Invading non-indigenous species in the United States cause major environmental damage, public health problems and cost the nation more than $122 billion per year. More than 30,000 species are foreign; this number is increasing as the human population grows and trade and travel increases. About 42% of the species on the Threatened or Endangered species list are at risk primarily because of non-indigenous species. Some of the introduced pests include purple loosestrife, zebra mussels, gypsy moths, various plant pathogens, rats, feral cats and pigs, and the AIDS virus.

Since Columbus first landed on North America, more than 30,000 non-indigenous species (NIS) are estimated to have been introduced into the United States. Introduced species, like corn, wheat, rice, and other food crops and domestic cattle, poultry, and other livestock now provide more than 98% of U.S. food supply with a value of more than $500 billion per year (USDA 1996). Other NIS have been used for landscape restoration, biological pest control, sport, pets, and food processing. Some NIS, however, cause major economic losses in agriculture, forestry, and several other segments of the U.S. economy as well as negatively impact the environment. One recent study reported approximately $97 billion in damages from 79 NIS from the period 1906 to 1991 (OTA 1993).

Estimating the full extent of the environmental damage caused by NIS and the number of species extinctions they have caused is complicated because approximately half of the estimated 750,000 native species in the United States are not described (Raven and Johnson 1992). Nonetheless about 400 of the 958 species on the Threatened or Endangered Species List are considered at risk primarily because of competition and predation by non-indigenous species (Nature Conservancy 1996). Other species are threatened by alien species and/or ecosystem changes. Estimating the economic impacts of non-indigenous pests affecting the United States is also difficult; yet, sufficient data are available to quantify some impacts on agriculture, forestry, and public health.

This study assesses the magnitude of environmental impacts and economic costs associated with the diverse non-indigenous species that have invaded the United States.

Most plant and vertebrate animal introductions have been intentional, whereas most invertebrate animal and microbe introductions have been accidental. In the recent past the rate and risk associated with NIS introductions have increased enormously because of rapid human population growth and alteration of the environment. In addition, large numbers of people are traveling faster and farther, and more goods and materials are being traded among nations (Bryan 1996, USBC 1996).

Introduced Crop Plants

Most alien plants now established in the United States were introduced for food, fiber, and/or ornamental purposes. An estimated 5,000 plant species have escaped and now exist in U.S. natural ecosystems (Morse et al. 1995), compared with a total of about 17,000 species of native U.S. plants (Morin 1995). Of the approximately 25,000 cultivated plant species, mostly ornamentals brought into Florida, more than 900 have become established in surrounding natural ecosystems (Frank and McCoy 1995a; Frank et al. 1997; Simberloff et al 1997). More than 3,000 plant species have been introduced into California (Dowell and Krass 1992).

In crop systems, including forage crops, an estimated 500 introduced plant species have become weed pests. Most of these were accidentally introduced with crop seeds, from ship ballast, or from various other imported plant materials. Some of these accidentally introduced weeds, like yellow rocket (Barbarea vulgaris) and Canada thistle (Cirsium arvense), are major weed pests.

Effects of Introduced Plants on Ecosystems

Most of the 5,000 non-indigenous plants established in U.S. natural ecosystems have displaced several native plant species (Morse et al. 1995). Non-indigenous weeds are spreading and invading approximately 700,000 ha per year of the U.S. wildlife habitat (Babbit 1998).

One of these pest weeds is the European purple loosestrife (Lythrum salicaria) that was introduced in the early 19th century as an ornamental plant (Malecki et al. 1993). It has been spreading at a rate of 115,000 ha per year and is changing the basic structure of most of the invaded wetlands (Thompson et al. 1987). The monotypic stands of purple loosestrife have reduced the biomass of 44 native plants and endangered wildlife that depend on these native plants (Gaudet and Keddy 1988). Loosestrife now occurs in 48 states and costs $45 million per year in control and forage losses (ATTRA 1977).

