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Nonindigenous Species—A Global Threat to Biodiversity and Stability

Daniel Simberloff
Department of Ecology and Evolutionary Biology, University of Tennessee, 569 Dabney Hall, Knoxville, TN 37996

The world's biota is being rapidly homogenized. This global change constitutes a major threat to biodiversity and to our ability to extract resources sustainably from many ecosystems. The threat was first recognized 50 years ago, but its extent is only now being realized as burgeoning tourism and unfettered international trade expand the opportunity for species to get from one region to another. In the past, a desired immigrant species or one furtively hitching a ride often had to survive a sea voyage of months. Now, over 280 million passengers use commercial airliners each year worldwide, as do millions of tons of cargo. The brown tree snake (Boiga irregularis) occasionally arrives in Honolulu in wheel wells and cargo bays of planes from Guam, where it has devastated forest birds after introduction from the Admiralty Islands (Rodda and others 1992). Similarly, mosquitoes arrive in Great Britain from Africa in airliner passenger cabins (Bright 1996), and the giant African snail (Achatina fulica), which has ravaged agriculture on many Pacific islands, was carried by a boy from Hawaii to Florida as a gift to his grandmother (Simberloff 1997a).

Of course, on every continent, many of the most venerated plants and animals were introduced intentionally. In many parts of the world, the major crop plants are almost all introduced, as are livestock. For example, of nine crop plants in the United States classified as “major” (USDA 1997), one (corn) is native and five were introduced from the Old World, one from the Andes, and two from Central America. Pets and ornamental plants are also usually of exotic origin. So what is the threat, exactly?

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Effects of Nonindigenous Species

The biggest threat posed by introduced species is the disruption of ecosystems, often by invasive plant species that replace the native species. The Australian tree Melaleuca quinquenervia, until recently increasing its range in southern Florida by more than 20 ha/day, replaces cypress and other native plants. It now covers about 200,000 ha, provides poor habitat for many native animals, affects the fire regime, and causes water loss (Schmitz and others 1997). South American water hyacinth (Eichhornia crassipes) now blankets many near-shore areas of Africa's Lake Victoria, blocking light and killing plants at the bottom of the food chain. The death and decay of plants that make up the water hyacinth mat remove still more oxygen from the water, and the major fisheries are in drastic decline. In addition to the ecological damage, water hyacinths are an economic nightmare, fouling engines and propellers of cargo ships and ferries, preventing docking, and clogging power-plant pipes and so causing numerous blackouts (McKinley 1996).

Introduced plants can also change an ecosystem without smothering the native plants. For example, on the island of Hawaii, the eastern Atlantic island shrub Myrica faya has invaded nitrogen-poor lava flows and ash deposits. A nitrogenfixer, it favors other introduced species over the native plants adapted to low nitrogen (Vitousek 1986). In much of the American West and in Hawaii, Old World grasses, such as cheatgrass (Bromus tectorum), increase the frequency and intensity of fires to the great detriment of native plants and the animals that use them (D'Antonio and Vitousek 1992; Macdonald and others 1989).

Entire marine ecosystems can be radically changed by the invasion of a single plant species. The Pacific seaweed Caulerpa taxifolia, released from the Oceanographic Museum of Monaco into the Mediterranean about 15 years ago, now covers over 4,000 ha and has locally smothered native seagrass beds that harbor many native animals (Boudouresque and others 1994; Simons 1997). Introduced red mangrove (Rhizophora mangle) trees from Florida on the coasts of the Hawaiian islands and Australian “pine” trees (Casuarina spp.) on the Florida coast have come to dominate their new homes, displacing native plants and animals (Schmitz and others 1997; Walsh 1967).

Just as an introduced plant can modify an ecosystem, a species that eliminates a plant can have a drastic effect. The Asian chestnut blight fungus (Cryphonectria parasitica), which arrived in New York City on nursery stock in the late 19th century, spread over about 100 million ha of the eastern United States in less than 50 years, destroying almost all chestnut trees (von Broembsen 1989). Chestnut had been the most common tree in many forests, making up one-fourth or more of the canopy trees, so the cascading ecosystem effects of this invasion were substantial. For example, several insect species that were host-specific to chestnuts were extinguished (Opler 1979); that chestnut leaves decompose faster than leaves of the oaks that largely replaced them suggests that the invasion greatly affected nutrient cycling (K. Cromack, Oregon State University, pers. comm.), although systematic data were not gathered. The North American pine wood nematode (Bursaphelenchus xylophilus) reached Japan in timber and spread

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among the islands, killing more than 10 million pine trees and affecting 25% of Japan's pine forests (von Broembsen 1989). The effects on other forest species must have been dramatic.

