because economic and environmental factors limit eradication or control options that may be appropriate in agricultural settings.

Only a small fraction of introductions of nonindigenous species result in invasions in the United States, but it is not obvious which nonindigenous plants, pathogens, and arthropods are benign and which will become important pests. Moreover, resources are not available to detect the introduction of every nonindigenous pathogen and arthropod or to monitor the fate of every imported plant, so alternative strategies to identify and eliminate pests are needed. The U.S. Department of Agriculture (USDA) asked the National Research Council’s Board on Agriculture and Natural Resources (BANR) to examine what is known about nonindigenous plant pests so that it could be determined whether there is sufficient information to list the species that are potential invaders in the United States. To study the issue, BANR created the Committee on the Scientific Basis of Predicting the Invasive Potential of Nonindigenous Plants and Plant Pests in the United States. The committee was charged to

  • Consider the historical record of weed, pathogen, and arthropod invaders, including pathways of their introduction.

  • Identify and analyze circumstances that could allow nonindigenous species to become invaders, considering the biotic and abiotic characteristics of potentially affected ecosystems, including agricultural systems, and the characteristics of nonindigenous plant pests that contribute to their potential invasiveness.

  • Determine the extent to which scientific principles and procedures can characterize the invasive potential of nonindigenous plant pests and determine the degree of uncertainty intrinsic in such characterizations.

  • Identify research that should be conducted to enhance the prediction of invasiveness.

The committee’s study assesses the state of knowledge about biological invasions of nonindigenous plant pests, examines current capabilities to predict invasions and the identity or characteristics of invaders, and recommends ways to improve those capabilities. The committee was not charged with evaluating current government practices or suggesting policy; therefore, it has limited its comments on the regulatory activities and other functions of federal agencies to the extent to which they can or do contribute to the scientific basis of predicting the invasive potential of nonindigenous plants, pathogens, and arthropods.

PREDICTIBILITY OF THE INVASION PROCESS

Few arriving populations become established, even fewer populations of established nonindigenous organisms expand and spread dramatically, and the environmental and economic impacts of the ones that do spread vary widely. The events that take place during the transition between the phases of the invasion



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