Many introduced plant species established in the wild are having an effect on U.S. parks (Hiebert and Stubbendieck 1993). In Great Smoky Mountains National Park 400 of approximately 1,500 vascular plant species are NIS, and 10 of these now threaten the park's natural biological resources (Hiebert and Stubbendieck 1993).

In Hawaii, there are a total of 2,690 plant species, with 946 of these being non-indigenous species (Eldredge and Miller 1997). About 800 native species are endangered and more than 200 endemic species are believed to be extinct (Vitousek 1988).

Sometimes one non-indigenous plant species overruns an entire ecosystem. For example in California, yellow star thistle (Centaurea solstitalis) now dominates more that 4 million ha of northern California grassland with disastrous results on the use of this grassland (Campbell 1994).

European cheatgrass (Bromus tectorum), is dramatically changing the vegetation and fauna of natural ecosystems. This annual grass has invaded and spread throughout the shrub-steppe habitat of the Great Basin in Idaho and Utah and predisposed the invaded habitat to fires (Kurdila 1995, Vitousek et al. 1996, Vitousek et al. 1997). Before the invasion of cheatgrass, fire burned once every 60 to 110 years and shrubs had a chance to become well established. Now, fires occur about every 3 to 5 years; shrubs and other vegetation are diminished and monocultures of cheatgrass now exist on 5 million ha in Idaho and Utah (Whisenant 1990).

An estimated 138 non-indigenous tree and shrub species have invaded native U.S. forest and shrub ecosystems (Campbell 1998). Introduced trees include salt cedar (Tamarix pendantra), eucalyptus (Eucalypus spp.), Brazilian pepper (Schinus terebinthifolius), and Australian melaleuca (Melaleuca quenquenervia) (OTA 1993, Miller 1995, Randall 1996). Some of these trees have caused displacement of native trees, shrubs and other vegetation types, which in turn has reduced populations of some associated native animal species (OTA 1993). For example, the melaleuca tree, is spreading rapidly throughout the vast forest and grassland ecosystems of the Florida Everglades (Campbell 1994). Currently the tree is spreading over 11,000 ha per year where it damages the natural vegetation and wildlife (OTA 1993).

Exotic aquatic weeds in the Hudson River basin, New York, number 53 species (Mills et al. 1997). In Florida, aquatic plants, like hydrilla (Hydrilla verticillata), water hyacinth (Eichhornia crassipes), and water lettuce (Pistia straiotes), are altering aquatic habitats and require control. These aquatic weeds are altering fish and other aquatic animal species, choking waterways, altering nutrient cycles, and reducing recreational use of rivers and lakes.

Economic Costs of Introduced Plants

In U.S. agriculture, weeds cause a reduction of 12% in crop yields or, in economic terms, about $36 billion in crop production annually, based on crop potential value of all U.S. crops of more than $300 billion per year (USDA 1996). Based on the estimate that about 73% of the weeds are non-indigenous (Pimentel 1993), it follows that about $26 billion of these crop losses are due to introduced weeds. In addition, approximately $4 billion in herbicides are applied to U.S. crops (Pimentel 1997), of which about $3 billion can be attributed to non-indigenous weed control. Therefore, the total costs for introduced weeds in the U.S. economy is about $29 billion annually.

In pastures, 45% of weeds are non-indigenous species (Pimentel 1993). U.S. pastures provide about $10 billion in forage crops annually (USDA 1996), and the estimated losses due to weeds is approximately $2 billion. Since about 45% of the weeds are non-indigenous (Pimentel 1993), we estimate that forage losses due to non-indigenous weeds are nearly $1 billion each year.

Some introduced weeds are toxic to cattle and wild ungulates, like leafy spurge (Euphoria esula) (Trammel and Butler 1995). In addition, several non-indigenous thistles also replace desirable native plant species in pastures, rangelands, and forests and thus reduce cattle grazing (Dewey 1991). According to Secretary Bruce Babbitt (1998), ranchers spend about $5 billion each year on controlling invasive non-indigenous weeds in pastures and rangelands and these weeds continue to spread.

