Biological Invasions: A Growing Threat
An army of invasive plant and animal species is overrunning the United States, causing incalcuable economic and ecological costs.
To the untrained eye, Everglades National Park and nearby protected areas in Florida appear wild and natural. Yet within such public lands, foreign plant and animal species are rapidly degrading these unique ecosystems. Invasive exotic species destroy ecosystems as surely as chemical pollution or human population growth with associated development.
In July 1996, the United Nations Conference on Alien Species identified invasive species as a serious global threat to biological diversity. Then in April 1997, more than 500 scientists called for the formation of a presidential commission to recommend new strategies to prevent and manage invasions by harmful exotic species in the United States.
Already, many states attempt to maintain their biological heritage, and a number of state and federal regulations restrict harmful species. Unfortunately, for a variety of reasons, such tactics have failed. Without greatly increased awareness and coordinated efforts, the devastating damages will continue.
Exotic species have contributed to the decline of 42 percent of U.S. endangered and threatened species. At least 3 of the 24 known extinctions of species listed under the Endangered Species Act were wholly or partially caused by hybridization between closely related exotic and native species. After habitat destruction, introduced species are the second greatest cause of species endangerment and decline worldwide-far exceeding all forms of harvest. As Harvard University biologist E. O. Wilson put it, "Extinction by habitat destruction is like death in an automobile accident: easy to see and assess. Extinction by the invasion of exotic species is like death by disease: gradual, insidious, requiring scientific methods to diagnose."
The cost of inaction
According to a 1993 report by the (now defunct) congressional Office of Technology Assessment (OTA), lack of legislative and public concern about the harm these invasions cause costs the United States hundreds of millions, if not billions, of dollars annually. This includes higher agricultural prices, loss of recreational use of public lands and waterways, and even major human health consequences. About a fourth of U.S. agricultural gross national product is lost to foreign plant invaders and the costs of controlling them. For example, leafy spurge, an unpalatable European plant invading Western rangelands, caused losses of $110 million in 1990. Such losses are likely to increase. Foreign weeds spread on Bureau of Land Management lands at over 2,300 acres per day and on all Western public lands at twice that rate.
Other effects on private land are more obvious. The spread of fire-adapted exotic plants that burn easily increases the frequency and severity of fires, to the detriment of property, human safety, and native flora and fauna. In 1991, in the hills overlooking Oakland and Berkeley, California, a 1,700-acre fire propagated by Eucalyptus trees planted early in this century destroyed 3,400 houses and killed 23 people.
Over the past two centuries, human population growth has substantially altered waterways and what remains of the natural landscape. Once contiguous across the entire United States, wetland and upland ecosystems are often mere remnants that are now being degraded and diminished by nonindigenous species invasions. This exacerbates the problem of conserving what remains of our country's biological heritage.
At the same time, nonindigenous crops and livestock, including soybeans, wheat, and cattle, form the foundation of U.S. agriculture, and other exotic species play key roles in the pet and nursery industries and in biological control efforts. Classifying a species as beneficial or harmful is not always simple; some are both. For example, many imported ornamental plants are used in manicured landscapes around our homes. On the other hand, about 10 percent of these same species have escaped human cultivation, some with devastating ecological or economic results.
Scientists wake up
Until the past decade or so, conservationists were often complacent about nonindigenous species. Many shared the views of Charles Elton in his 1958 book The Ecology of Invasions of Plants and Animals, which introduced generations of biologists to invasion problems. He contended that disturbed habitats, because they have fewer or less vigorous species, pose less "biotic resistance" to new arrivals. Conservationists now realize that nonindigenous invaders threaten even species-rich pristine habitats. The rapidly increasing conservation and economic problems generated by these invasions have resulted in an explosion of interest and concern among scientists.
In the United States, invasive plants that constitute new habitats and dramatically alter a landscape or water body have some of the greatest impacts on ecosystems. On land, this could be the production of a forest where none had existed before. For example, sawgrass dominates large regions of Florida Conservation Area marshes, providing habitat for unique Everglades wildlife. Although sawgrass may be more than 9 feet tall, introduced Australian melaleuca trees are typically 70 feet tall and outcompete marsh plants for sunlight. As melaleuca trees invade and form dense monospecific stands, soil elevations increase because of undecomposed leaf litter that forms tree islands and inhibits normal water flow. Wildlife associated with sawgrass marshes declines. The frequency and intensity of fires change, as do other critical ecosystem processes. The spread of melaleuca and other invasive exotic plants in southern Florida could undermine the $1.5-billion effort to return the Everglades to a more natural state.
