CRS Report: RL30123 - Harmful Non-Native Species: Issues for Congress

A variety of abundant non-native species have had severe economic impacts on U.S. industries and the natural environment. The accelerating number of costly introductions is causing pressure on Congress to develop new responses to the problem. In a 1993 study, just 79 of over 4500 non-native plants and animals in the United States caused over $97 billion in damage between 1906 and 1991. 5 A January 1999 estimate put damage at $123 billion annually. 6 Damages vary by species, and can span an enormous range of effects, including power outages; loss of farmland property value; contamination of grain; spread of disease; increased operating costs; inefficient irrigation; collapse of buildings; loss of sport, game, or endangered species; ecosystem disturbance; etc.

Some non-native plants have been notorious for years for causing both economic and ecological damage; kudzu, melaleuca, tamarisk, purple loosestrife, spotted knapweed, and Russian thistle are just a few examples of unwanted plants now causing ecological and economic harm in large areas of the United States. Their damage includes lowering water tables, competing with native plants, poisoning livestock, and increasing pest control costs. A serious infestation can cause substantial losses in property values. 7

Non-native invertebrate pests are also well-known: gypsy moths, Japanese beetles, Asian long-horned beetles, fire ants, Africanized honeybees, and zebra mussels are among the most well-known. Introduced vertebrate pests (e.g., walking catfish, cane toad, brown tree snakes, monk parakeet, starlings, feral goats, rats, etc.) can also have serious economic and ecological impacts. These effects can also include the introduction of various human parasites and diseases. 8

In some of these cases (e.g., kudzu, melaleuca, gypsy moths, Africanized bees, zebra mussels, and starlings), the source of the introduction is either known or strongly suspected. Introducers of some species, such as kudzu, melaleuca, and starlings, actually intended that their imports proliferate in the wild. Their purposes ranged from the practical (kudzu for erosion control) to the quixotic (starlings for a purported desire to bring all of the birds mentioned in Shakespeare's works to the United States). Whatever the motive, society lost the importers' Faustian bargains as the organisms proliferated to the detriment of the economy, as well as native fauna, flora, and ecosystems. Had they foreseen the damage these species would later cause, governments at all levels might have worked to prevent these introductions.

Introductions of non-native species may be intentional or unintentional. Some intentional introductions produced benefits (or at least low levels of harm), as in the case of the ring-necked pheasant, a game bird native to Asia. Introduced species may be present for years or decades before their presence is even recognized, and the harm, if any, that they do is measured or observed. 9 However, catastrophic results can come from both accidental introductions (e.g., zebra mussels), and intentional introductions (e.g., hogs to Hawaii).

A myriad of intentional and generally beneficial introductions must be recognized. These include such food sources as cattle, wheat, honeybees, kiwi fruit, and soybeans, and such ornamentals as tulips, chrysanthemums, and dawn redwoods, to name only a few. In each case the introduction of these species was very much intentional and their propagation was more or less controllable. The benefits conveyed by these species are vast, and probably exceed the $123 billion figure cited above for the annual costs of non-native species.10 U.S. agriculture would have a far different appearance if it were limited to the several dozen food crops known to have been cultivated in North America before 1492 rather than the hundreds of crops grown today. These non-native report, but should not be forgotten in discussions of harm caused by other imported species.

Even in the case of non-native species conveying obvious benefits, the introductions of non-natives are not without ancillary dangers, sometimes to the interests of those importing the target species: cattle can bring in seeds and new diseases, tubers can bring in insect pests, and soil from roots or hooves can harbor diseases for native plants. Examples of these attendant risks and problems will be considered below. This paper will focus on species prone to (a) escaping human control or whose potential for escape is unclear, or (b) carrying in other undesirable species. Predicting either of these risks is one of the most difficult problems in addressing invasions by non-native species.

What's In A Name?

Many names have been used to describe species that have become able to survive and reproduce outside the habitats where they evolved or spread naturally. Among them are alien, exotic, injurious, introduced, invasive, non-indigenous, and non-native. Alien tends to be applied to species from other regions that are also harmful and likely to proliferate wildly in their new habitat. Injurious is generally not confined to species outside of their normal range, and is used roughly synonymously with invasive. Both terms are applied to species that proliferate wildly, whether native (like white-tailed deer and barnacles) or non-native (like leafy spurge or brown tree snakes). In the legal arena, these terms are found in various laws and defined in various ways. This report generally uses the term non-native to emphasize the geographic aspects of the issues posed by these species, and to avoid the somewhat pejorative connotations of some of the other terms.

