tual basis exists for understanding invasions that could be developed into predictive principles.

The committee was not convinced that broadly successful methods for predicting invasiveness have been established. No reliable, all-inclusive, and tested list of potential invaders has been developed for plants, arthropods, or plant pathogens, nor has sufficient requisite information (species traits, circumstances of species’ entrance, persistence, spread, rankings of new ranges by degree of vulnerability) been assembled to permit the identification of invaders. And there is not adequate information to quantify the uncertainties in the available predictive schemes.

Expert judgment—the synthetic ability of investigators who have long experience in monitoring the fate of species in new ranges—is now used to bridge the gap between a body of largely qualitative data and a determination of the probability of an organism’s arrival and ability to establish. The collective experience of experts suggests that there is a conceptual basis for understanding invasions; the challenge is to transform judgment into transparent, repeatable, quantitative, and comprehensive predictive capability.

Conclusion 3. The inability to predict accurately which nonindigenous species will become invasive stems from a lack of comprehensive knowledge of the events that dictate species’ immigration (arrival), persistence (survival), and invasion (proliferation and spread) in new environments. The requisite knowledge would be based on critical observation of the natural history of nonindigenous species and experiments designed specifically to evaluate nonindigenous species in the stochastic environments they encounter in new ranges.

It is not now possible for experts or predictive systems to be more unequivocal or definitive about invasiveness, because only part of the needed information is available. The committee found that specific, relevant information on the performance of species in new ranges is lacking, especially the results from experiments designed to evaluate the ability of nonindigenous species to persist and proliferate in a broad range of environmental conditions. Heretofore there has been insufficient attention to the fate of immigrant populations in demographic terms, except for scattered accounts of organisms released as agents of biological control and some plant pathogens.

Conclusion 4. Some data on the natural history of plant pests exist, but they often reside in grey literature and in datasets that are not easily accessible. Data on events that potentially lead to invasions are frequently collected by federal or state agencies in the course of pest surveys and inspections and after releases of biocontrol agents. Such collections need to be more comprehensive and need to be implemented on a quantitative, statistically sound