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Updated 11 November 2004

Consequences Vol. 2, No. 2, 1996



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Impacts of Introduced Species in the United States

by Daniel Simberloff

Since our country was first colonized, several thousand foreign plant and animal species have established themselves in the U.S., as have hundreds of animal and plant disease agents, or pathogens. Many of these species--such as crop plants and some game animals--are beneficial, while others--such as most pets and ornamental plants--are harmless amenities. At the same time, some 15 percent of these interlopers have become invaders, causing widespread problems that can prove serious and exceedingly costly. They can devastate farms and forests, impede waterways, foul lakes and ponds, affect human health, and invade natural areas and replace native species.

Introduced species cause disasters that one would never have foreseen. It might not seem surprising that the spread of fire-adapted, exotic plants that burn easily has increased the frequency and severity of fires, to the detriment of property, human safety, and native plants and animals. But would one have guessed that, in 1936, the town of Bandon, Oregon would be destroyed and eleven citizens killed by a fire propagated by gorse, a highly flammable plant introduced, seventy years earlier, from Europe? Or that dense, flammable plants such as Australian melaleuca, the so-called Australian pine, Asian cogongrass, and Brazilian pepper, introduced for roadside planting in Florida, have become costly hazards because of water loss through increased transpiration, increased fires, and blocked vision? The problem is so severe that the Florida Department of Transportation is removing 27,000 Australian pines along the Florida Turnpike.

The cost to taxpayers of introduced species in the U.S. was estimated, in a 1993 report of the Congressional Office of Technology Assessment (OTA), to range from hundreds of millions through billions of dollars each year. These estimates do not include effects on native ecosystems, such as extinction of native species that are of no immediate economic concern.

Best documented are costs to agriculture: about a fourth of this country's agricultural gross national product is lost each year to foreign plant pests and the costs of controlling them. In the case of cotton, the total accumulated cost of the boll weevil, which arrived in the U.S. from Mexico in the 1890s, now exceeds 50 billion dollars. Leafy spurge, an unpalatable European plant that has invaded western rangelands, caused losses of $110 million in 1990 alone. In eastern forests, losses to European gypsy moths in 1981 were $764 million, while the Asian strain that has invaded the Pacific Northwest has already necessitated a $20 million eradication campaign. To keep U.S. waterways clear of such plants as Sri Lankan hydrilla and Central American water hyacinth, about $100 million is spent annually. The cost of Eurasian zebra mussels, which clog pipes in water systems such as cooling systems in power plants, is predicted to be hundreds of millions of dollars annually.

Costs of introduced pathogens and parasites to human health and the health of economically important species have never been comprehensively estimated, but must be enormous. A recent example is the Asian tiger mosquito, introduced to the U.S. from Japan in the mid 1980s and now spreading in many regions, breeding largely in water that collects in discarded tires. The species attacks more hosts than any other mosquito in the world, including many mammals, birds, and reptiles. It can thus vector disease organisms from one species to another, including into humans. Among these diseases are various forms of encephalitis, including the La Crosse variety, which infects chipmunks and squirrels. It can also transmit yellow fever and dengue fever. The exotic disease brucellosis, probably introduced into the U.S. in cattle, is now a major economic and ecological problem, for it causes miscarriages in bison and elk as well as domestic livestock.

Introduced diseases can arrive through circuitous routes. When the North American rainbow trout was introduced to Europe, for example, it suffered epidemics of whirling disease, caused by a parasite carried by the European brown trout. Indiscriminate transport of rainbow trout from one breeding facility to another subsequently spread the disease to other parts of the world, including back to America.

Ecological Impacts

Domination of ecosystems

Costs to natural systems, although not easily translated into dollars, are staggering and diverse. Perhaps the greatest impacts are caused by plant species that come to dominate entire ecosystems. Melaleuca, which is increasing its range in south Florida by some 35 acres each day, replaces cypress and other native plants and provides poorer habitat for numerous animals. In many regions in and near the Everglades, it forms vast, dense monocultures where no other plant can grow. Asian salt-cedar and Russian olive are introduced plants that have formed new forests or replaced native plants along many rivers in the West, to the detriment of numerous bird species. Red mangrove from Florida, planted in Oahu in 1902, has spread to fill many formerly-open coastal areas, creating dense forests up to seventy-five feet high. Because each acre of mangrove swamp typically drops more than four tons of leaves each year, and because their roots trap sediment and provide habitat for various marine animals, this introduction has had enormous ecological impacts. Some of these may be positive, although most are as yet unstudied.