Throughout the world, such invasions threaten biodiversity. In Australia, invasion by Scotch broom led to the disappearance of a diverse set of native reptiles and to major alteration of the composition of bird species. On the island of Hawaii, the tall Atlantic shrub Myrica faya has invaded young, nitrogen-poor lava flows and ash deposits on the slopes of Mauna Loa and Mauna Kea. Because it fixes nitrogen, it inhibits colonization by native plants, favoring other exotic species.
Plant communities offering little forage value ultimately lower wildlife abundance or alter species composition. Invading plant species often exclude entire suites of native plants but are themselves unpalatable to native insects and other animals. Two Eurasian plants-spotted knapweed, which infests 7 million acres in nine states and two Canadian provinces; and leafy spurge, which occupies 1.8 million acres in Montana and North Dakota alone-provide poor forage for elk and deer. Likewise in Florida, the prickly tropical soda apple from Brazil and Argentina excludes native palatable species. Losses to the local cattle industry are over $10 million per year, or about 1 percent of gross revenues.
Bird, reptile, and amphibian invasions may also devastate individual native species but generally do not cause as much damage as exotic plants. Herbivorous mammals and insects are often far more troublesome. In the Great Smoky Mountains National Park, feral pigs descended from a few that escaped from hunting enclosures in 1920 devastated local plant communities by selectively feeding on plants with starchy bulbs, tubers, and rhizomes and by greatly changing soil characteristics. In parts of the southern Appalachians, two related insects, the hemlock woolly adelgid and the balsam woolly adelgid, defoliate and kill dominant native trees over vast tracts. Host trees have not evolved genetic resistance, and native predators and parasites of the insects are ineffective at slowing their advance.
The zebra mussel from the former Soviet Union has clogged the water pipes of many electric companies and other industries, particularly in midwestern and mid-Atlantic states. It also threatens the existence of many endemic native bivalve molluscs in the Mississippi Basin. Infestations in the midwest and northeast cost power plants and industrial facilities nearly $70 million between 1989 and 1995.
Death by disease
Introduced animal populations can also harm their native counterparts by competing with them, preying on them, and propagating diseases. For example, a battery of introduced Asian songbirds are host to avian pox and avian malaria in the Hawaiian Islands; native birds are especially susceptible. Introduced species can also gradually replace native species by mating with them, leading to a sort of genetic extinction.
Pathogens are among the most damaging invaders. Plant pathogens can change an entire ecosystem just as an introduced plant can. The chestnut blight fungus, which arrived in New York City in the late 19th century from Asia, spread in less than 50 years over 225 million acres of the eastern United States, 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 always obvious. Several insect species restricted to chestnut are now extinct or endangered.
We have no precise figures on the enormous costs of introduced pathogens and parasites to the health of humans and of economically important species. One such invader is the Asian tiger mosquito, introduced from Japan in the mid-1980s and now spreading in many regions, breeding in stagnant water left in discarded tires and backyard items. It attacks more hosts than any other mosquito, including many mammals, birds, and reptiles. It is a vector for various forms of encephalitis, including the La Crosse variety, which infects chipmunks and squirrels, and the human diseases yellow fever and dengue fever.
Almost every ecosystem in the United States contains nonindigenous flora and fauna. Particularly hard hit are Hawaii and Florida because of their geographic location, mild climate, and reliance on tourism and international trade. In Florida, about 25 percent of plant and animal groups were introduced by humans in the past 300 years, and millions of acres of land and water are infested by invaders. In Hawaii, about 45 percent of plant species and 25 to 100 percent of species in various animal groups are 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 dominated by introduced species.
In western states, invasions have harmed native plant diversity and the production capability of grazing lands. Although the percentage of introduced 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. All regions of the United States are under assault.
Damage by exotic species is often best documented on public lands and waterways because taxpayers' dollars are used for management. However, the problem is at least as pronounced on private properties. The Nature Conservancy, which operates the largest private U.S. reserve system, views nonindigenous plants and animals as the greatest threats to the species and communities its reserves protect. It can ill afford the increasing time and resources that introduced-species problems cost, and the progress it makes on its own properties is almost always threatened by reinvasion from surrounding lands.