In addition to ecosystem effects, nonindigenous species have myriad effects on particular native species or groups of them. They can eat them, for example. The Nile perch (Lates niloticus), after introduction into Lake Victoria, eliminated many species of endemic cichlid fishes, which had undergone perhaps the greatest evolutionary radiation that scientists have studied (Goldschmidt 1996). Introduced rats (Rattus spp.) on many islands have destroyed at least 37 species and subspecies of island birds (Atkinson 1985; King 1985). The impact of the brown tree snake on the Guam avifauna is noted above. Introduced herbivores can similarly drive species to extinction, especially on islands where plants are less likely to have a refuge, an area that herbivores cannot reach. For example, goats introduced to St. Helena in 1513 almost certainly eliminated over 50 endemic plant species, although only seven were scientifically described before they disappeared (Groombridge 1992).

Introduced pathogens, often carried by introduced plants and animals, can also devastate native species. The chestnut blight was noted above. As another example, in the Hawaiian islands, the extensive introduction of Asian songbirds has brought avian pox and avian malaria, which have contributed to the decline and extinction of numerous native forest-bird species (van Riper and others 1986). The introduction into Africa of the virus rinderpest, native to India, in cattle in the 1890s led to the infection of many native ungulate species; mortality in some species reached 90%, and the distribution of some species is still affected by the virus (Dobson 1995).

Nonindigenous species can compete with native ones, although competition for resources is often difficult to demonstrate. Some well-studied examples provide good evidence. The house gecko (Hemidactylus frenatus) has invaded many Pacific islands; this has led to drastic declines in the population of some native gecko species. Experiments suggest that at least one of the natives, Lepidodactylus lugubris, avoids the larger house gecko, thereby suffering food shortage (Petren and others 1993), and that the invader depletes the insect food base sufficiently to reduce the food available for the native (Petren and Case 1996). The continuing replacement in the United Kingdom of the native red squirrel (Sciurus vulgaris) with the introduced American gray squirrel (S. carolinensis) is now attributed largely to the greater foraging efficiency of the invader and concomitant lowering of food available to the native (Williamson 1996).

Many instances are known in which introduced species affect native ones by interfering with them directly rather than indirectly through resource depletion. The South American fire ant (Solenopsis invicta), which has spread throughout the southeastern United States, attacks individuals of native ant species and is replacing the latter in many habitats (Tschinkel 1993). In a plant analogue of aggression, the African crystalline ice plant (Mesembryanthemum crystallinum) accumulates salt, which remains in the soil when the plant decomposes. In California, this plant thus excludes native plants that are intolerant of such salty soil (Vivrette and Muller 1977).

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Nonindigenous species also eliminate native species by mating with them; this threat is especially strong if the native species is much less numerous than the introduced one. For example, the New Zealand gray duck (Anas superciliosa superciliosa) and the Hawaiian duck (A. wyvilliana) are threatened with a sort of genetic extinction because of rampant hybridization and introgression with the introduced North American mallard (A. Platyrhynchos) (Rhymer and Simberloff 1996). Likewise, Europe's rarest duck, the white-headed duck (Oxyura leucocephala), is threatened in its last redoubt in Spain by hybridization and introgression with North American ruddy ducks (O. jamaicensis), which were introduced into England as an amenity, escaped, and made their way to Spain (Rhymer and Simberloff 1996). This sort of threat is far more common in regions that exchange closely related species (such as Europe and North America) than in those whose species are so distantly related that they are unlikely to be able to mate and exchange genes (such as Australia and either Europe or North America). A native species can be threatened by hybridization with an introduced one even if no genes are exchanged, simply by the reproductive reduction effected by fruitless matings. Females of the endangered European mink (Mustela lutreola) mate with male introduced American mink (M. vision); although the embryos are aborted, the loss of reproduction by the European mink exacerbates their population decline (Rozhnov 1993).

Slowing the Flow

The first line of defense against nonindigenous species is to keep them from being introduced. There are both practical and legal impediments to doing so. The sheer volume of tourism and trade dictates that inspection is destined to miss many inadvertent immigrants. Agricultural pests insinuate themselves into foodstuffs, woodboring beetles into timber, rodents into cargo containers—virtually any product shipped in bulk can carry many hitchhikers. Routine purging of ship's ballast water has released hundreds of nonindigenous species in waters throughout the world (Carlton and Geller 1993). Tourists can easily import species inadvertently in baggage, even if they heed warnings about which items are the most likely carriers of immigrants. In 1990, about 333 million nonindigenous plants were imported into the United States through Miami International Airport alone (OTA 1993). Economic resources are insufficient to examine everything that crosses a nation's borders.