Other damage to ecosystems

An introduced plant species can bring great changes to an entire ecosystem without dominating in either numbers or biomass. On the island of Hawaii, the tall shrub Myrica faya, a native of islands in the eastern Atlantic, has invaded young, nitrogen-poor lava flows and ash deposits on the slopes of Mauna Loa and Mauna Kea. Because it fixes nitrogen, it modifies the normal colonization by other plants, favoring other introduced species.

Introduced animal species can also change the entire character of an ecosystem, through their effects on vegetation and on which plants will survive. In the Great Smoky Mountains National Park, a population of wild, or feral hogs, established by the few that escaped from hunting enclosures in 1920, has devastated local plant communities--selectively feeding on plants with starchy bulbs, tubers, and rhizomes, and greatly changing the soil by thinning the naturally heavy litter, and mixing organic and mineral layers together. The European periwinkle snail, introduced to Nova Scotia as a source of food around 1840, spread southward, feeding on rhizomes of marsh grasses and the algae that covered the rocks at the shoreline. In areas sheltered from waves, it was key to a gradual transformation of the New England coast--into the bare rocks that most people interpret today as a natural condition.

Plant pathogens can equally overwhelm an entire ecosystem. The chestnut blight fungus arrived in New York City in the late 19th century on nursery stock from Asia and in less than fifty years had spread over 225 million acres of the eastern U.S., destroying virtually every chestnut tree. Because chestnut had comprised a quarter or more of the canopy of tall trees in many forests, the effects on the entire ecosystem were staggering, although not all were obvious. For example, several insect species that live only on the chestnut are now extinct or endangered. A less predictable result is an increase in the oak wilt disease in many native oak species, since the trees that replaced many of the chestnuts were red oaks, which are particularly susceptible to the blight.

Introduced species can drastically affect specific plants or animals even when the invaders do not modify the entire ecosystem--feeding upon a native species, for example, to the point of its extinction. The semaphore cactus, which was found until recently in the lower Florida Keys, is such a victim. Now a candidate endangered species, it has all but disappeared as an indirect result of steps taken forty years ago to control another cactus that is also native to the region. In 1957 the South American cactus moth was introduced to the island of Nevis in the Lesser Antilles to control a pest cactus. Hopping from island to island, and possibly aided by inadvertent transport in cut flowers, the moth reached the Florida Keys by 1990 and soon eliminated most of the semaphore cactus found in the wild. Any that remain today must be protected by cages. Meanwhile, the cactus moth has found other host plants in the Keys and has continued to grow in number.

Other examples abound. The balsam woolly adelgid, a European aphid, has killed almost all firs, once a dominant tree at higher elevations of the Great Smoky Mountains National Park. The sea lamprey was able to colonize the upper Great Lakes when the completion of the Welland Canal between Lake Ontario and Lake Erie allowed this parasitic fish to bypass Niagara Falls and subsequently to work its way into lakes Huron, Michigan, and Superior. It proceeded in ensuing years to devastate populations of lake trout, burbot, and whitefish, at great economic loss to the region.


Introduced species can also propagate diseases that harm native species. In the Hawaiian islands, for example, a battery of introduced Asian songbirds are host to avian pox and avian malaria. The diseases are transmitted to native birds by introduced mosquitoes, and they have contributed greatly to an almost total elimination of native birds, unique to Hawaii, in lowland forests.


Introduced species can compete with native ones. In Florida, the decline of the native wasp Pseudhomalopoda prima --of value as a major enemy of a pest scale insect--corresponded closely with the arrival of another wasp from southern Asia, Chrysomphalus aonidium, which had been introduced in 1990 for the same purpose. In many parts of the U.S., the European brown trout has been introduced as a game fish, and it is suspected of contributing to the declining numbers of some native fish species, such as the golden trout of California.

Introduced plants can also harm native ones by producing and releasing chemicals. The African crystalline ice plant, for example, has devastated native coastal vegetation in California. The ice plant is an annual that accumulates salt, which leaches from its leaves when the plant dies at the end of the season. The salt, not surprisingly, suppresses the growth and germination of native plants in these well-used but fragile habitats. And introduced animals can simply attack native species, as does the South American fire ant, which arrived in Mobile, Alabama around 1940 and has since spread throughout the Southeast, replacing large parts of the native ant community.