Lawn, garden, and golf course management costs about $36 billion per year (USBC 1996). A significant proportion of management costs is related to pest control. In addition, Templeton et al. (1998) estimated that about $1.3 billion (included in $36 billion) is spent on outdoor residential weed, insect, and disease pest control each year. We estimate that $500 million is spent on residential NIS weed control and an additional $1 billion is invested in non-indigenous weed control on golf courses.

Weed trees are also a major problem. From $3 to $6 million per year is being spent in efforts just to control the melaleuca tree in Florida (J. Richardson, 1998, personal communication, Duke University).

Aquatic weeds are a significant problem in many regions of the nation. For instance, Florida spends about $14.5 million each year on hydrilla control (Center et al. 1997). Despite this large expenditure, hydrilla infestations in just 2 Florida lakes have prevented their recreational use, causing $10 million annually in losses (Center et al. 1997). In the United States, a total of $100 million is invested annually in aquatic weed control (OTA 1993).

Large Mammal Introductions

Mammal introductions include dogs, cats, horses, burros, cattle, sheep, pigs, goats, and deer (Layne 1997). Several of these animals have escaped or were released into the wild and many have become pests by either preying on native animals, grazing on vegetation, or intensifying soil erosion.

For example, goats (Capra hirus) introduced on San Clemente Island, California are responsible for extinction of 8 endemic plant species as well as the endangerment of 8 other native plant species (Kurdila 1995). Horses (Equus caballus) and burros (Equus asinus) released in western United States have attained populations of approximately 50,000 animals (Pogacnik 1995). These animals graze heavily on native vegetation, allowing non-indigenous annuals to displace native perennials (Rosentreter 1994). Furthermore, burros inhabiting the northwestern United States diminish the primary food sources of native bighorn sheep and seed-eating birds, thereby reducing the abundance of these native animals (Kurdila 1995). In general, large populations of horses and burros are cost the nation an estimated $5 million per year (Pimentel et al. 1998).

Pigs (Sus scrofa), native to Eurasia and North Africa, have been introduced into some U.S. parks for hunting, like the California coastal prairie, and have substantially changed the vegetation (Kotanen 1995). In Hawaii, more than 80% of the soil is bare in regions inhabited by pigs (Kurdila 1995). This disturbance allows annual plants to invade the overturned soil and intensifies soil erosion. Pig control per park in Hawaii (~1,500 pigs/park) (Stone et al. 1992), costs about $150,000 each year based on $100 spent per pig for control. Assuming that the 8 parks in Hawaii have similar pig control problems, the total is $1.2 million per year (R. Zuniga, 1998, personal communication. Cornell University, Ithaca, NY).

Pigs have also become a serious problem in Florida where the population has risen to more than 500,000 (Layne 1997). In Florida and elsewhere, pigs damage grain, peanut, soybean, cotton, hay, and various vegetable crops (Rollins 1998). Pigs also transmit and are reservoirs for serious diseases of humans and livestock, like brucellosis, pseudobrucellosis, and trichinosis (Davis 1998).

Nationwide, there are an estimated 2 million feral pigs (Mayer and Brisbin 1991). Based on control costs of $100 per pig (Katahira et al. 1993, Bach and Conner 1998), we estimate that attempts to control feral pigs costs at least $100 per pig per year. Assuming 2 million feral pigs inhabit the United States, the yearly cost is about $200 million per year. This is a conservative estimate because pigs cause significant environmental damages and diseases that can not be translated into dollar values.

Small Mammal introductions

Many small mammals have been introduced into the United States. These species include the European (black or tree) rat (Rattus rattus), Asiatic (Norway or brown) rat (Rattus norvegicus), house mouse (Mus musculus), European rabbit (Oryctolagus cuniculus), cat (Felis cattus), and dog (Canis familiaris) (Layne 1997).