This paper focuses on those non-native species which have caused or seem likely to cause substantial economic harm. The authors recognize that many species (e.g., cattle, olives, wheat, tulips, etc.) were introduced with either little known adverse ecological impact, or with impacts (even if some are adverse) beneficial enough to make it likely that most people would not consider them harmful.

If a new kind of tree grows in the forest, no one hears of it, at least not for a long time. When any kind of new organism is introduced to a new site, it must find habitat suitable to its needs, as well as a mate, and must avoid predators and diseases. As a result, scientists agree, the great majority of biological introductions, whether caused by humans or occurring naturally, tend to fail. Of those that succeed, only a small fraction go on to become serious pests. A new species can exist in an area for decades without its presence being noticed. For that reason, the number of non-native species actually counted in an area (if such counts are made) is likely to be an underestimate of the number of non-native species in the area.

The 1993 OTA study cited above summarized information on the numbers of non-native species in the United States. It divided the species into those of foreign origin, and those of U.S. origin that had been introduced outside of their native ranges. As the OTA study noted, "These numbers should be considered minimum estimates. Experts believe many more [non-native species] are established in the country, but have not yet been detected." A more recent report estimated 30,000 non-native species in the United States.11

The first human introduction of non-native species into North America is lost in antiquity. Nomads crossing the Bering Strait brought their dogs with them over 10,000 years ago. Polynesians landed in Hawaii bearing pigs, rats, and crop seeds over 1500 years ago. Nearly a thousand years ago, Icelandic settlers in what is now Greenland brought cattle, sheep, and goats with them (and perished with them, possibly due to a reliance on European livestock unable to find fodder in an increasingly harsh climate). In the l7th century, Puritan colonists released domestic pigs into the New England forests to fend for themselves and provide food for the colonists. By the 1840s the descendants of the Puritan's pigs were as common "as grains of sand on the sea-shore" in Midwestern forests.12 The colonists also brought smallpox, measles, brucellosis, and other undesirables, to the severe detriment of native populations, both human and non-human. Some, like dandelions and Norway maples, have been in this country for so long that their non-native status is virtually forgotten.

In recent decades, introductions have swelled dramatically. Contributing factors include increases in the number of people traveling, the speed and methods of travel, trade generally, improved ability and speed in moving living plants and animals so that more of them survive the journey, the increase in modes of transport for hitch-hiking organisms (such as bilge water, pallet wood, and airplane wheel wells), the desire to have familiar sport and game animals in new areas, trade in horticultural and garden plants, trade in pets and aquarium animals, etc. So vast is this "bioinvasion" (as some have termed it), that only rough estimates can be made of the numbers of non-native species now in North America, much less the rest of the world.

Table 1. Estimated Numbers of Non-Native Species in the United States

Species with Origins Outside the United States

(U.S. Congress. Office of Technology Assessment. Harmful Non-Indigenous Species in the United States. 1993. p.92. Various original sources cited in report.)

Plants and animals tend to survive best in a new site when that site is similar to their original habitats. Formosan termites arriving in New Orleans are much more likely to thrive than Formosan termites whose next stop is Anchorage, and a northern European grass seed traveling inside a prize bull is much more likely to survive in the organic matter of the Chicago area than the same species would in Miami. Thus, the plants and animals of northern Europe, Korea, northern China, Japan and New England are more likely to be a threat to the stability of each other's ecosystems, than any of them is to the fauna and flora of Miami or Singapore. Regions with similar climates and soils around the world are tending toward increasing biological homogenization as plant and animal species spread and the new arrivals thrive at the expense of natives. This reversal of the normal evolutionary pattern of ever-greater species divergence between two regions from biodiversity to "biosimilarity" has led some to call the spread of non-native species "evolution in reverse." 13

While the focus of this report is on importing non-native species into the United States, it must be noted that this country is itself a source of species which threaten ecosystems elsewhere. The gray squirrel of the eastern and northern states was introduced to Britain, where it has severely threatened populations of the English red squirrel. In 1958, Pan Am Airways and a local hotel introduced largemouth bass into scenic Lake Atitlan in Guatemala. The introduction led, in the next 25 years, to the crash of a sustainable crab fishery and of several native fish, contributed to the near-elimination of the commercial harvest of reeds used in local handicrafts, and forced the extinction of a bird found nowhere else. Moreover, the harvest of the bass has since plummeted.

