ABSTRACT
When species move (or are moved) from their natural ranges to new areas and become established, they are variously termed “nonindigenous,” “alien,” “introduced,” or “invasive”; the technical or legal de¢nitions for these terms are still in developmental stages. In the United States, an invasive species was legally de¢ned by executive order in 1999 as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health. (p. 6186)” This limiting de¢nition has apparently worked for legal purposes but has made the technical literature somewhat confusing. Some authors have used “invasive” and various synonyms to focus on injurious species not native to the United States; others include indigenous species introduced to new ecosystems with the potential to cause harm,1,2 and still others have focused on all nonindigenous species, including those presumed innocuous or desirable.3
