Why are alien species regarded as an environmental problem? As the case studies show, while some alien species have negative impacts, others have mixed or even highly positive impacts. One problem with alien species is that their impacts are difficult to predict, so that species thought of as useful have turned out to be pests (for example, water chestnut, carp). Even worse, the impacts of alien species have scarcely been considered when conducting activities that bring in alien species (for example, shipping, canal-building, the pet trade). Thus, we have been flooded with a largely indiscriminant (from the point of view of impacts) group of alien species. Alien species have high risks of undesirable impacts; the Office of Technology Assessment (1993) estimated that one-third of alien species in North America have been harmful. The problem with alien species is not so much that some species have undesirable impacts as that the human activities that bring in species do not adequately separate the desirable from the undesirable species. Because many alien species have large, probably irreversible impacts, the absence of sound controls and screening of species introductions has serious long-term ecological consequences.
What routes are open to reduce the undesirable effects of alien species? Three lines of action should be pursued: (1) selective control or eradication of aliens with clearly undesirable impacts, in cases where such programs are economically sensible and environmentally acceptable; (2) aggressively reducing the numbers of aliens that are unintentionally carried around the globe by humans; and
(3) adopting more stringent criteria for allowing the intentional introduction of species.
Once alien species become firmly established, they usually are difficult to control or eradicate. Biological control of alien species has been successful in some cases. In biological control, enemies (predators, parasites, competitors) of the pest are introduced or encouraged. Biological control is attractive because it can suppress a pest over large areas and long periods of time without harmful chemicals, but requires careful matching of the enemy to the target pest species. Biological control is being attempted or considered for pest species like purple loosestrife (Malecki et al., 1993), water chestnut, and the zebra mussel (Molloy et al., 1997). Nevertheless, biological control has been successful in only 10-20 percent of the cases in which it has been attempted, and is never attempted in many cases, especially for aquatic animals. Further, the longstanding image of biological control as environmentally benign has recently been challenged by critics who suggest that effects of biological control agents on non-target species has been vastly understated (e.g., Strong and Pember-ton, 2000; Henneman and Memmott, 2001).
It usually is simpler and far more effective to prevent the arrival of an alien species than to control it after it is established. The major vectors that bring aquatic aliens into the Northeast today are ballast water and unintended releases of species used for pets, bait, and aquaculture, both of which could be brought under better control. Ballast water management is currently an active area of policy change and research (Carlton and Holohan, 1998). Ballast water is water that is taken on by ships to improve their stability and performance. Because ships carry large volumes of ballast water, which is not usually treated to exclude or kill organisms, ballast water is a major vector for species introductions worldwide (e.g., Carlton andGeller, 1993;National Research Council, 1996). Currently, under the National Invasive Species Act of 1996, ballast water of ships entering the Great Lakes or Hudson River (above the George Washington Bridge) must be treated to kill organisms, retained within the ship, or exchanged in the open ocean, which prevents spread of freshwater organisms. Ships entering other parts of the United States are asked to participate in a voluntary ballast-water exchange program. Further, research is pro ceeding on better ways to prevent moving organisms around in ballast water (Carlton and Holohan,
1998). Wider application of existing methods of ballast management and development of better methods of ballast management could substantially reduce the number ofnewinvasions of aquatic aliens.
The second major group of vectors includes releases leading to unintended establishment of alien species, including the release of unwanted pets, the release of unused bait, and the escape of organisms from aquaculture. Plants and animals sold as pets may be released into the wild when the owner tires of them or they outgrow the aquarium. Many species have been established in North America as a result of such releases (Crossman and Cudmore, 1999a; Mackie, 1999). Anglers sometimes release unused bait at the end of a day of fishing. These organisms (which may include contaminant species other than those purchased) may establish a self-sustaining population (Litvak and Mandrak, 1999; Goodchild, 1999). For example, the European rudd is now appearing widely through eastern and central North America as a result of bait-bucket releases (Fuller et al., 1999). Releases from aquaculture may occur when the animal that is being raised escapes from captivity (Crossman and Cudmore, 1999b). For example, three species of carp (grass, silver, and bighead carp) have established breeding populations in North America, probably from animals that escaped from cultivation (Fuller et al.,
1999). Alternatively, aquaculturists may inadvertently bring in undesirable aliens with the species that are intended to be cultured. Throughout the world, many species have been transported with living oysters or with oyster shells used to reestablish oyster beds (Carlton, 1992). Thus, attempts to reinvigorate oyster populations in the lower Hudson by bringing large volumes of old shell into New York Harbor (Revkin, 1999) may accidentally bring more alien species into the river. It may be possible to reduce rates of these unintentional introductions with better laws and improved enforcement of existing laws about the pet, bait, and aquaculture trades. Ultimately, though, reducing inadvertent introduction of aliens through releases will require better education of the public as to the risks of introducing alien species. People need to realize that releasing foreign plants and animals into the wild is an act of environmental recklessness comparable to tossing a lighted match into a forest.
Next, we need better controls over the deliberate establishment of alien species. Many species thatwere deliberately established in North America have had undesirable effects. In fact, the Office of Technology Assessment (1993) estimated that negative effects resulted as frequentlyfrom species that were intentionally introduced as from those that were unintentionally brought to North America! This suggests that past screening procedures have been nearly useless. The problem lies partly with the weakness of existing laws and screening procedures (Ruesink et al., 1995), which tend to allow importation of alien species if they are not known to have deleterious properties (i.e., the species are considered to be innocent until proven guilty). The case of the black carp (Mylopharyngodon piceus) is a particularly appalling example of the shortcomings of existing legal controls on species importation into the United States (Ferber, 2001; Williams, 2001). This species, which probably will escape from cultivation and establish wild populations throughout North America, was allowed into the United States on the authority of the Mississippi Department of Agriculture, despite strong opposition from twenty-eight states in the Mississippi River basin and several groups of professional biologists and a negative risk assessment by the United States Fish and Wildlife Service (Nico and Williams, 1996). More effective controlover undesirable alien species canbe achieved if species are imported only after being shown that the risk of deleterious effects is minimal (e.g., Townsend andWinterbourne, 1992).
Finally, attempts to deal with alien species nationally or internationally have been hampered by the patchwork of state and Federal programs, usually not coordinated with one another, that claim authority over various aspects of alienspecies management (Ruesink et al., 1995). A recent executive order (number 13112, signed 3 February 1999) establishing the Invasive Species Council to expand and coordinate programs of Federal agencies to combat alien species could be a step in the right direction.
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