Some rodents have become serious pests on farms, and in industries and homes (Layne 1997). On farms, rats and mice are particularly abundant and destructive. On poultry farms there is approximately 1 rat per 5 chickens (D. Pimentel unpublished). Using this ratio, the total rat population on U. S. poultry farms may easily number more than 1.4 billion (USDA 1996). Assuming that the number of rats per chicken has declined since these observations were made, we estimate that the number of rats on poultry and other farms is approximately 1 billion. With an estimated 1 rat per person (Wachtel and McNeely 1985), there are an estimated 250 million rats in U.S. urban and suburban areas (USBC 1996). Combined there is a total of approximately 1.25 billion rats in the United States.

If we assume, conservatively, that each adult rat consumes and/or destroys large amounts of grain (Chopra 1992, Ahmed et al. 1995) and other materials valued at $15/yr, then the total cost of destruction by rats in the United States is more than $19 billion per year. In addition, rats cause fires by gnawing electric wires, pollute foodstuffs, and act as vectors of several diseases including salmonellosis and leptospirosis and to a lesser degree plague and murine typhus (Richards 1989). They also negatively affect some native invertebrate and vertebrate species (Amarasekare 1993, Hadfield et al. 1993).

The Indian mongoose (Herpestes auropunctatus) was first introduced into Jamaica in 1872 for biological control of rats in sugarcane (Pimentel 1955). It was soon introduced to Puerto Rico, other West Indian Islands, and Hawaii. Moreover, the mongoose preyed heavily on ground nesting birds and subsequently reduced their numbers (Vilella and Zwank 1993). It also preyed on amphibians and reptiles beneficial in biocontrol pest control, causing a minimum of 7 to 12 extinctions in Puerto Rico and other islands of the West Indies (Henderson 1992). In addition, the mongoose emerged as the major vector and reservoir of rabies and leptospirosis in Puerto Rico and other Islands (Everard and Everard 1992). Based on public health damages, killing of poultry, extinctions of amphibians and reptiles, and destruction of native birds, we estimate that the mongoose in Puerto Rico and the Hawaiian Islands is causing approximately $50 million in damages each year (D. Pimentel, unpublished data; and David Foote, 1998, personal comunication, Hawaii Volcanoes National Park).

An estimated 63 million domestic cats now inhabit the United States (Nassar and Mosier 1991), plus as many as 30 million feral cats (Luoma 1997). In addition to preying on the introduced house mouse, cats prey on birds (Fitzgerald 1990), plus small mammals, amphibians, and reptiles. Estimates are that feral cats in Wisconsin and Virginia kill more than 3 million birds in each state per year (Luoma 1997). Assuming that the 63 million pet cats capture about one-quarter the number of birds as feral cats, then approximately 200 million birds are killed by cats annually. Based on a value of $30 per bird (Pimentel and Grenier 1997), total damage to U.S. bird population is approximately $6 billion per year. This cost does not include small mammals, amphibians, and reptiles that are killed by feral and pet cats (Dunn and Tessaglia 1994).

Dogs introduced into the United States have been domesticated but also have escaped into the wild. Some of these wild dogs run in packs and kill deer, rabbits, and domestic cattle, sheep, and goats. Carter (1990) reported that feral dog packs in Texas cause more than $5 million in livestock losses each year. Dog packs have also become a serious problem in Florida (Layne 1997). In addition to Texas, assuming $5 million damages for the other 49 states, total losses in livestock kills per year would be approximately $10 million per year.

An estimated 4.7 million people are bitten by dogs annually, with 800,000 cases requiring medical treatment (Sacks et al. 1996). Approximately 430,000 children are brought to hospital emergency rooms each year with wounds from dog bites and this amounts to $30 million/yr in treatment costs (August 1988). Assuming that the remaining 370,000 cases involve adults and their medical treatment is also $70 per attack, these added costs are about $26 million/yr. In addition, dog attacks cause between 11 and 14 deaths per year with 80% of the victims being small children (CDC 1997). If each of these deaths has a conservative value of $2.2 million the cost would be approximately $30 million per year (Pimentel and Grenier 1997).