Mating between some introduced and native species can lead to an extinction of the native species by replacing some of its genes. For example, mallard ducks introduced to the Hawaiian islands for hunting have hybridized extensively with the native, endangered Hawaiian duck, greatly complicating recovery plans for the latter species. On the U.S. mainland, mallards migrate to Florida in the winter. Although they formerly bred only while in the North, domesticated mallards released to the wild in Florida for hunting have bred with the native Florida mottled duck, whose existence may now be threatened by hybridization. Rainbow trout introduced into western watersheds as sport fish hybridize extensively with the Gila trout and the Apache trout--two species that are listed under the Endangered Species Act. The native Pecos pupfish in Texas has hybridized so massively with the sheepshead minnow, a widely used bait fish, that "pure" Pecos pupfish may no longer exist.

Plants can also fall prey to the same insidious phenomenon. An example is Lantana depressa, which is found on a few dune and limestone ridge habitats of peninsular Florida. It hybridizes with Lantana camara, the descendant of several Latin American or West Indian species that were brought to Europe as ornamentals in the seventeenth century, hybridized by horticulturists, and then introduced by the late 18th century into the New World. And in Texas, the Department of Transportation has hybridized the native white firewheel to extinction in its eagerness to beautify roadsides with Indian blanket--a plant that is not found naturally in the range of the firewheel.

Even if no genes are exchanged between the hybridizing species, the process can threaten the existence of one of them. Introduced brook trout are today displacing native bull trout in parts of the West. Although there is extensive hybridization between the two species, the hybrid offspring are sterile, so they cannot transmit brook trout genes back into the bull trout population. But the loss of productive mating opportunities by the rarer species, the bull trout, contributes to its displacement.

At least three of the twenty-four known "extinctions" of species listed under the Endangered Species Act have been wholly or partially caused by hybridization, and there seems to be no limit to other possible consequences. Johnson grass was originally introduced to the U.S. around 1800 as a forage crop for cattle and is now viewed as one of the worst weeds. Among other noxious traits, it hybridizes with cultivated sorghum to produce "shattercane," which is agriculturally worthless. Even worse outcomes are possible. For example, North American cordgrass, which entered England in the holds of ships in ballast soil, hybridized there with an innocuous native species to produce new plants that proved sterile. This might have ended the story, had not one of them undergone a mutation, yielding a new species that turned out to be a fertile, invasive weed.

The Geography of Introduced Species

All parts of the U.S. now host troublesome introduced species, but not all states are affected equally. Particularly hard hit are Hawaii and Florida, owing to their geographic location, mild climate, and their reliance on tourism and international trade. Biologically, neither state is what it was 100 or 200 years ago.

In Florida, about 25 percent of many plant and animal groups are not native but introduced by humans in the past 300 years, and millions of acres of land and water are dominated by invasive introduced species. In Hawaii, about 45 percent of plant species and 25 to 100 percent of species in various animal groups were similarly introduced. As a result, all parts of the Hawaiian islands except the upper slopes of mountains and a few protected tracts of lowland forest are today dominated by introduced species. Other such regions of the world, including New Zealand, are populated by similarly high fractions of introduced species.

Four features common to Florida and Hawaii account for their disproportionate number of introduced species. The first is their geographic insularity: Hawaii is an archipelago of islands, while the Florida peninsula is a "habitat island" bounded on three sides by water and on the fourth side by frost. A typical feature of islands is an impoverished native flora and fauna, relative to mainland areas of equal size.

Second, in both Florida and Hawaii land cover is dominated by human-produced habitats, such as agricultural and residential tracts. This is particularly so in the Hawaiian lowlands and throughout the southern part of Florida, which are the regions most affected by introduced species. Land under heavy agricultural and residential use is often unfavorable to native plant and animal species but is suitable for a variety of non- indigenous species.

Third, both states have large tropical or subtropical areas--particularly, once again, in the lowlands of Hawaii and the southern part of Florida--and the absence of freezing temperatures allows many species to survive. It is no accident that the northern limits of the ranges of many of Florida's introduced species are precisely where overnight freezes occur. Moreover, the tropical climate of the two states has encouraged the introduction and dissemination of an immense number of non-native species that are sold as ornamental plants or pets. The accidental escape of an exotic tropical ornamental such banana poka or Brazilian pepper, or of pets like bulbuls and tropical fish, would be innocuous in most regions of the U.S., because of the local climate. In Florida and Hawaii, they thrive and spread.

The fourth distinguishing feature of these two states is that they are hubs for transportation and popular destinations for tourists. Most visitors from Latin America enter through Miami, and many planes and ships from other regions land there as well. Fully 85 percent of all plant shipments into the U.S. pass through Miami; in 1990 the total was 333 million plants--over 1000 for every man, woman, and child in the city, had they been asked to help screen them. Hawaii is extremely popular with tourists and a center for both civilian and military traffic moving through the Pacific.

