ABSTRACT
The invasion of ecosystems by exotic species is a major environmental problem that has become widely recognized (Culotta, 1991; Mack et al., 2000; Malakoff, 1999). This phenomenon is occurring globally and causing changes to ecosystems, along with associated economic impacts. The most important issue with the invasion of exotics is the replacement of native species, in terms of either reduction of their relative abundance or, in the extreme, their outright extinction. Associated costs to human economies from the invasion of exotics include losses of value derived from the natives they replace, direct damages caused by them, and expenditures for control programs directed at exotics (Pimentel et al., 2000). The invasion of exotic species occurs because of introduction by humans, either intentionally or unintentionally. Of course, intentional introductions are undertaken in an effort to add a useful species to an ecosystem, and there are positive examples of this action such as the introduction of honey bees as a pollinator for crop species. Problems arise, however, when intentionally introduced species take on unintended, expanded, and negative roles in ecosystems or when this occurs with unintentional introductions.