The 1993 OTA report concluded that the federal framework is largely an uncoordinated patchwork of laws, regulations, policies, and programs and, in general, does not solve the problems at hand. Federal programs include restricting entry of harmful species, limiting their movement among states, and controlling or eradicating introduced species
Most of the federal money goes toward efforts to keep foreign species out of the United States. The U.S. Department of Agriculture (USDA) spent at least $100 million in FY 1992 for agricultural quarantine and port inspection. However, most of this effort is aimed at preventing the introduction of agricultural diseases and disease vectors. Moreover, federal efforts to prevent introduction fail because entry is denied only after a species is established or known to cause economic or environmental damage elsewhere.
The Federal Noxious Weed Act of 1974 and the Lacey Act of 1900-the two major laws that restrict entry of nonindigenous species-use blacklists. That is, they permit a species to be imported until it is declared undesirable. Excluding a plant species requires its addition to the Federal Noxious Weed list, a time-consuming process with no guarantee of success. It took more than five years to list the Australian melaleuca tree, and that happened only with the support of the entire Florida congressional delegation. At least 250 weeds meeting the Federal Noxious Weed Act's definition of a noxious weed remain unlisted. In addition, USDA's Animal and Plant Health Inspection Service (APHIS) simply failed to act on listings for years, wishing to avoid controversy and research effort. Now there is interest within APHIS in listing noxious weeds, but the agency lacks the necessary staff and funds to conduct the risk assessments needed to justify a listing.
In 1973, a "white" or "clean" list approach was proposed for the Lacey Act. Importing a species would be legal only if it posed a low risk. However, in 1976, the U.S. Department of the Interior abandoned the plan under pressure from pet-trade enthusiasts and parts of the scientific community. The pet trade did not want to assume the burden of demonstrating harmlessness and particularly feared loss of income from new tropical fish. Some scientists thought the approach might exclude certain zoo and research animals even though the proposal specifically allowed permits for scientific, educational, or medical purposes.
Listing a species on a black or white list can also be scientifically challenging. If a suspected harmful species has not received the necessary taxonomic research to distinguish it from closely related species, especially native ones, the process can be difficult at best. Overall, the Lacey and Federal Noxious Weed acts fail to prevent the interstate shipment of listed species and are only marginally effective in preventing new invasions.
Because Americans demand new exotic plants and animals for aquariums, homes, gardens, and cultivated landscapes, the pet and ornamental plant industries wield enormous political influence at federal and state levels. A 1977 executive order issued by President Carter instructed all federal agencies to restrict introductions of exotic species into U.S. ecosystems and to encourage state and local governments, along with private citizens, to prevent such introductions. The U.S. Fish and Wildlife Service was to lead in drafting federal regulations. When attempts to implement this order met with strong opposition from agriculture, the pet trade, and other special interest groups, the formal regulatory effort was largely abandoned.
Even when states take the lead in attempting to prohibit harmful exotic species, special interest groups have effectively undermined this effort. Recently, the pet industry essentially blackmailed the Colorado Division of Wildlife into grandfathering an extensive list of exotic species from future regulations. The threat was legislative action that could strip the division of its authority, such as shifting its function to the Colorado Department of Agriculture.
Because of the political power of vested interests, federal and most state agencies use blacklists and do not demand that importers of plants and animals demonstrate that an introduction will prove innocuous. White lists are also problematic because it is extremely difficult to determine if a species will become invasive in any given locale. The precise reasons why some species become invasive and disruptive are usually unknown. Occasionally, there is a long time lag between introduction and when a species becomes troublesome. Brazilian pepper, for example, introduced during the 19th century, became noticeable in south and central Florida only in the early 1960s, but it is now a widespread scourge. Long time lags may be related to factors such as unnoticed population growth, with some sites acting as staging areas for long periods of time; habitat change, rendering waterways and landscapes more prone to invasions; and even genetic mutations, adapting a species to previously inimical local conditions. Synergism between species can also account for long time lags. Several fig species imported as landscape ornamentals into southern Florida during the 1920s have now become invasive because their host-specific fig wasps have independently emigrated, and their seeds are dispersed by introduced parrots.