Obviously, the likelihood of inadvertent introduction of alien species is enormous in both states. The upshot is that in lowland Hawaii and southern Florida, large areas are now dominated by ecosystems comprised of species introduced from all over the world, interacting in new ways and often facilitating one another's existence. For example, several fig species imported into south Florida as ornamentals have become invasive because the wasps that pollinate them have independently immigrated into the country, and the resulting fruits may be dispersed by parrots introduced from various continents. Red-whiskered bulbuls from Asia roost in these trees and eat fruit of exotic plants such as Brazilian pepper, loquat, and ornamental jasmines, contributing to the spread of these introduced species. In the Hawaiian islands, feral pigs disperse many non-indigenous plants.

Other parts of the country are also under heavy assault by introduced species. Most of the sorts of problems that have arisen in Florida and Hawaii have cropped up in other states, and the local and regional economic and ecological costs may even exceed those of invaders in Florida and Hawaii. The invasion of leafy spurge in western rangelands was cited earlier. Purple loosestrife, a weed from Eurasia, now covers wetlands in much of the contiguous U.S. and is particularly troublesome in the Midwest, while the inter-mountain West has been radically transformed by invasions of Eurasian annual herbs, such as cheatgrass, and the tendency of hoofed livestock to interact synergistically with these plants. Although the percentage of introduced, exotic species in California is not as high as in Florida and Hawaii, large portions of the state, including grasslands and many dune systems, are dominated by exotic plants, and exotic fishes threaten many aquatic habitats.

Can the tide of invasions be stemmed?

Stemming The Tide: Keeping Them Out

The first line of defense against invasive exotics is not to allow them into the country in the first place. Organisms have always moved around the globe, but the volume of traffic has increased continuously and dramatically in the last three centuries with the increase in human travel and the movement of goods. There is no real national policy to stem the tide, despite President Carter's Executive Order 11987 of 1977, which simply forbids the import of any exotic species. Institutions and legislation to implement the order have not been forthcoming, while a host of interest groups-- some private, some governmental--have fought to introduce more species.

Existing regulations

Under the Lacey Act of 1900, the U.S. Fish and Wildlife Service (FWS) is ordered to restrict the entry of fish or wildlife that threaten humans, agriculture, horticulture, forestry, or wildlife. At present, however, the FWS views very few species as posing such threats, and, in any event, a certain number of prohibited species are smuggled in. The Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture prohibits the introduction of several other vertebrate species because they may carry pathogens that affect poultry or livestock, and the U.S. Public Health Service prohibits importation of other vertebrate species because they may carry pathogens that threaten human health. But APHIS inspections are often perfunctory, as anyone who has entered the continental U.S. on an airplane can readily confirm.

The Non-Indigenous Aquatic Nuisance Prevention and Control Act of 1990 authorizes FWS and the National Oceanic and Atmospheric Administration to regulate introductions of aquatic nuisance species such as the zebra mussel. Still, the Act has not been extensively implemented, and such wholesale introductions that occur, for example, with the routine purging of ballast water from ships remain virtually unstemmed. Present proposals to require at-sea (rather than in-port) pumping of ballast tanks would probably greatly lessen the influx of viable individuals of introduced species. APHIS also has the authority to prohibit the entry of plants, under the Federal Noxious Weed Act of 1974 and the 1939 Federal Seed Act, but it has been slow to exercise the authority and narrow in its interpretation of "noxious." Such documented scourges as purple loosestrife, Brazilian pepper, and Eurasian watermilfoil, for example, are not yet prohibited.

The troubles with introduced species do not always originate at the borders of the country: major problems often ensue when a species from a state in which it is native moves to another in which it is not naturally found. FWS has no authority to regulate interstate transport, beyond the enforcement of state laws that ban entry of specific species. In any event, FWS is underfunded and understaffed, so that such regulation takes a very low priority. APHIS has the authority to regulate interstate movement but exercises it only to prevent the movement of pathogens that might affect livestock or poultry.

Very generally, federal and most state agencies have adopted a policy of assuming that a species will pose no problems unless proven otherwise and, therefore, have not demanded that individuals wishing to import a plant or animal species demonstrate that its introduction will prove innocuous. Worse, many agencies actively promote the import and spread of invasive exotic species, such as multiflora rose and Russian olive, without any serious consideration of their potential consequences. Between 1935 and 1942 the U.S. Soil Conservation Service (SCS) grew 85 million kudzu seedlings, promoted them for erosion control, and paid farmers to plant them, with a result that has become legendary. State wildlife commissions fund expensive programs to import new game species with little serious consideration given to the damage they might inflict. In general, interstate movement of species, including fish and game animals, is viewed with even less concern than importation from outside the country. By accident or design, funding for the enforcement of the meager regulations that now govern the movement of exotics is woefully inadequate.

