keywords invasive species, ecological niche models, geographic information systems, predictive modeling, niche evolution abstract
Species' invasions have long been regarded as enormously complex processes, so complex as to defy predictivity. Phases of this process, however, are emerging as highly predictable: the potential geographic course of an invasion can be anticipated with high precision based on the ecological niche characteristics of a species in its native geographic distributional area. This predictivity depends on the premise that ecological niches constitute long-term stable constraints on the potential geographic distributions of species, for which a sizeable body of evidence is accumulating. Hence, although the entire invasion process is indeed complex, the geographic course that invasions are able to take can be anticipated with considerable confidence.I NVASIVE SPECIES have become an issue of great concern in fields as diverse as biology, agriculture, transportation, and economics (Carlton 1996;Kareiva 1996;Williamson 1996Williamson , 1999Enserink 1999;Higgins et al. 1999; NAS 2002). The basic point is that with ever-increasing scales of human movement, species are finding ever-increasing opportunities to move as well, resulting in many introductions of species to novel landscapes (NAS 2002). Although many colonizations fail (Williamson and Fitter 1996), for reasons of demography or of chance, this increased opportunity for colonization certainly plays a major role in increasing numbers of successful invasions.Several reviews of the natural history and ecology of invasive species have been conducted in the hope of identifying factors key in determining the success of invasions that could serve as the basis for a predictive understanding of which species represent potential invaders (Honig et al. 1992;Perrins et al. 1992;Scott and Panetta 1993;Lonsdale 1994;Carlton 1996;Rejmánek and Richardson 1996;Williamson 1996Williamson , 1999Reichard and Hamilton 1997;Ricciardi and Rasmussen 1998;Enserink 1999; NAS 2002). Unfortunately, the list of such identified key factors is short, consisting principally of characters associated with weedy habit and prior history of successful invasion. More than anything, with a few exceptions (Rejmánek and Richardson 1996), the conclusion has been that 420Volume 78 THE QUARTERLY REVIEW OF BIOLOGY few or no factors allow consistent prediction a priori of success or failure of invasions. As a result, efforts to combat species' invasions have been largely reactive in nature: a new invader arrives, and a plan must then be developed to combat it.A distinct approach toward predicting the behavior of invasions, however, can be referred to as "climate-matching" (NAS 2002). This approach is based on the concept of ecological niches as a constraint on the distributional potential of species (Grinnell 1917(Grinnell , 1924, and involves a logical extension of the basic niche concept-that species will be able to establish populations only in areas that match the set of ecological c...