Furthermore, liberalization of trade through such treaties as the General Agreement on Tariffs and Trade (GATT) and the North American Free Trade Agreement (NAFTA) is bound to increase the flow of nonindigenous species, and not only as a result of the increased volume. Under GATT and NAFTA, restrictions claimed as environmental measures can be challenged on the grounds that they are protectionist. The relevant regulatory authority must then adjudicate the dispute. Aside from the overwhelming appeal of free trade, both GATT and NAFTA require that species exclusions be based on risk assessments. However, risk-assessment procedures for introduced species are in their infancy and

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do not appear to be scientific, often resting on undefended judgments by experts and on arbitrary algorithms for combining risks (Simberloff and Alexander 1998). Furthermore, these risk assessments are expensive; one conducted by the US Department of Agriculture (USDA) on risks associated with importing larch from Siberia into the United States (USDA 1991) cost $500,000 (Jenkins 1996). It is difficult to imagine finding funding sources sufficient to mount risk assessments for all the challenges that might appear even to an educated layperson to be justified on prima facie grounds.

Virtually every specialist in invasion biology who has examined the matter concludes that aspects of the ecological impact of a nonindigenous species are inherently unpredictable (for example, Hobbs and Humphries 1995), and many scientists argue that every species should be considered a potential threat to biodiversity and sustainability if it were to be introduced (for example, Ruesink and others 1995). That implies that every species proposed for deliberate introduction, whether or not it appears superficially to be innocuous, necessitates some formal risk assessment. The cost would be staggering if the USDA process (USDA 1991) were the model.

In addition, many parties introduce species not inadvertently, but deliberately. These range from the boy smuggling giant African snails to his grandmother, who released them in her yard in Miami (Simberloff 1997a), to such large industries as the pet and ornamental-pet trades, which lobby vigorously against many restrictions. In the United States, recommendations that all species proposed for introduction must be on “white lists”—lists of species whose invasive potential has been assessed and has been approved for introduction—have been systematically attacked by those interest groups. Rather, the major laws that restrict entry of species use “black lists”lists of species that have already been shown to be damaging or are strongly suspected of being dangerous; a species is prohibited only if it is on such a list (Schmitz and Simberloff 1997). Rarely is blacklisting forward-looking.

Thus, there will always be a flow of nonindigenous species. However, the flow can be lessened. Undoubtedly, increased public education as to the risks would lead to fewer deliberate and inadvertent introductions as people strive to be good environmental citizens. The Convention on Biological Diversity mandates that its signatories “as far as possible and as appropriate . . .prevent the introduction of . . .those alien species which threaten ecosystems, habitats, or species.” Bean (1996) suggests that this statement reflects widespread recognition that nations are obliged to attempt to prevent introductions, and he cites as an example New Zealand's 1993 Biosecurity Act, which subjects all incoming persons and goods to rigorous inspection and prevents the importation of any species not already cleared by government authorities for inclusion on a white list. He also notes the increasing international and national regulation of purging of ballast water and points out that the considerable legal framework and effort that many nations use to attempt to prevent agriculturally harmful introductions could be adapted and expanded to prevent ecologically harmful ones. The problem is educating the public sufficiently that they demand regulation of nonindigenous species.

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Managing Nonindigenous Species

Once a species enters a new region, there are several options for managing it. The most obvious one is to attempt to eradicate it. This approach is often feasible if the invasion is recognized and targeted early enough (Simberloff 1997a), but several factors militate against its success. Perhaps foremost, almost no countries have an early-warning system in place that is charged with determining when an invasion has occurred, much less a procedure to generate a rapid, coordinated response while the invasion is still restricted geographically. The reaction is usually only after an invasion has existed for so long that it has become noticeable, and by then eradication is often impossible (Schmitz and Simberloff 1997). Second, for species deliberately introduced, the same forces that conspired to allow introduction in the first place act to prevent eradication. In addition, many invasions appear innocuous for long periods (Crooks and Soulé 1996; Williamson 1996); by the time they are recognized as ecologically or economically damaging, they are so widespread that they cannot be eradicated.

Minimizing ecological and economic damage if eradication proves impossible is usually attempted by one or more of three routes (Simberloff 1996): chemical, mechanical, and biological control. The environmental and human health effects of broad-spectrum pesticides are legendary. Although some newer chemicals have far fewer side effects, their high cost and the necessity of repeated application and the frequent evolution of resistance by the target pest have led to great interest in alternative methods. Also, if pesticides were used to prevent damage by introduced species both to vast areas of natural habitats and to agriculture, all the above problems would be exacerbated.

Mechanical methods, either alone or in concert with pesticides, are sometimes feasible. For example, water hyacinth has been successfully controlled in Florida for over 20 years by a combination of mechanical harvesting and treatment with the herbicide 2,4-D (Schardt 1997). However, mechanical devices are often expensive and would be less likely to work on widespread invasions.