It is far more difficult to deal with introduced species once they are established than it is to keep them out, as will be discussed below. Yet current federal laws are only invoked after a species is already in the U.S.--in other words, too late. The 1993 OTA report and a recent paper by a team of biologists from the University of Washington both propose far more stringent regulations on what importation should be allowed, pointing to a policy of "guilty until proven innocent," rather than assuming that a species will be harmless. They further recommend that interstate transport should be much more carefully regulated. Whether any of these laws or recommendations become a reality must await the public will to do so, for a comprehensive approach to the problems caused by introduced species will require sufficient means to ensure that the greatly tightened regulations are enforced.

Eradicating Them

Trying to eradicate every single individual of a harmful introduced species is a seductive but controversial goal. One would like to eliminate ongoing, and sometimes increasing, damage, but the development of eradication technology may prove daunting, and a failed attempt may be exceedingly costly and invoke colossal damage to non-target species.

For example, the attempt to eradicate the introduced fire ant from southern states proved disastrous. In 1957, a well- meaning Congress authorized $2.4 million for the project, but the initial heptachlor applications caused wildlife and cattle deaths. Researchers next developed mirex bait, but the ant rapidly re-invaded areas from which it had once been eliminated, and mirex residues were discovered in many non- target organisms. These findings resulted in a ban by the Department of Interior on use of the insecticide on its lands. Registration of mirex was finally canceled by the EPA in 1977, but by then the costs of the applications had climbed to about $200 million and the range of fire ants had only expanded, dramatically, during the eradication campaign.

It is often said that eradication can be successful only if an unwanted species is detected early and while it lives within a restricted area. Eradication campaigns in Florida against the Asian citrus blackfly seem to confirm this view. The insect was found in Key West in 1934 and never left this island during a $200,000, three-year campaign in which a mixture of paraffin oil, whale oil, soap, and water was widely sprayed. The last blackfly was seen three years later, in 1937. The insularity of Key West was crucial, and other circumstances that greatly reduced traffic of humans and commodities between Key West and the mainland prevented the insect from spreading. The same species was discovered in 1976 in a much larger area centered on Fort Lauderdale. An eradication campaign was again mounted, but the area was too large and low-level infestations recurred. In 1979 the eradication effort was abandoned in favor of a more modest program of maintenance control, or "containment."

The case of the Giant Snail

The giant African snail, Achatina fulica, about three inches long, has been introduced widely in Asia, to islands in the Pacific and Indian Oceans, and recently to the West Indies. It is seen as a serious agricultural pest, and predatory snails such as Euglandina rosea that were introduced to attack it have only added to the problem by extinguishing many native snail species. It was, however, eradicated in Florida--although neither easily nor cheaply.

In 1966, a boy returning from Hawaii smuggled three of the snails into Miami, and his grandmother released them in her garden. Reproduction ensued, and in 1969 the Florida Division of Plant Industry (DPI) was alerted, leading to an immediate survey. The state Commissioner of Agriculture notified the news media about the giant snail, mailed over 150,000 copies of an attractive brochure, and called for public assistance in reporting and eliminating it. An area covering about forty-two city blocks was quarantined, but within days, a second infestation was discovered--in Hollywood, 25 miles north of Miami and well outside the initial quarantine zone.

The ensuing eradication campaign relied primarily on hand-picking, plus a granulated chemical bait. There were frequent surveys, and by 1971 in a six months period only forty-six snails were found-- compared to 17,000 in the previous sixteen months. In Hollywood, seventeen months after its initial infestation, only one adult snail was found. But less than a month after the effort seemed to have succeeded, a third infestation, probably three years old, was discovered three miles south of the original Miami site, with over 1,000 live snails on one block. The block was quarantined, and a large buffer zone was surveyed and treated. Nine months later, a fourth infestation, again about three years old, was found two miles north of the original one, followed by a fifth, about half a mile north of the initial infestation.

Although profoundly disappointed, the DPI persisted. By 1973, seven years after the three snails were brought into the city, more than 18,000 had been found, and many eggs. In the first half of that year, by contrast, only three snails were collected, in two sites. By April of 1975, no live specimens had been found for almost two years, and the campaign--which had cost over $1,000,000--was judged successful. Frequent surveys were continued for many months, along with the application of bait and chemical drenching. As a result, the giant African snail has not been found again, anywhere in the state.

