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.

Perhaps because it is an environmental problem caused by excessive growth or "biology gone wrong," the invasion of exotics has become sensationalized by environmentalists and the news media with seemingly good reason. This situation is reflected in titles of news stories about exotics such as "Unstoppable Seaweed Becomes Monster of the Deep" (Simmons, 1997) and other evocative descriptions such as "the Frankenstein effect" (Moyle et al., 1986) and the need to consider exotics as "guilty until proven innocent" (Ruesink et al., 1995; Simberloff and Stiling, 1996). A further example is the announcement of "America's Least Wanted" (Table 7.1), which is a list of the dirty dozen of the country's worst exotics, according to the Nature Conservancy (Flack and Furrlow, 1996). The problem of invasion of exotics has captured the imagination of the public and the scientific community and is receiving greater and greater attention. Figure 7.1 illustrates this growing interest by plotting the number of books published on exotics by decade since World War II (Appendix 1). Although this listing may not be complete, the pattern is clear with relatively little publishing until the 1980s and especially the 1990s when there was an explosion of writing about exotics. This growing literature includes mostly the standard scientific writing but also popular books (e.g., Bright, 1998), books commissioned by the federal government (National Research Council [NRC], 1996a; Office of Technology Assessment, 1993), and even a children's book (Lesinski, 1996). The latter clearly reflects a trickle-down effect and a growing awareness of the issue. This trend is also seen in a growing body of policy and legislation such

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