And even at the ends of the earth, human fecal coliform bacteria contaminate MeMurdo Sound, the major U.S. scientific research base in Antarctica. In an area thought of as earth's most nearly pristine environment, the bacterium Clostridium perfringens contaminates over 80% of sea urchins near the undersea outfall of the untreated sewage from the base. The scientist who discovered this contamination plans to study the effect, if any, this sewage contamination is having on the plants and animals of the Sound. Analysts have identified no laws designed expressly to prevent the spread and proliferation of U.S. species in other countries, except in agricultural areas.

Near neighbors are less likely to be a source of pests. Canada, whose northern forests are contiguous with those of the United States, is not likely to be a source of forest pests, since its forests have no meaningful biological barricade separating them from our own. Consequently, if a native Canadian species could survive in a U.S. forest, it is probably already here. Similarly, desert plants of the southwest are probably shared with Mexico, and neither country's desert fauna or flora poses much of a threat to the other. 14

To some extent, pathways of invasion between countries can be predicted. The arrival of zebra mussels and their attendant damage to city water supplies and electric utilities in the Great Lakes area focused much attention on the ballast water of cargo ships as a pathway for biotic invasion. Similarly, the propensity of brown tree snakes to hide in the wheel wells of airplanes has done much to focus attention on air stowaways. The recent attention to the arrival of Asian long-horned beetles may play a similar role in focusing attention on pallet wood, packing crates, and airport warehouses as pathways and centers of biotic invasion. In general, any arrival of living or untreated material (water, wood, soil, etc.) at frequent intervals should not be overlooked as a possible source of biotic invasion.

Within countries, certain paths for species invasions are quite predictable. In the 19th century, the railroad tracks over which cattle were transported were a major path for the establishment of new plants. In the 20th century, the zebra mussel quickly escaped the drainage of the Great Lakes, and began its invasion farther and farther south into the Mississippi River drainage. In Africa, the spread of AIDS through countries and villages where it had not been previously known coincides with the flow of truck traffic and troop movement.

In addition to the accidental introduction of non-native species, introductions may occur from species deliberately brought into the country. In some cases, the importer does not intend for the imported plant or animal to escape to the wild, and in other cases, the purpose of importation is to promote spread into natural habitats to achieve some desirable goal. In the first category are the importations of non-native pets and plants. The importer is hardly ever interested in seeing the imported organisms escape. But once the specimens are sold, control is lost, and purchasers may release unwanted non-native aquarium fish, for example, into local lakes or streams, often feeling they are doing a good thing by letting the fish go.15 A garden or greenhouse plant may scatter shoots or seeds far more widely than expected. Water hyacinths were brought from South America in the late 19th century as pool ornaments. The plant now covers thousands of acres in the southern United States, as well as parts of Africa and Asia. Moreover, the non-native animal or plant may harbor microorganisms that pose a danger to other species, even if the animal or plant itself does not survive in the wild.

People deliberately release organisms into the wild for a variety of purposes. Tamarisk (or salt cedar) was introduced from Central Asia into the desert Southwest in the early 19th century, in part to control erosion along river banks. The tree now forms dense thickets on more than a million acres of riparian habitat. The thickets have generally little value for most native animals, and the trees are estimated to absorb more water each year than all the cities of southern California.16

The first line of defense is not control but prevention. Inspections and quarantines are a key component of prevention, and a major responsibility of the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture. Various states, particularly California, Arizona, and Florida, mount their own inspection efforts; these are aimed at agricultural pests. If exclusion and quarantines fail to keep a non-native out of an area, and a species becomes established, the problem shifts to control of the pest.