Other attempts

Even widely distributed pest species have been eradicated. For example, the citrus canker, a bacterium, arrived in the Gulf states around 1910. By 1914 it had spread as far as Texas to attack grapefruit, orange, and other citrus trees. A strict quarantine, chemical controls, and a major educational effort to keep citrus workers from spreading the bacterium led in time to total eradication. By 1916, it had been reduced to mostly local outbreaks, and the last infestation was seen about thirty years later. No new infestations were found through 1984, when the effort to monitor the bacterium was effectively ended.

It is possible that new technologies of gene manipulation--in either a pest species or an organism that might attack it--could lead to the eradication of introduced pests, including those that seem immune to more traditional methods. However promising, the reliance of such genetic engineering projects on venture capital--plus the economic reality that, if such a method worked, there would be no continuing market for it--militate against this method, at least as genetic engineering is currently practiced in the U.S.. In Australia, where a somewhat different approach is taken, the national government now sponsors the genetic development of organisms to eradicate introduced rabbits and foxes.

Finally, eradication campaigns are occasionally beset by public disagreements over either the desirability of eliminating an introduced species or the proposed methods. The attempt by the state of California in the 1980s to remove exotic blue gum eucalyptus trees from Angel Island, a state park in San Francisco Bay, led to vociferous organized protest by those who found the trees attractive. The opponents were either unoffended by the fact that the species was an alien or unconvinced that, as the state had shown, the blue gum eucalyptus was a threat to native species. Attempts in recent years to eradicate non-native mountain goats from Olympic National Park in Washington and feral pigs from Nature Conservancy land in Hawaii have run into stiff opposition from animal rights activists--again, in the face of evidence that the introduced animals were harming natural ecosystems. Proposals to remove feral horses and burros in California and elsewhere have sparked similar responses.

Controlling Them

If eradication is impossible, the population of an introduced species can often be controlled in order to contain the level of economic and/or ecological damage. Such methods are called "maintenance control."


Chemicals, both insecticides and herbicides, are widely used for this purpose, although they must be carefully chosen. The massive detrimental effects to the environment and to human health of broad-spectrum pesticides such as DDT are widely known. Some of the newer pesticides, however, have far fewer side effects. For example, glyphosate-based herbicides have been used with some success to control melaleuca and Brazilian pepper in south Florida and cheatgrass in the West. The major drawbacks are their higher cost, the necessity of repeated application, and the fact that they affect plants other than the pest species for which they are intended.

Similarly, insecticides are now available that, when used properly, pose no hazards to human health. Some of these, such as those based on insect hormones, can minimize damage to non-target species. As with herbicides, the newer insecticides are expensive. Moreover, a potential problem with both herbicides and insecticides lies in the ability of targeted species to develop resistance against them. The pest can evolve physiological or behavioral modifications that remove some or all of the intended impact. For example, on Long Island, New York, the Colorado potato beetle has developed resistance to all major classes of insecticides.

Mechanical methods

Introduced pests can often be controlled mechanically. Water hyacinth, introduced from South America to Florida at the end of the 19th century, spread to cover more than 120,000 acres of public waters by 1960, smothering beds of submersed native vegetation and producing many other detrimental ecological effects. Coverage today is maintained at around 3,000 acres by a combination of mechanical harvesting and treatment with the herbicide 2,4-D. The key to this successful effort in maintenance control is to attack the plant continually and not to wait until particular infestations reach problem levels. A key component of a Nature Conservancy maintenance control project for yellow bush lupine--an invasive nitrogen-fixing species introduced to a northern California dune preserve--is periodic "bush bashes," in which supervised public volunteers rip out as many lupine bushes as they can find.

Biological control

Perhaps the major method of maintaining acceptable levels of introduced pest species is biological control, in which a natural enemy, typically from the intruder's native region, is introduced to keep the pest under control. The approach is a venerable one, for we know that green weaver ants were used by citrus growers in China to control caterpillars and beetles as early as AD 900. A strikingly successful American project in 1889 entailed importing the predatory Australian vedalia beetle to control the Australian cottony-cushion scale--a bug that was then devastating California citrus groves.

Some introduced pest plants can also be controlled at acceptable levels by introduced natural enemies. St. Johnswort, also known as Klamath weed, a native of Europe, Asia, and northern Africa, was introduced to Pennsylvania in 1793 and by 1900 had spread to California. By the mid-1940s, about five million acres of rangeland in North America had been rendered worthless for grazing. Two beetles, one from Eurasia, the other from Europe and northern Africa, were released at that time, and within ten years St. Johnswort had become an unimportant roadside weed, less than 1 percent as abundant as in the peak years of the 1940s. Microbial pathogens are also used. For example, Bacillus popilliae now plays a key role in controlling Japanese beetles in the eastern U.S.