Worse, many state and federal agencies are schizophrenic about exotic species. Not only do they have control programs aimed at harmful invaders, they also actively promote the import and spread of potentially invasive exotic species, while giving the potential long-term consequences only minimal consideration. Probably the best example of agency promotion of potentially harmful exotic species is USDA's Natural Resources Conservation Service, formerly the U.S. Soil Conservation Service, which has a policy of introducing nonindigenous plant species suitable for erosion control. During the 1930s, the agency distributed approximately 85 million kudzu seedlings to southern landowners for land revitalization. By the 1950s, kudzu was a nuisance species, and by 1991, it infested almost 7 million acres in the region. After this disaster, the agency modified its policy and now provides general guidance to its 20 U.S. plant-material centers on testing species for toxicity and for their propensity to become agricultural pests. Still, current review processes fail to screen out potential environmental pests. At least 7 of the 22 nonindigenous plant species released between 1980 and 1990 had invasion potential.
Even when invasive exotic species are federally listed and found in the United States, federal control efforts are often virtually nonexistent. For example, for FY 1998 APHIS has a budget of only $408,000 ($325,000 after overhead and administrative costs) for survey and control efforts for 45 noxious weed species. Similarly, the National Park Service has only $2 million to remove invasive species from its parks this year, despite $20 million in management needs identified by its biologists. Federal agencies' failure to manage harmful species on their lands can have long-term impacts on abutting state, local, and private lands and can undermine state programs to manage invaders.
Eradicate or control?
Because invasive species do not respect jurisdictional boundary lines, efforts to eradicate or limit them usually require an enormous degree of cooperation among federal, state, and local government agencies as well as the participation of private interests and broad public support. Eradication of plants, insects, and other vertebrate and invertebrate animals is often feasible, particularly early in an invasion. For example, the Asian citrus blackfly was found on Key West, Florida, in 1934 and was restricted to the island during a successful $200,000, three-year eradication effort. The insularity of Key West was a crucial factor in preventing the fly's rapid spread. However, in 1976, this same species was discovered in a much larger area centered in Fort Lauderdale. This time eradication did not work; the area infested was too large, and low-level infestations recurred. In 1979, a more modest program of maintenance control or containment replaced eradication. This approach is often the only practical way to limit ecological or economic damage when eradication fails.
However, eradication and even maintenance control often require strong political will. Eradication and control activities that employ insecticides, herbicides, and poisons must be shown not to harm nontarget organisms and humans, and normal scientific standards of proof may not suffice with large elements of the public. Of course, the use of any pesticide today can be controversial.
Pesticides have successfully controlled some invaders, such as melaleuca in Florida and European cheatgrass in the West. However, pesticides are generally expensive, and many organisms evolve resistance to them. Some introduced species can be controlled mechanically, and some, such as water hyacinth, by a combination of herbicide and mechanical harvesters. With enough volunteers or cheap labor, handpicking or hunting can sometimes maintain animals and plants at acceptably low levels, at least locally.
Probably the main method of maintaining acceptable levels of introduced pest plants and animals is biological control: the introduction of a natural enemy (predator, parasite, or disease), often from the pest's native range. Many biological control programs have achieved permanent low-level control of agricultural pests, and yearly benefits in the United States are around $180 million. However, a biological control agent is also an introduced species, and many survive without controlling the target pest. Whether or not they exert the desired control, some may attack nontarget organisms. In several instances, rare nontarget species have been attacked, and inadvertent extinction may even be attributed to some biological control projects. For example, a cactus moth introduced in 1957 in the Lesser Antilles to control a pest cactus island-hopped to Florida, where it nearly destroyed the desirable semaphore cactus.
Some estimates for insects introduced to control other insects are that 30 percent establish populations, but only a third of these effectively control the targets. For insects introduced to control weeds, about 60 percent establish populations, but again only a third control the target plant. Currently, there is insufficient monitoring to know the impacts of these surviving biological control agents on native species, but it is almost certain that once they are established they cannot be eradicated.
Because of the various problems with the different methods of control and their economic and potential political costs, Congress and state legislatures have resisted creating programs with broad authority to control invasive nonindigenous species. A good example is the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990, which was reauthorized and broadened in 1996. It establishes substantial hurdles that control programs must overcome, including the need to cooperate with other interested or affected parties. The zebra mussel invasion in the Great Lakes spawned this act, and it is really the first federal legislative effort that is specifically designed to prevent, monitor, conduct research on, and manage invasive nonindigenous species in natural areas.