Control of non-native species is divided into two related tasks: eradication where that is possible, and reduction to bearable levels where eradication is not possible. No single method of control is likely to be a panacea. Few, if any, promise eradication under any conditions in which a species is well-established, but several in combination and used indefinitely 17 might reduce some target species' populations to tolerable levels. On the other hand, if many methods are used intensively and in combination, eradicating very small populations that threaten to become established may be possible.

For any introduced species, the range of control actions falls into seven basic categories: (1) baits and attractants; (2) fumigants, repellents, and barriers designed for confined spaces; (3) traps; (4) poisons; (5) biological control; (6) bounties and commercial exploitation; and (7) mechanical removal. To apply any of these basic strategies of control, substantial knowledge of the target species' behavior, biochemistry, prey preferences, diseases, or other aspects of its total biology may be essential. A number of species (e.g., the brown tree snake and the Mediterranean fruit fly) have been the focus of several of these strategies where they have become established. The pros and cons of these seven strategies, and the information needed to apply them, are described below.18

Baits and Attractants. Baits and attractants may be used to draw unsuspecting individuals of a target species toward a potential source of food or mates; once there, they can be counted, killed, or studied. In some species, chemicals are given off that allow males and females to find each other; these chemical communication signals are called "pheromones." The discovery and use of pheromones of a target species could be a powerful species-specific control method. Difficulties with baits and attractants commonly include the need to check the traps and the need to prevent non-target species from being harmed by or interfering with the bait. Baits and attractants seem most promising when the area needing protection has a clear boundary or well-defined area, and a significant density of the target species. Sex pheromones hold the added possibility of functioning even when target species densities are low as well as being highly species-specific. For example, "Disparlure", the commercially synthesized sex pheromone of the female gypsy moth, is used to trap male gypsy moths.

Fumigants, Repellents, and Barriers for Confined Spaces. Substances might be found which kill individuals of the target species in a confined small area or prevent them from crossing some bottleneck. These methods can be used either to create pest-free "islands" in a zone of infestation, or in conjunction with a quarantine program to prevent non-natives from leaving an infested area via boxes, cargo holds, etc. Some species are known to try to evade certain substances, such as tear gas or gasoline. Obviously, these substances can be used only to a limited extent, and probably not over large areas, or where these spaces require frequent human access. Fumigants can be used to kill or exclude individuals of a target species from confined areas such as cargo containers and the like. The Environmental Protection Agency approved methyl bromide 19 as a fumigant for the brown tree snake. Light is also known to repel some nocturnal animals. Submerged surfaces have been electrified at water and power facilities to discourage the settling of zebra mussel larvae. Physical barriers can be used to prevent access to new habitats, such as the electrical barrier being constructed in the Chicago Sanitary and Ship Canal to prevent the round goby from migrating from the Great Lakes into the Mississippi River drainage.

Traps. The use of traps is limited primarily by cost, time to service the traps, and inability of traps to control the target species over a very large area. Various designs can be used, and most are used in combination with some sort of bait. For confined areas such as cargo holds, buildings, etc., traps may be relatively successful. On the other hand, they have obvious drawbacks in an open situation with either abundant alternative prey or very low target species densities. Sticky traps have been used in rodent control and for brown tree snakes. Trapping methods are comparatively safe for humans, although they require some care when individuals of the target species in the trap are killed and removed. Chinese mitten crabs are effectively trapped when they encounter irrigation screens during downstream migration to saltwater, with thousands of crabs annually clogging and blocking such screens in California's Sacramento River drainage.

Poisons. Where poisoning is an option, broadly toxic substances risk harm to non-target species, so poisons registered specifically for control of certain species are strongly preferable. For example, TFM (3-trifluoromethyl-4-nitrophenol) is the product of many years of research and is very specific for larval stage of lamprey. A variety of aquatic herbicides are used to control hydrilla and water hyacinth. Even if poisons are highly specific, it is likely that they would be used in conjunction with baits, thereby further reducing risks to pets, children, and other non-target organisms even further, to reduce the chance of previously undiscovered effects, and to convey the toxin to the target organisms most efficiently.

