A though plants and animals have always spread across the globe, expanding their ranges and invading new territories, human civilization has accelerated these biological invasions to an extraordinary degree. Nonindigenous species represent a major threat to the integrity of natural systems, because they can alter nutrient regimes, develop into monocultures, and drive native species to extinction (e.g., the nitrogen-fixing tree Myrica faya [Vitousek and Walker 1989], the zebra mussel Dreissena polymorpha [Mills et al. 1994, Roberts 1990], the purple loosestrife Lythrum salicaria [Mercer 1990], and the brown tree snake Boiga irregularis [Burdick 1994]). Although the US government recognizes the risks posed by nonindigenous species and for nearly a century has had legislation in place aimed at reducing harm-