About 97 of the 1,000 birds in the United States are NIS that have become established are song birds (Temple 1992). In Hawaii, 35 of 69 non-indigenous birds introduced between 1850 and 1984 are still extant on the islands (Moulton and Pimm 1983, Pimm 1991).

Of the 97 introduced bird species, only 5% are considered beneficial, while most (56%) are pests (Temple 1992). Several species, including chickens and doves, were introduced into the United States for agricultural purposes.

The common myna (Acridotheres tristis), introduced into Hawaii, helped control of pest cutworms and armyworms in sugarcane (Kurdila 1995). However, it became the major disperser of seeds of the introduced pest-weed, Lantana camara. To cope with the weed problem, Hawaii was forced to introduce insect-biocontrol agents (Kurdila 1995).

The English or house sparrow (Passer domesticus) was introduced into the United States intentionally in 1853 to control the canker worm (Laycock 1966, Roots 1976). By 1900, the birds were considered pests because they damage plants around homes and public buildings and consume wheat, corn, and buds of fruit trees (Laycock 1966). Furthermore, English sparrows harass robins, Baltimore orioles, yellow-billed cuckoos, and black-billed cuckoos (Laycock 1966). They also displace bluebirds, wrens, purple martins, and cliff swallows from their nest sites (Roots 1976, Long 1981), and are associated with the spread of about 29 diseases of human and domestic animals (Weber 1979).

Similarly, European starlings (Sturnus vulgaris) are serious pests and are estimated to occur at densities of more than 1 per ha in agricultural regions (Moore 1980). They are capable of destroying approximately $2,000/ha worth of cherries (Feare 1980). In grain fields, starlings consume about $6/ha of grain (Feare 1980). Therefore, assuming approximately $5/ha for all damages to agriculture crop production in the United States, the total loss would be approximately $800 million/yr.

In addition, the aggressive starlings have displaced numerous native birds (Laycock 1966). Starlings have been implicated in the transmission of 25 diseases including parrot fever and other diseases of humans (Laycock 1966, Weber 1979).

Thus, if we assume $800 million per year costs for starlings and additional $200 million for house sparrows and other pest birds (not including the common pigeon), the damages from non-indigenous pest birds are estimated to be $1 billion per year.

The domestic pigeon, (Columba livia), exists in most cities of the world including those in the United States (Robbins 1995). Pigeons are considered a nuisance for their fouling of buildings, statues, cars, and sometimes people, and for feeding on grain (Long 198, Smith 1992). The economic impacts caused by fouling are estimated to be at least $9 per pigeon per year (based on $9 control costs per pigeon per year [Haag-Wackernagel 1995]). Assuming there is 1 pigeon per ha in urban areas (Johnston and Janiga 1995) or approximately 0.5 pigeons per person in the urban areas, common pigeons are causing at least $1.1 billion in damages each year. This does not include their role as reservoirs for over 50 diseases, including parrot fever, ornithosis, histoplasmosis and encephalitis (Weber 1979, Long 1981)

Although amphibians and reptiles introduced into the United States number about 53, the negative ecological impacts of a few of these species have been enormous (McCoid and Kleberg 1995, Lafferty and Page 1997). All species occur in states where it seldom if ever freezes; Florida is now host to 30 species and Hawaii to 12.

The brown tree snake (Boiga irregularis) was accidentally introduced to snake-free Guam immediately after World War II when military equipment was moved onto Guam (Fritts and Rodda 1995). Soon the snake population reached densities of 100 per ha, and dramatically reduced native bird, mammal, lizard populations. Of the 13 species of native forest birds originally found on Guam, only 3 species still exist (Rodda et al. 1997); of the 12 native species of lizards, only 3 retain the possibility of surviving (Rodda et al. 1997). The snake eats chickens, eggs, and caged birds causing major problems to small farmers and pet owners. This snake, crawls up trees and utility poles, and has caused 1,500 power outages on the island. One island-wide power outage caused by the snake cost the power utility more than $250,000 (Teodosio 1987). Even local outages that affect business are estimated to cost from $2,000 to $10,000 per commercial customer (Coulehan 1987). With about 86 outages per year (BTSCP 1996), our estimate of the cost of power outages is conservatively $1 million/yr.