Roughly one in five of all recent biological control projects have led to economically significant control of the target pest, and with virtually no continuing expenses. Yearly benefits from biological control programs in the U.S. are estimated to exceed $180 million. Still, the method is not without its dangers. An imported predator, herbivore, or parasitoid could potentially attack a non-target species, and several instances are known, particularly in earlier projects, in which biological control agents caused major damage.

Most examples of biological control projects gone awry involve the introduction of predators, often vertebrates, that attack a variety of prey species. The small Indian mongoose, for example, was introduced to the Hawaiian islands in 1883-1885 to control introduced rats in sugarcane fields. Its taste for prey of all kinds contributed to the decline of native birds in the Hawaiian archipelago, and especially ground-nesting species. The predatory snail Euglandina rosea, introduced to the Hawaiian and many other Pacific islands in a futile attempt to control the giant African snail, has devastated populations of several native Hawaiian snails. The mosquito fish (Gambusia affinis and G. holbrooki ) has been introduced worldwide for mosquito control, and through predation has brought about the decline of several native fish species. In Texas, mosquito fish have hybridized with a restricted native species and now threaten its genetic integrity.

The hazards in employing biological control are like those of fighting fire with fire. Biological control agents are, by choice, introduced species, and in early attempts, particularly, many of their ensuing ecological impacts would have been difficult to predict. Moreover, an introduced biological control cannot simply be recalled if it is found to have an unintended impact, as one can simply stop disseminating a chemical pesticide.

Today the professional biological control community has far more rigorous safety standards, especially for agents proposed to control pest plants. There is often extensive laboratory testing of the palatability of non-target species, such that generalized predators like the mongoose or Euglandina rosea would probably never be endorsed by the professionals who today deal in biological control. One that slipped by, more recently, is grass carp, which is often released for biological control of introduced aquatic weeds. This Asian fish devours native vegetation as well, to the detriment of native aquatic communities. Recently devised techniques allow the grass carp that are introduced to be genetically altered, as eggs, so that the fish are largely sterile. Controlling their subsequent reproduction in this way may lessen the potential damage from this generalized herbivore.

Nonetheless, there may still be major inherent risks in biological control. For one, there is often little monitoring after the release of a biological control agent to see just what it is doing, and this is particularly true when the introduced species feeds on insects. Insect species are not routinely monitored, and it would be difficult to know if a beneficial non- target insect was harmfully affected--particularly if it is far from the site of release of a parasitoid or predator. Second, biological controls, because they are living organisms with built-in mechanisms to disperse, can end up far from the intended theater of operations. The appearance of the cactus moth in the Florida Keys, more than thirty years after its release in the Lesser Antilles, is a good example. Lastly, biological control agents are sometimes released by groups or individuals acting on their own, without the benefit of professional counsel or review. Publicity regarding biological control can easily inspire such action. In Florida, homeowner associations and individual citizens have released the goldenhorn marisa snail in attempts to control hydrilla and water hyacinth infestations, even though the marisa feeds indiscriminately on many desirable native plant species and harms aquatic animals that use these plants as habitats.

Concluding Thoughts

Introduced plants, animals, and pathogens often pose an initially hidden but eventually monumental problem in the U.S.--or any other country whose doors are opened to traffic from other lands. Some costly invaders remained inconspicuous in this country for decades, and then spread swiftly. Their harmful effects are often subtle and surreptitious, but the eventual impacts on the economy or natural environment are no less real, and often disastrous and even irreversible, as when native species disappear.

On a general level, introduced species often interact with the destruction and fragmentation of habitat, the other major cause of our national conservation crisis. Areas of land that have been cleared are often colonized not by the plants that were removed but by introduced species. Moreover, in what is now a typical landscape of forested patches interspersed with farms and residential areas, the forests are much more easily invaded by exotic plants and animals than was the case when trees alone prevailed. Another major form of habitat destruction occurs when fire-prone introduced plants foster catastrophic fires that eliminate native communities of plants and animals.

The staggering economic implications of this tide of invaders are often unappreciated. For example, in the western rangelands alone, more than 16 million acres are heavily affected by alien plants--at great cost to cattle-raising--and 2,300 additional acres are invaded each day. The alien species are consistently much less nutritious than the native ones, if not totally unpalatable. Many of the ecological impacts cannot easily be given dollar values, but they are likely to be enormous. Of no inconsiderable value are the impacts on native songbird populations that have followed the introduction of the English or house sparrow in 1850, the starling in 1890, and the house finch in 1940.

