Although many nonindigenous species arrive in new ranges, few persist; Chapter 3 examines this important issue. Understanding the processes that allow persistence can have important implications for restricting the establishment of nonindigenous species before they become invasive. In Chapter 4, the committee examines the forces that facilitate or impede the proliferation and geographic spread of a plant pest, a stage at which a species can be said to be invasive. For most organisms, this stage is often the first process that can be observed directly; arrival and persistence usually occur without detection. Invasion is also the stage in which organisms can exhibit adverse impacts on the surrounding plant life and the ecosystem in general. The impacts of invasions have received the most attention by researchers and are examined in Chapter 5.

There is some overlap between components of this analysis, but the approach nonetheless serves as a common denominator among taxa, whether they are plants, arthropods, or plant pathogens. Even in the face of terminological, theoretical, and methodological differences among the scientific disciplines that study these organisms, the conceptual model allowed us to acknowledge the differences and facilitate their integration. In Chapter 6, the committee examines predictive methods and provides its own set of questions as a framework for using scientific information to predict the invasive potential of plants and plant pests. The chapter summarizes the ability to use scientific principles and processes to predict invasiveness and discusses implications of that capability for risk assessment.

Summaries of key scientific findings are listed at the ends of Chapters 2-6. In Chapter 7, the committee presents its overall conclusions and makes recommendations regarding research that is needed to improve the ability to predict invasiveness.

This report focuses on the threats of introduced plant pests to the plant resources of the United States. It does not explicitly include genetically modified organisms, although much of the same analysis would be relevant. Vertebrate pests are not included in the committee’s charge, and aquatic plant resources, such as aquatic ecosystems and submerged aquatic vegetation, have largely been excluded. Otherwise, all forms of vascular plants–whether in agriculture, in forestry, or in natural areas in the United States–are topics of concern in this report.

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