Biological Controls. A species used in biological control preys on, parasitizes, or causes disease in a targeted pest species. Ideally, it attacks that species and no others. Considerable knowledge of both the target species' and the control species' basic ecology would be necessary to select a suitable control. One must be especially careful with biological control, since species introduced to provide the control may turn to feeding on other nontarget species, often leading to further endangerment of native fauna. The history of some successful mongoose introductions, for example, is matched against introductions that backfired when the released mongooses proliferated then failed to confine themselves to the prey their importers wanted them to eat, and instead wrecked havoc on various birds and other native animals. Similarly, the house sparrow (Passer domesticus, or English sparrow) was brought from Europe to control the canker worm. This pest bird now crowds out native birds and damages crops. And canker worms are still plant pests.

A disease or a selective parasite may be an attractive option in some circumstances, but again care must be taken to minimize the chance that the disease or parasite will attack a nontarget species. Field work in a target species' native habitat is usually necessary to find diseases or parasites to which the target species may be susceptible. Although the requisite research might be expensive, biological control holds out great hope for long term control. A variety of insects are being investigated for their ability to control non-native aquatic plants, such as hydrilla and water hyacinth. The brown tree snake of Guam could be a possible candidate for biological control, since there is only one snake native to the island, and its habitat is quite different from the tree snake. As a result, the chance of an introduced parasite or disease affecting a non-target species is diminished.

Bounties. Under a bounty system, someone is paid to catch the target species. High bounties may have to be paid to encourage control that results in a substantial effect on the target species' population. From the species' point of view, bounties and commercial exploitation of wild organisms are identical: the animal or plant is killed and removed from the population. The remover is paid to do it in either case, but the final disposition of the dead organism (eaten, skinned, incinerated, etc.) is biologically irrelevant. If either bounties or commercial prices are too high, there may be financial incentive to grow or spread the organism a particular risk as the population dwindles to very low levels and prices go up. In early 1999, the California Department of Fish and Game was debating whether permitting commercial harvest of Chinese mitten crabs would contribute to their control or encourage further introductions.

Mechanical Removals. Mechanical controls may be used to collect and remove a large volume of non-native species, usually plants. Mechanical harvesters may be used in the management of non-native aquatic vegetation, such as hydrilla and water hyacinth, but are ineffective alone for control on large bodies of water. The recently introduced Asian long-homed beetles (still perhaps at low enough levels to have some slight chance of eradication) are currently being controlled to some degree by removing all trees on which the pests might feed in neighborhoods where they have been found. Such a drastic strategy would be unworkable if this pest becomes well-distributed.

Harmful non-native species occur throughout the United States, but Some areas are more susceptible than others. Mild climate, geographic isolation, disturbance of the natural landscape, and a high rate of exposure to non-native Species are all factors which can make a habitat particularly susceptible to invasion, Islands and other long-isolated areas with unique plants and animals are generally more Susceptible to Invasive species than continental habitats.

Hawaii and Florida, for example, each have many such Species. Both states were long biologically isolated and have large numbers of native species found nowhere else. The mild climates of Florida and Hawaii make it easier for many Species to survive, and make the states attractive to businesses that import and maintain or even breed non-native animals and plants, such as tropical fishes and ornamental plants. In Florida, the number of non-natives seemed overwhelming to a local reporter:

Both Hawaii and Florida are major travel destinations and transportation hubs, so they are more likely to be subjected to inadvertent introductions. In both states, large areas have been cleared of native plants. It is often easier for non-native Species to establish themselves in such disturbed habitats in fact, many such Species are weeds that have evolved to exploit such land and then "hitchhike" to freshly disturbed areas.

Another factor putting some environments at risk is the sheer number of "attempts" at new introductions. Seaports, in which ships have exchanged ballast water daily for decades or even centuries are at severe risk of invasions. Even if only a tiny proportion of newly arriving non-native species survive in the new habitat of San Francisco Bay, Chesapeake Bay, or Boston Harbor, the actual number of successful, invasive species may be very large. Airports, with increasing volumes of international traffic and tourism are also at risk. In addition, the greater the similarity of the point of origin, the more likely the invasion is to be successful. For example, biotic invasion of the Hawaiian islands is more likely to come from a plane originating in Guam than one originating in Anchorage.