Cooperation is usually needed for successful prevention and control. However, agencies are notoriously jealous of their programs. They may not participate in or may even object to initiatives by others because of policy or resource impact concerns, or just because of the personalities involved. When chemical control is proposed, concerns about human health and the effects on nontarget organisms can quickly derail a program. Also, the ecological impacts of a nonindigenous species, especially if recently introduced, are usually incompletely understood or are a matter of scientific debate. This lack of knowledge can prevent agencies from responding quickly to eradicate or contain an invader. For example, the ruffe, a small perch-like European fish, became the most abundant fish species in Duluth/Superior Harbor since its discovery there in 1986. A program to prevent its spread eastward along the south shore of Lake Superior called for annually treating several streams flowing into the lake with a lampricide. Cooperation between various agencies foundered at the last moment because of turf issues, environmental concerns, and limited information about effects, and the ruffe is now expected to expand its range and become established in the warmer, shallower waters of Lake Erie. There it will probably negatively affect important fisheries such as that of the native yellow perch.
Aggressive state action
To control and manage such invasions, states must adopt rigorous white lists, despite the difficulties of doing so. Every proposed introduction must receive the scrutiny currently reserved for species known to have caused harm elsewhere. The literature and databases on introduced species are not sufficiently developed to allow state officials to determine easily whether a species has been problematic elsewhere; this fact alone dooms blacklists to failure. Further, evidence that a species is not problematic elsewhere is no proof that it will not cause damage. The Indian mynah bird is a pest in the Hawaiian islands, where it feeds on crop plants, is a vector for parasites of other birds, and spreads the pestiferous weed lantana. In New Zealand, it is equally well established but not seen as a serious pest. However, the fact that a species need not have the same impact wherever it is introduced can serve to make white lists less onerous. A plant that cannot overwinter in northern states, for example, might be white-listed there as long as federal or state restrictions on its shipment exclude it from states where it could be invasive.
A second major generic problem with state approaches to biological invasions is the lack of a coordinated rapid response. The adage "what is everybody's business is nobody's business" is all too true as it relates to the problem of invasive exotic species at the state and federal levels. The lessons of Florida's successful efforts to control widespread exotic plants in its waterways illustrate the problems and solutions.
Before 1971, aquatic plant management activities were fragmented and piecemeal. Given the diverse ownership of public lands and their varying uses, many state agencies manage exotic species, but they tend to act without coordinating efforts, without adequate funding, and most important, without considering entire ecosystems. To succeed, a state must first do what Florida did in designating a lead agency to coordinate the efforts of local, state, and federal agencies and private citizens.
With such an approach, Florida has reduced water hyacinth infestation from 120,000 acres to less than 2,000. Other invaders of the state's waterways and wetlands are in or near maintenance control. These low levels reduce environmental impacts, pesticide use to control them, and costs to taxpayers.
Unfortunately, vast areas of Florida are still being invaded by exotic plants, in large part because of a third problem: inadequate and inconsistent funding. States are often more committed to land acquisition than to proper land management, particularly if pest damage is not obvious or the record of introduction elsewhere is not dramatic. If maintenance control of a weed knocks the level back sufficiently that the public ceases to recognize it as a problem, state funding correspondingly drops. Once controls relax, an introduced species may spread rapidly, presenting a more expensive problem than if funding and management efforts had remained. Further, eradication is far more likely during the initial phase of an invasion than after a species is widely established.
Of course, removing an invasive species from public lands does little good if reinvasion quickly occurs from adjacent private lands. Legislatures must develop incentive programs to encourage private citizens to help control invasive exotic species. Tax incentives for removing exotics seem to be the most acceptable way to deal with this problem. If such incentives fail, legislatures should enact penalties, much as some cities require citizens to clear their sidewalks of ice and snow.
Finally, states must make strong educational efforts to ensure that the public understands the threats from nonindigenous species. Without an educated public and legislature, special interest groups can undermine the ability of state agencies to put a harmful species on a blacklist or to keep one off a white list.
More than 20 federal agencies have jurisdiction over the importation and movement of exotic species, introductions of new ones, prevention or eradication of exotic species, and biological control research and implementation. However, no overall national policy safeguards the United States from biological invasions, and often federal and state agency policies conflict with one another. The Federal Interagency Committee for the Management of Noxious and Exotic Weeds has recently taken a small positive step by devising a National Strategy for Invasive Plant Management. This document promotes effective prevention and control of invasive exotic plant species and restoration or rehabilitation of native plant communities. More than 80 federal, state, and local government agencies, nonprofit organizations, scientific societies, and private sector interests have endorsed this nonbinding resolution. Although an important first step, it is basically educational and does not suggest specifically how to deal with weed problems on the ground. It still falls far short of an effective national program and does not address invasions by nonindigenous animals.