Of the 632 species and subspecies recently officially listed as "endangered" under the Endangered Species Act, forty-five are primarily threatened by competition from introduced species. The key threats to approximately 100 species on the list are introduced species that prey or feed on them. Five are threatened chiefly by exotic diseases carried by introduced species. For 424 of the 632, habitat destruction is the main threat, but the nature of the threat for many of the plants that are included is the domination of their new habitat by introduced species. And a contributing threat for several of those listed as endangered is hybridization with introduced plants or animals.

Indeed, the great majority of introduced species do not cause problems of any sort. Most ornamental plants do not establish themselves outside gardens, and most species of discarded or escaped pets cannot survive in the wild. Of the minority of introduced species that do live for long outside human- dominated habitats, many are not invasive.

What is important is that some introduced species do create problems--a fraction that has been estimated as one in every seven that enter the country as aliens. And as we have seen, some of those problems are costly indeed. One would like to be able to declare in advance which introductions will be innocuous and which will prove disastrous. But ecologists and evolutionists, despite intensive research, are not always able to do this. Sometimes they can spot a potential problem quite accurately. At other times, they can only rationalize, post facto, the observed impacts, and learn from them. And they can occasionally point to classes of organisms--such as insects that attack a wide variety of plants--that have a particularly high probability of proving problematic. But there will always be exceptions to rules so generally drawn, and surprises, as from exotic plants or animals where the risk would have been seen as quite low.

Because of these inevitable uncertainties, it seems unconscionable that the introduction of species and their interstate transport are so minimally regulated. The complexity of ecological interactions, the history of past introductions, and the potential ecological and economic costs of new ones strongly support the recommendations that every proposed introduction be viewed as potentially problematic until substantial research suggests otherwise. This reversal of the present policy--or lack of one--would replace the present operating tool of "blacklists" of prohibited species with an approved "whitelist" of those species that have been studied intensively enough to allow a reasonable judgment that no harm is likely. By this more prudent method of control, only species on the whitelist could be introduced. There would still be occasional problems--whitelisted species may turn out to cause problems. But the number of problem introductions would be far lower than today.

The patchwork arrangement of federal and state laws and agencies that now regulate introductions of plants and animals came into being, over time, in response to isolated alarms and is wholly inadequate to address what is now known to be a broader and more pervasive problem. It is no match for the heavy commerce in today's smaller and ever more connected world. Needed is the designation of a single, lead agency, with a comprehensive mission and with overall regulatory authority, if we are truly to confront this biological invasion in a meaningful way. Plants, vertebrates, invertebrates, and pathogens all interact in synergistic and complex ways, and any alien that is released into the natural environment becomes a part of this interconnected web. We cannot hope to address the potential problems of any introduced species when, as is the case today, one organization regulates only certain plants, another monitors a limited set of microbes, another is in charge of selected game animals, and yet another governs mollusks.

Reviewed by James Drake, Richard Mack, and Harold Mooney

For Further Reading

Biological Invasions: A Global Perspective. Edited by J. A. Drake, H. A. Mooney, F. di Castri, R. H. Groves, F. J. Kruger, M. Rejmanek, and M. Williamson. Wiley, Chichester, U.K., 1989.

Biological Pollution: the Control and Impact of Invasive Exotic Species. Edited by B. N. McKnight. Indiana Academy of Science, Indianapolis, 1993.

Eradication of Exotic Pests. Edited by D. L. Dahlsten and R. Garcia. Yale University Press, New Haven, 1989.

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


Prof. James Drake is an ecologist in the Department of Ecology and Evolutionary Biology at the University of Tennessee in Knoxville. His interests are in understanding the causes of structure in ecological systems, and the study of nonlinear dynamics and complex systems.

Prof. Richard Mack is an ecologist on the faculty of the Department of Botany at Washington State University in Pullman and a member of the Invasive Species Specialist Group of the International Union for Conservation of Nature and Natural Resources. His research deals with the causes of plant naturalizations and invasions as examined through the population biology of alien species.

Prof. Harold Mooney is a plant ecologist in the Department of Biological Sciences at Stanford University, a member of the National Academy of Sciences, and a former chairman of the academy's Committee on Global Change Research. For many years he has led international research activities that deal with global environmental change, including a program on Ecology of Biological Invasions for the Scientific Committee on Problems of the Environment (SCOPE) and a new SCOPE program on A Global Strategy for Invasive Species.

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