotic forest pests may be at least as dangerous to the forests as poor management. Once a consensus for action has begun to take shape, it should be directed at the following goals.

Quarantine officials, ecologists, agronomists, and others who deal with exotics should be invited to pool their findings in a set of databases accessible to anyone with an interest in the subject. Such a project would be a logical extension of many efforts already underway; ultimately it could provide a far more effective basis for policy advocacy - and for action - than exists today.

Planned imports and releases of exotics should be regulated by a "clean list" approach, in which an organism must be explicitly approved as harmless - "clean" - before it can be legally introduced. (The more common approach uses a "dirty list," which catalogs organisms that have been banned because they are known to be harmful, and admits unlisted organisms.)

Importers should be required to accept liability for any damages their imports cause. This requirement should apply to first-time imports of exotics, and to releases of both exotics and genetically engineered organisms. The same principle of liability should be extended to importers of commodities known to be important pathways for dangerous exotics.

When a government funds an introduction - whether it's a new fish for aquaculture or a forage plant for livestock - it should require the studies necessary to determine the ecological effects of the new arrival. This policy should apply to both domestic introductions and to foreign ones which may, for instance, be carried out in the course of a foreign aid project.

Unfortunately, no single set of regulations will slow the rate of inadvertent introductions. That task will require an array of specific technologies and procedures built around the pathways involved. For example, alternative ballast water systems have been designed to reduce the rate of ship-borne introductions. The principal political task here will be to build a broad public mandate for the development and application of these responses. In the process, the public should be acquainted with the possibility that some forms of commerce may never be worth the risks they entail - Siberian timber imports into the United States, for instance. (The spruce bark beetle (Ips typographus), one of the forest pests associated with Siberian timber, has already killed millions of trees in Japan and Europe.)

Uninvited exotics that get past these barriers should be subjected to immediate evaluation and, where warranted, control. Since exotics are most vulnerable in the incipient stages of an invasion, protocols for responding should be as free from red tape as possible - and tied as firmly as possible to a scientific evaluation of the threat.

Finally, governments should adopt a general policy favoring the use of native species - and wherever possible, the use of native populations - in government activities that involve the release of organisms. Such activities include landscaping, roadside planting, erosion control, forestry, range management, fish and game stocking - even some foreign aid programs. The policy could be combined with a clean list approach to permit the use of exotics when no suitable natives can be found. By pushing industries towards the use of natives, such a policy could help to shape them into more environmentally benign forms, just as the U.S. federal commitment to recycled paper helped boost the use of recycled fiber in paper production. The policy could also advance ecological research, educate the public about exotics, and speed up landscape restoration programs. In developing countries, it could be a tool for helping local economies and indigenous peoples. A couple of examples show how broad the payoffs could be:

In the U.S. southeast, the Asian sawtooth oak (Quercus acutissima) has been widely introduced as a wildlife food plant - despite the fact that native oaks appear to be in decline. But in the state of Illinois, foresters can care for both wildlife and native forests by planting the 15 native oaks now grown at state nurseries. The Illinois Department of Conservation now uses plants native to the state almost exclusively for its planting programs a precedent that other land management agencies would do well to consider.

In Honduras, the Inter-American Development Bank is funding a reforestation project for the ruined watershed around the immense El Cajon Dam. Plans call for the introduction of Eucalyptus species and other exotic trees which critics fear could disrupt what is left of the area's ecosystem. Yet in Costa Rica, a number of local nurseries are doing a brisk business selling thousands of mostly native tree species for reforestation there.

Both cases come down to conserving particular local resources - the things that make a community distinct. These are the resources that are most vulnerable to exotic invasions. And in both cases, a knowledge of those resources could be an important weapon against invasion. For as exotics spread from one community to the next, homogenizing millions of years of intricate variety, they challenge our ability to value diversity - and to use it wisely. If we are to stem the rising tide of biological pollution, we must do more than just keep exotic species out; we must recognize the value of the species that belong where they are.

KEY SOURCES

Faith Thompson Campbell and Scott E. Schlarbaum, Fading Forests: North American Trees and the Threat of Exotic Pests (New York City: Natural Resources Defense Council, January 1994).

Donald L. Dahlsten and Richard Garcia, eds., Eradication of Exotic Pests: Analysis with Case Histories (New Haven, Connecticut: Yale University Press, 1989).

J.A. Drake et al., eds., Biological Invasions: A Global Perspective, SCOPE (Scientific Committee on the Problems of the Environment) 37 (Chichester, U.K.: Wiley, 1989).

R.H. Groves and F. Di Castri, eds., Biogeography of Mediterranean Invasions (Cambridge, U.K.: Cambridge University Press, 1991).

Bill N. McKnight, ed., Biological Pollution: The Control and Impact of Invasive Exotic Species (Indianapolis, Indiana: Indiana Academy of Science, 1993).

P.S. Ramakrishnan, ed., Ecology of Biological Invasions in the Tropics (Proceedings of an International Workshop Held at Nainital, India), (New Delhi: International Scientific Publications, 1989).

Aaron Rosenfield and Roger Mann, eds., Dispersal of Living Organisms Into Aquatic Ecosystems (College Park, Maryland: Maryland Sea Grant College, 1992).

United States Congress, Office of Technology Assessment, Harmful Non-Indigenous Species in the United States, OTA-F-565 (Washington, D.C.: U.S. Government Printing Office, 1993).

Chris Bright is Associate Editor of World Watch.