importation of a commodity or to impose phytosanitary requirements, such as fumigation, that could exterminate hitchhiking pests. Risk assessments are most reliable when the identity and biological characteristics of a pest likely to be associated with a commodity are well documented and when the availability and vulnerability of potentially affected plants and their communities in the United States are known. Risk assessments are less reliable when it is not known which species might accompany a commodity or when known organisms are likely to be present but have an unknown capability to invade ecosystems and to harm plants in the United States. Even when much is known about the biological characteristics of a species in its native range, it can be difficult to predict the results of its entry into a new range. Intentional introductions of nonindigenous species for biological control of some specific pest, for example, have on occasion resulted in the introduced species’ unexpectedly attacking species in addition to the targeted ones. Biological control species sometimes compete with and displace native species and have other unexpected effects (Louda et al. 1997, Lockwood et al. 2001). Consequently, release of these organisms requires careful prerelease screening and postrelease evaluation.

A parallel problem is posed by the importation of nonindigenous plant cuttings and seeds for horticulture and other uses. Some plants known to be invasive are on the federal noxious weed list, and these species are consequently prohibited from importation. If no information on their invasive potential is available, propagative materials are allowed to enter virtually without restriction.

The importation of propagative materials and commodities in the absence of definitive knowledge of their invasiveness weakens or even undermines the national system of plant protection. Even at the outset of any examination of these issues, it is clear that the predictability of invasiveness requires more scientific information and more effective use of existing information. The capability to use scientific principles and procedures to make such a prediction is urgently needed, not only to protect plant life, but also to fulfill U.S. commitments under international trade agreements.

The United States helped to establish the World Trade Organization (WTO) in 1995 to liberalize world trade opportunities. With more than 142 nation members, WTO maintains a code of trade rules and is a forum for discussing and adjudicating trade disputes. Disputes between nations can involve market access, export subsidies, domestic support schemes, and the sanitary and phytosanitary measures used by nations to dictate whether foreign goods may enter their borders on the basis of concerns for the health and safety of their citizens, plants, and animals. Before the creation of WTO, no international rules distinguished legitimate importation restrictions from trade barriers erected under the guise of protecting health or safety. Under WTO, the exclusion of any trade item must have a scientific basis (USDA/FAS 1999). The requirements apply as well to controls that the United States places on the movement of materials to protect this country from invasive plants and plant pests.

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