Biological control—the introduction of a natural enemy of the pest—has seemed an extremely attractive alternative to chemical and mechanical control on both ecological and economic grounds. Many biological-control projects have provided continuing suppression of a pest to acceptably low levels with the sole costs being those of the initial exploration to find natural enemies and the testing for efficacy and safety. Odour (1996) cites the control of water hyacinth in Sudan by three South American insects, of prickly pear cactus (Opuntia inermis and O. stricta) in Australia by the moth Cactoblastis cactorum from Argentina, and of the South American cassava mealybug (Phenacoccus manihoti) in Africa by a South American encyrtid wasp. In each instance, the natural enemies maintain populations in perpetuity without further human intercession.

More recently, biological control has been subjected to critical scrutiny on the grounds that nontarget species, some of conservation concern, have been attacked and even driven to extinction (Howarth 1991; Simberloff 1992). Early biological control projects using vertebrates, such as the small Indian mongoose or the cane toad, and the widespread dissemination of the New World predatory snail

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Euglandina rosea to control the giant African snail were disastrous, and biological control professionals now eschew the use of vertebrates, except for fishes. However, insects tested for host specificity have also attacked nontarget species. For example, the Eurasian weevil Rhinocyllus conicus, introduced into North America to control musk thistle (Carduus nutans), is now attacking native nonpest thistles, including narrowly restricted endemic species in nature reserves (Louda and others 1997). Although the extent of such problems is controversial, the fact that biological control agents can both disperse and evolve, just as any other introduced species can, suggests great caution in their use and extensive preliminary testing before their release.

Action Needed Now

Burgeoning international interest in invasive nonindigenous species has led to several international meetings (for example, Sandlund and others 1996), new monographs (for example, Williamson 1996; Simberloff and others 1997), increased news coverage (for example, McKinley 1996; Simons 1997), and widespread appeals for action (for example, Glowka and de Klemm 1996). Nevertheless, there is no evidence that the flow of exotics is decelerating under the pressures of increased trade and tourism described above. What else must be done?

Glowka and de Klemm (1996) feel that inclusion of nonindigenous species as a priority item for the Conference of Parties to the Convention on Biological Diversity, which has been ratified by 172 nations, is necessary to prevent a fragmented approach to the problem. Schmitz and Simberloff (1997) see the effort in the United States as also bedeviled by fragmentation. In short, as long as one program deals with aquatic plants, another aquatic animals, another agricultural weeds, and yet another bird introductions, the effort is bound to be frustrated if only because species often interact synergistically to generate an environmental or economic problem (Simberloff 1997b). Furthermore, because nonindigenous species do not recognize political boundaries, both regulatory and management responsibility must also cross for them to be effective. Thus, the Convention on Biological Diversity, an international instrument, is highly appropriate as one locus of action. It is important to observe, however, that, even if no species were henceforth able to cross a national border, introduced species would still be a major problem. In the United States, for example, interstate movement of introduced species has the same effect as importing such species from other countries: ecosystems are subjected to invasion and disruption by species that have evolved elsewhere. And, within-country transport can threaten invasion of neighboring countries.

A major current lacuna is a comprehensive database on introduced species that is associated with an early-warning system and a rapid-response team. For most taxa in most countries, someone who finds a species suspected to be nonindigenous and potentially invasive has nowhere to turn to examine this possibility. There is no emergency telephone number to use to determine whether it is a newly recorded species or a species that is spreading after introduction. Even if

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there were an organization charged with receiving such queries, there is no list of species to which it could turn to give an answer. For most species, there is no systematic effort to record where they have been introduced or their suspected effects. And there is rarely a procedure in place to respond rapidly to a newly recorded invasion, partly because of the fragmentation of authority described above.

The white-list approach advocated by Ruesink and others (1995) and Wade (1995) and discussed above needs to be adopted in some form both nationally and internationally. Black lists have never worked well, and the inherent unpredictability and idiosyncrasy of introductions dictate that all potential introductions be subjected to scrutiny—with no blanket exceptions. That requirement, of course, would mean that funding would be needed to process applications and to give them the necessary attention. Whether the costs of white listing are borne by the party wishing to import a species or by society as a whole will have to be addressed. For that matter, so will the costs of an unforeseen disaster if a white-listed species turned out not to be innocuous. Should an applicant be required to post a bond? Should an applicant be able to be indemnified by purchasing disaster insurance? Should society as a whole bear the cost? These matters have barely been broached.

How a species proposed for introduction should be assessed is yet another crucial issue that has been at best cursorily considered. As noted above, standard risk-assessment procedures for chemical and physical stressors do not appear to work well for biological introductions, for which the probabilities of such events as evolution and long-distance dispersal are so difficult to evaluate as to be mere guesses (Simberloff and Alexander 1998). The concatenation of guesses and arbitrary assignment of risk categories that pervades the current USDA risk-assessment procedure (see, for example, USDA 1991) hardly seems scientific, but no general alternative has been widely considered (O'Brien 1994). Having agreed that risk assessment will be the appropriate procedure to adjudicate disputes, we must determine how to do risk assessments en masse for nonindigenous species.

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