Lacking at the federal level are leadership, coordination of management activities on public lands, public education, and a strong desire to prevent new invasions. A parallel may be seen in the Centers for Disease Control and Prevention, with its missions of preventing new invaders, monitoring outbreaks, conducting and coordinating research, developing and advocating management practices, recommending and implementing prevention strategies, dealing with state and local governments, and providing leadership and training. Perhaps the federal government could develop an analog for invasive plants and animals. A high-level interdepartmental committee might serve much the same function-perhaps an enlarged version of the Federal Interagency Committee for the Management of Noxious and Exotic Weeds or the Aquatic Nuisance Species Task Force with a greatly expanded mission.
Independently of such structural changes, we must enhance state and federal programs in order to use agency personnel more effectively, develop nationwide consistency and cost effectiveness, conduct risk analysis, review and develop legal and economic policies, lower administrative costs, and eliminate duplication of effort. For instance, because APHIS budgets are prepared two years in advance, it is difficult for the agency to fund adequately an immediate response campaign. Also, basic research on an introduced species reflects the curiosity and idiosyncrasies of individual academicians and is not focused or coordinated very well.
Complicating the policy issues is international trade, the single greatest pathway for harmful introduced species, which stow away in ships, planes, trucks, containers, and packing material. Increased trade produced by the North American Free Trade Agreement (NAFTA) and the General Agreement on Tariffs and Trade (GATT) is bound to increase the problem. Of 47 harmful species introduced into the United States between 1980 and 1993, a total of 38 came in via trade.
Under NAFTA and GATT, restrictions claimed as measures to protect the environment can be challenged before the relevant regulatory body, which will decide whether the restriction is valid or simply protectionist. In GATT's case, the body is the World Trade Organization (WTO), which ruled in an analogous case that the European Union could not prohibit imports of beef from cattle treated with hormones. The WTO ruled that evidence of a health threat was insufficient.
For NAFTA and GATT, species exclusions are to be based on risk assessments, many of which require judgment calls by researchers. The effects of introduced species are so poorly understood and the record of predicting which ones will cause problems is so bad that one can question how much credence to place in a risk assessment. Also, the growing complication of risk assessment methods makes them less meaningful to the lay public and perhaps less responsive and relevant to policy needs. Particularly in controversial cases, as in many concerning introduced species, agreement by all parties is unlikely. Further, assessments are expensive, costing as much as hundreds of thousands of dollars, and funding sources are not established.
To address these trade issues, the federal government must be committed to limiting the import of exotic pests and must present a coordinated federal strategy to support restrictions. As a first step, the National Research Council should convene a high-level scientific committee to review the generic risk assessment processes produced by USDA and the Aquatic Nuisance Species Task Force. Also, all federal agencies that have a role in the trade process must have a common policy on what risk assessment to use and how to pay for it.
The growth of international trade only exacerbates a dire situation. A growing army of invasive exotic species is overrunning the United States, causing incalculable economic and ecological costs. Federal and state responses have not stemmed this tide; indeed, it has risen. Only a massive reworking of government policies and procedures at all levels and a greatly increased commitment to coordinating efforts can redress this situation.
B. N. McKnight, ed., Biological Pollution. The Control and Impact of Invasive Exotic Species. Indianapolis. Ind.: Indiana Academy of Sciences, 1993.
O. T. Sandlund, P. J. Schei, and A. Viken, eds., Proceedings of the Norway/UN Conference on Alien Species. Trondheim, Norway: Directorate for Nature Management and Norwegian Institute for Nature Research, 1996.
U.S. Congress, Office of Technology Assessment, Harmful Non-Indigenous Species in the United States. Washington, D.C.: U.S. Government Printing Office, 1993.
M. Williamson, Biological Invasions. London: Chapman & Hall, 1996.
Don C. Schmitz is wetland and upland alien plant coordinator of the Florida Department of Environmental Protection's Bureau of Aquatic Plant Management. Daniel Simberloff is Robert O. Lawton Distinguished Professor in the Department of Biological Science at Florida State University. They are the editors (with Tom C. Brown) of Strangers in Paradise: Impact and Management of Nonindigenous Species in Florida (Island Press, 1997)