In addition, the brown tree snake is slightly venomous, and has caused public health problems, especially when it bites children. At one hospital emergency room the hospital treated about 26 people per year for snake bites (OTA 1993). Some bitten infants required hospitalization and intensive care; assuming that 13 cases are children and 13 are adults, total health costs might be approximately $25,000 per year (T. Fritts, 1998, personal communication. U.S. Geological Survey). The total costs of endangered species recovery efforts, environmental planning related to snake containment on Guam and other programs directly stemming from the snake's invasion of Guam constitute costs in excess of an additional $1 million per year; in addition, up to $2 million per year is invested in research and control of this serious pest (T. Fritts, 1998, personal communication. U.S. Geological Survey, 1998 federal budget). Hawaii's concern about the snake has prompted the federal government to invest $1.6 million per year in brown tree snake control (Holt 1997-1998).

A total of 138 non-indigenous fish species has been introduced into the United States (Courtenay et al. 1991, Courtenay 1993, 1997). Most introduced fishes have been established in states with mild climates, like Florida (50 species) (Courtenay 1997) and California (56 species) (Dill and Cordone 1997). In Hawaii, 33 non-indigenous freshwater fish species have become established (Maciolek 1984). Forty-four native species of fishes are threatened or endangered by non-indigenous fish (Wilcove and Bean 1994). An additional 27 native species of native fish species are negatively affected by introductions (Wilcove and Bean 1994).

Introduced fish species frequently alter the ecology of aquatic ecosystems. For instance, the grass carp (Ctenopharyngodon idella) reduces natural aquatic vegetation, while the common carp (Cyprinus carpio) reduces water quality by increasing turbidity. These changes have caused the extinctions of some native fish species (Taylor et al. 1984).

Although some native fish species are reduced in numbers, are forced to extinction, or hybridized, some benefits are measured by improved sport fishing. Sport fishing contributes $69 billion to the economy of the United States (Bjergo et al. 1995, USBC 1996). However, based on the more than 40 NIS that have damaged native fishes and other aquatic biota, we estimate that these economic costs to be conservatively more than $1 billion annually.

Approximately 4,500 arthropod species (2,582 species in Hawaii and approximately 2,000 in continental United States) have been introduced (see below). Also, 11 earthworm species (Hendrix 1995), and nearly 100 aquatic invertebrate species have been introduced (OTA 1993). More than 95% of these introductions were accidental, many gaining entrance via plants, and soil and water ballast from ships.

Insects and Mites

Approximately 500 non-indigenous insect and mite species are pests in crops, stored-food products, and structures. Hawaii has 5,246 identified native insect species and an additional 2,582 introduced insect species (Howarth 1990, Frank and McCoy 1995a, Eldredge and Miller 1997). Introduced insects account for 98% of the pest insects in the state (Beardsley 1991). In addition to Florida's 11,500 indigenous insect species, 949 immigrant species have invaded the state (42 species were introduced for biological control) (Frank and McCoy 1995b). In California, the 600 introduced species are responsible for 67% of all crop losses (Dowell and Krass 1992). Some of the California pests include the cottony cushion scale (Icerya purchasi) and alfalfa weevil (Hypera postica).

Each year pest insects destroy about 13% of potential crop production representing a value of about $36 billion in U.S. crops (Pimentel 1997). Considering that about 40% of the pests were introduced (Pimentel 1993), we estimate that these pests cause about $14 billion in crop losses each year. In addition, about $1.2 billion in pesticides are applied for all insect control each year (Pimentel 1997). The portion applied against introduced pest insects is approximately $0.5 billion. Therefore, the total cost for introduced non-indigenous insect pests is approximately $14.5 billion annually.

