Biological Invasions: A Growing Threat

[Guest guest]

http://www.nap.edu/issues/13.4/schmit.htm

Don C. Schmitz

Simberloff

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. 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 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.

After habitat destruction introduced species are the second greatest cause

of species endangerment and decline worldwide.

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.

Federal failure

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 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.

About a fourth of U.S. agricultural GNP is lost to foreign plant invaders

and the cost of controlling them.

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.

The CDC's management of human pathogens could serve as a model for

controlling invasive species.

ation 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. ation 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.

Federal leadership

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.

Recommended reading

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. on, 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.

Simberloff is 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)