new environments, and novel circumstances that can produce biological invasion. The last 10 years has seen the emergence of a broad consensus that the prediction of biological invasion is of pressing national need (U.S. Congress 1993, Clinton 1999). It will take some time, however, to generate the predictive principles on which policy-makers, regulators, the scientific community and the public can have confidence.

On the basis on the current state of prediction of the fate and effect of nonindigenous species, the committee concludes that action needs to be initiated now to replace the largely retrospective or anecdote-based prediction process with a system derived from experimentation. (In a larger sense, science has always progressed in this manner—first a long period of natural history and observation, and then continuing, deliberate, question-based experimentation.) This sense of urgency in moving in a deliberate manner from expert judgment to prediction on the basis of experimentation is driven by the pressing societal need to deal fairly, adequately, and swiftly with an unparalleled volume and scope of trade involving nonindigenous species. These are policy issues, based in science, that cannot be left lingering and cannot be resolved through happenstance investigation.