One recent study put annual costs and damages due to non-native species at $123 billion per year. 21 A handful of species selected in this study alone cause annual losses over $3.5 billion, and two (Formosan termite and Asian clam) are responsible for $1 billion each in control, diminished property values, and other damages. Even if new importations of non-natives were completely halted (a near impossibility), the costs of controlling established non-native species would continue.

The value and the cost of prevention are difficult to assess. The first several thousand harmful non-native species collectively were estimated to cause about $123 billion annually in costs and damages; in the absence of any other way to predict economic impact, it is reasonable to assume that the next several thousand to arrive and become established could cause roughly the same level of economic damage. The damages could include the same types of damages already known to affect economies and ecosystems (lower outages; loss of farmland property value; contamination of grain; spread of disease; increased operating costs; inefficient irrigation; collapse of buildings; loss of sport, game, or endangered species; ecosystem disturbance; etc.). There could also be effects on industries or ecosystems that have not yet been markedly harmed by non-natives (e.g., the threat to fall tourism in New England from Asian long-horned beetles (see below), which attacks and kills a variety of tree species, but is particularly fond maple trees and their relatives).

Targeting each newly arriving non-native species individually is likely to be costly. It is possible a focus on high-risk pathways could be more cost-effective over the long term, but a pathways approach itself has costs, as evinced by debate over the arrival of the Asian long-homed beetle. In an effort to control this widespread Asian species, controls were placed on its suspected major pathway raw wood packing material of imports from China (rather than from all countries in which this species is common). The consequent threatened trade war has economic implications for U.S. industries doing business with China. It has also escalated federal government attention to the problem of non-natives to the highest levels of government. (See Asian Long-Horned Beetle, below.)

The controversy illustrates the dilemma for policy-makers. Specifically, the high cost to many industries (even seemingly unlikely ones, such as imported computer parts, steel, or other products without obvious connection to living organisms), economic interests, and ecosystems from unwanted species is balanced against the effects of protective measures on commerce in general. There will also be debate over the needs of domestic industries (horticulture, agriculture, pet trade, etc.) directly dependent on the importation of non-natives.

While the damage from some non-native species can be great, few have proved to be economically harmful, and many are beneficial. Some industries rely heavily on non-natives. For example, nearly all food plants and animals in the United States are not native to the areas where they are now grown. Besides agriculture, industries relying significantly on non-native species include the nursery, aquaculture, and pet industries.

Most woody invasive plants in the United States were originally introduced by the landscape industry. 22 The giant hogweed, whose toxic sap can cause severe scarring, was introduced as an ornamental but escaped cultivation and is now widely listed as a noxious weed. 23 As a result of this and similar escapes, the nursery industry has been subjected to increasing criticism. States are increasing their regulation of potentially invasive species. 24 Some of those species are economically important to the nursery industry. Some industry groups have been working to develop voluntary controls to lessen the risk of inadvertent introduction of invasive plants.

Production from private aquaculture more than doubled from 1985 to 1997 and is now worth more than $750 million per year. 25 Many cultured species are not native. There is concern about the escape and establishment of the cultivated species that may be harmful to native ones. Examples include the establishment of the Atlantic salmon in the Pacific Northwest; and the inadvertent introduction of diseases, such as dermo and MSX in oysters; or of other pest species. Concerns are similar for the aquarium and exotic pet trades, which rely heavily on tropical species.

Responsibilities for native flora and fauna and the ecosystems in which they live, are scattered the wards of many or of none. Generalized effects on ecosystems (e.g., the Great Lakes, suburban trees, Texas lawns) are not the specific responsibility of one federal agency. With so many pathways for the entry of non-natives, so many possible entering species, and so many possible and nearly unknowable injured species, the natural ecosystem as a whole has no specific guardian. This scattered responsibility is a result of the evolving legal history of species protection, 26 agriculture, and import regulation.

U.S. law concerning native wild plants and animals is not a comprehensive body at the federal level. Under our system, inherited from English legal tradition, and stated very simply, the State regulates the "take" of native wild animals generally, and the landowners control the native (and other) plants growing on their lands. A wild deer walking across a pasture does not "belong" to the landowner but is rather the State's to regulate; the bush it eats belongs to the landowner. Thus, the colonial governments regulated the native wild animals, and after the U.S. Constitution was ratified, the states retained the rights they already had to control the wildlife within their boundaries.

