1. It must be transparent, be open to review, and have been evaluated by peers.

  2. It must have a logical framework that includes independent factors— identified through critical observation, experimentation, or both—important in the invasion process.

  3. Use of the framework must be repeatable and lead to the same outcome, regardless of who makes the predictions.

This chapter examines approaches for predicting the invasiveness of plants and the arthropod and pathogenic pests of plants in the United States. Most predictive systems rely principally on observational data (sometimes coupled with traits, origins, and the like). The data may be grouped taxonomically on coarse or fine levels (for example, all plants, animals, or microorganisms; or according to family, genus, or species), by characteristic (such as reproductive mode, mode of propagation, or environmental range), or by locale and climate of origin. The committee suggests a broader framework for organizing information used in predictive systems; the framework is based on the sequential steps of invasion: arrival, persistence or establishment, and proliferation and spread.

This chapter also addresses systems of prediction that are augmented by an assessment of the value or character of invasions and an assessment of the certainty of or confidence in their occurrence. Predictions based on this broader dimension of information and analysis are risk assessments, specifically ecological risk assessments.

Predicting biotic invasions has been based largely on identification of species that already have a record of invasiveness. This approach seems almost obvious, especially for plant pathogens. For example, the fungal pathogens of cereals are equally likely to be infectious on a given wheat variety in the United States, in Australia, and in western Europe. It would be imprudent to assume otherwise unless there were specific information to the contrary. Similar examples occur for arthropod pests. Even though the Russian wheat aphid has not yet produced the devastation in the United States that it produces elsewhere, it is a pest of wheat in many new ranges (Hughes 1996). No prolonged monitoring was deemed necessary to enforce control of the aphid once it appeared in the United States. Plant species known to be invasive have routinely sparked awareness of their invasive potential in unexploited new ranges. The World’s Worst Weeds, the compendium by Holm et al. (1991), is empirical testimony to the recurrence of some species as invaders in many ranges. The first detection of one of these species in a new range should spark immediate eradication efforts. The record of a plant’s invasiveness in other geographic areas beyond its native range remains the most reliable predictor of its ability to establish and invade. The same is true for arthropods and pathogens if host plants they can use elsewhere also occur in the United States.

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