FIGURE 7.1 Exponential increase in the publication of books about exotic species. (See Appendix 1 of this chapter for a list of titles.)

as the National Invasive Species Act of 1996 (Blankenship, 1996) and the proposed Species Protection and Conservation of the Environment Act (Paul, 2002).

Although interest and concern about exotics have recently exploded, the problem is an old one, probably as old as human civilization. For example, Haemig (1978) describes introductions by pre-Colombian people in Mexico several thousand years ago. Modern awareness about exotic species as an environmental impact dates to

Charles Elton's monograph from 1958. Elton defined biological invasions as occurring when species move from an area where they evolved to an area where they did not evolve, and this still may be the best definition of the concept. Although some of the approaches Elton used to explain invasions may be outdated by standards of current ecological theory, his book was clearly far ahead of its time. Recent interest in exotics by ecologists dates to the 1970s when W. E. Odum coined the term living pollutants to describe the problem (W. E. Odum, 1974). Also, Courtenay and Robins published what may be the first general paper on exotics in 1975. Finally, Holm et al. (1977) may have presaged the Nature Conservancy's Dirty Dozen list of exotics with their listing of "The World's Worst Weeds."

The greatest fear from exotics for environmentalists, conservation biologists, and natural resource managers is "the homogenization of the world" (Culotta, 1991; Lockwood and McKinney, 2001). In this view a relatively few exotics spread throughout the world's ecosystems reducing native biodiversity. This phenomenon has already occurred with humans, who are exotics in most ecosystems. The fear of homogenization of the world's biodiversity seems real as exotics are clearly occurring as a global environmental problem (Schmitz and Simberloff, 1997; Soule, 1990; Vitousek et al., 1996). This fear cannot be denied but there is still much to understand about the ecology of exotic invasions. For example, MacDonald and Cooper (1995) suggest that alien-dominated ecosystems may be unstable over long time periods and therefore perhaps only a temporary problem. Many new ecosystems, which need to be described and explained, are being formed by the combination of exotics and natives. The prevailing view of exotics as negative additions to ecosystems has been accepted rather uncritically by the scientific majority, and the small amount of published literature on any controversy has been largely ignored (Lugo, 1988, 1990, 1994). Alternative views of exotic species can be imagined (Table 7.2) and some of these are examined in this chapter. The study of exotic species seems to be a wave of the future, and it will be a challenge to ecological theory for some time.

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