Lawn, garden, and golf course management activities cost about $36 billion annually (USBC 1996). Most of this cost is related to the expenses of pest control. Assuming the presence of 40% non-indigenous insect pests, we estimate the control costs in lawns, gardens, and golf courses to be at least $1.5 billion each year.

About 360 non-indigenous insect species have become established in American forests (Liebold et al. 1995). Approximately 30% of these are now serious pests. Insects cause the loss of approximately 9% of forest products amounting to $7 billion per year (Hall and Moody 1994, USBC 1996). Because 30% of the pests are non-indigenous pests, annual losses attributed to NIS is about $2.1 billion per year.

The gypsy moth (Lymantria dispar), intentionally introduced into Massachusetts in the 1800s, has developed into a major pest of U.S. forests and ornamental trees, especially oaks (Campbell and Schlarbaum 1994). The U.S. Forest Service currently spends about $11 million annually on gypsy moth control (Campbell and Schlarbaum 1994).

The introduced balsam woolly adelgid (Adelges piceae) inflicts severe damage in balsam-fir natural forest ecosystems (Jenkins 1998). According to Alsop and Laughlin (1991), this aphid is destroying the old-growth spruce-fir forest in many regions. Over about a 20-year period, it has spread thoughout the southern Appalachians and has destroyed up to 95% of the fraser firs (H.S. Neufeld, 1998, personal communication. Appalachian State University). Alsop and Laughlin (1991) report the loss of 2 native bird species and the invasion by 3 other species as a result of adelegid-mediated forest death.

Other introduced insect species have become pests of livestock and wildlife. For example, the red imported fire ant (Solenopsis invicta) kills poultry chicks, lizards, snakes, and ground nesting birds (Vinson 1994). A 34% decrease in swallow nesting success was reported as well as a decline in the northern bobwhite quail populations due to ants (Allen et al. 1995). Fire ants can be extremely abundant with as many as 367 nests per ha (Banks et al. 1991). The estimated damage to livestock, wildlife, and public health caused by fire ants in Texas is estimated to be $300 million/yr. An additional $200 million is invested in control per year (Vinson 1992, TAES 1998). Assuming equal damages in other infested southern states, the fire ant damages total approximately $1 billion per year.

Green Crab

The European green crab (Carcinus maenas) has been associated with the demise of the soft shell clam industry in New England and Nova Scotia (Lafferty and Kuris 1996). It also has destroyed commercial shellfish beds and preys on large numbers of native oysters and crabs (Lafferty and Kuris 1996). The annual estimated economic impact of the green crab is $44 million/yr (Lafferty and Kuris 1996).

Non-Indigenous Mollusk

Eighty-eight species of mollusks have been introduced and established in the U. S. aquatic ecosystems (OTA 1993). Two of the most serious pests are the zebra mussel (Dreissena polymorpha) and Asian clam (Corbicula fluminea).

The European zebra mussel was first found in Lake St. Clair having gained entrance via ballast water released in the Great Lakes from ships that traveled from Europe (Benson and Boydstun 1995). The zebra mussel has spread into most of the aquatic ecosystems in eastern United States and is expected to invade most freshwater habitats throughout the nation (Benson and Boydstun 1995). Large mussel populations increase water clarity and reduce food and oxygen for native fauna. In addition, zebra mussels have been observed completely covering native mussels, clams, and snails, thereby further threatening their survival (Benson and Boydstun 1995, Keniry and Marsden 1995). Mussel densities have been recorded as high as 700,000/m2 (Griffiths et al. 1991). Zebra mussels also invade and clog water intake pipes and water filtration and electric generating plants, for which an estimated $3 billion is spent annually for control and cleaning (OTA 1993).

Though the Asian clam grows and disperses less rapidly than the zebra mussel, it too, is causing significant fouling problems and is threatening native species. Costs associated with its damage are about $1 billion per year (Isom 1986, OTA 1993).

The introduced shipworm (Teredo navalis) in the San Francisco Bay has caused serious damage since the early 1990's. Currently, damages are estimated to be about $205 million/yr (Cohen and Carlton 1995).