Aside from special rules for lands owned by the federal government, federal native wildlife law can be thought of as a series of exceptions to the general concept that states regulate wild animals, and landowners manage (or don't manage) wild plants. Some of the major exceptions to that generalization include federal laws controlling migratory birds (pursuant to treaties), marine mammals, and endangered species. The great majority of species of native wild plants and animals do not fall into any of these categories and therefore are not direct federal responsibilities.

Native wild flora and fauna are frequently protected as a consequence of protecting something else agriculture and endangered species, for example. Where there have been specific injuries to other industries or interests (utility intakes, for example) the pathway by which the harmful species arrived may be regulated to prevent other non-natives arriving via that pathway. Natural ecosystems, benefiting from the threats to other interests, may face an incidental reduced risk of harm arriving by a pathway that is controlled.

5 U.S. Congress. Office of Technology Assessment. Harmful Non-Indigenous Species in the United States. OTA-F-S65Washington, D.C.: U.S. Government Printing Office, September 1993. p.3-5. (Hereafter referred to as "OTA Report.")

7 A serious infestation of leafy spurge (Euphorbia esula) on an Oregon ranch caused a drop in property value from about $125-$150 per acre to $22 per acre over 10 years. (FICMNFW report, p.27.)

8 While the transfer of human pathogens such as smallpox, syphilis, measles, AIDS, and malaria from continent to continent has enormous human, ecological, and economic consequences, human pathogens and parasites will not be covered in this report, though a few human disease vectors (species that can transport diseases between hosts, but are not themselves pathogens) are mentioned.

9 Measurements of the number of non-native species that do no harm is exceedingly difficult: by being innocuous, they escape study.

12 C. Dickens, 1842, p.165 in From Coastal Wilderness to Fruited Plain, by Gordon G. Whitney. Cambridge, England. Cambridge University Press. 1994.

13 Bright, Christopher. Life Out of Bounds: Bioinvasion in a Borderless World. 1998. New York. W. W. Norton & Co, p.17. (Hereafter referred to as "Bright.")

14 However, the statement does not apply to all ecosystems. For example, the higher elevation forests of the Sierra Madre in Mexico have been separated for millennia by hundreds of miles of desert from ecologically very similar forests of the Rockies. Pests, both plant and animal, could evolve in comparative isolation in these two areas, and be transported only recently with greater links of trade and traffic. Thus, the flora and fauna of either disconnected area could pose a threat to the other while the more continuously connected desert species are much less likely to do so.

15 According to one author, "By far the most ecologically disruptive sector of the pet industry is the aquarium trade.... Of those exotic fish species established in the United States that are completely foreign to the country, about 65 percent arrived through the aquarium trade." (Bright, p.162-163.)

17 In the absence of eradication, control practices cannot stop, due to the residual pest populations which could otherwise reproduce and build back up to problem levels.

18 The discussion below draws heavily on U.S. Dept. of the Interior. Fish and Wildlife Service. The Brown Tree Snake Boiga irregularis, A Threat to Pacific Islands. Biological Report 88(31). Washington, DC: September 1988. p.18-20.

19 For more information, see CRS Report 98-590 STM, Methyl bromide and stratospheric ozone depletion policy issues.

20 W. Belleville. 1994. "Critter patrol." Florida (a news magazine of the Orlando Sentinel Star) 29 May 1994; 8-12, 15.

21 Pimentel report, p.1. No information about the overall costs of excluding non-natives was discovered. However, it appears that over-abundant non-natives are more costly than rare natives: one estimate of costs of the regulatory burden of the ESA is about $530 million annually ($3.706 billion over7 years). (P. 214 in Committee on Resources, Ser. No.104-65. April 17, 1996.)

22 S.H. Reichard and C.W. Hamilton, "Predicting Invasions of Woody Plants Introduced into North America," Conservation Biology 11 (1997):193-203.

26 For a general discussion of early development of federal wildlife law, see The Evolution of National Wildlife Law, 3rd ed., by Michael J. Bean and Melanie J. Rowland. Praeger Publishers, Westport, CT. p.7-14.

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