Expert judgment of a species’ history in new ranges has several limitations. First, expert judgment is subjective and given the same information, experts’ evaluations of the threat of an invasion can differ. Second, some species considered potential invaders in the United States simply on the basis of their record elsewhere have yet to become problems here despite their repeated entry (Reed 1977). The failure so far to be a problem has multiple potential causes. On the basis of the information in Chapters 3 and 4, it is apparent that there is a high degree of stochasticity in the outcome of any immigration. A species with a record of invasion could eventually become invasive in an additional new range once stochastic forces were overcome or avoided. Finally, a further dilemma is posed by arthropod and plant pathogens that cause little or no recognizable damage to plants in their native habitats and therefore have been ignored but have the potential to cause substantial damage to those plants’ susceptible North American relatives.

PREDICTION BASED ON CLIMATE-MATCHING

Even before ecology was formalized as a science, the ability to predict the geographic and ecological ranges of species by using climatic similarities was actively sought (von Humboldt and Bonpland 1807, Grace 1987). The extension of this reasoning to predict the potential new range of an introduced species and even whether it would survive at all in a new locale has long been practiced (Johnston 1924, Wilson et al. 1992). Such reasoning is supported by comparison of the climates in native and naturalized ranges for some species. Most striking in this regard is the extensive list of plant species that are native along the border of the Mediterranean Basin and are now naturalized in locales with climates similar to the Mediterranean Region: coastal areas of southern California and Chile, southwest Australia, and the Cape of Good Hope. Furthermore, species from each of the other four regions have become naturalized in one or more of the others (Kruger et al. 1989, Fox 1990).

There have been repeated attempts to develop “climate-matching” models by comparing the voluminous records of the earth’s climates collected at numerous locations (Busby 1986, Panetta and Dodd 1987, Panetta and Mitchell, 1991, Cramer and Solomon 1993). The challenge (aside from verifying the accuracy of data from meteorological stations scattered around the globe) has been to integrate these data in an ecologically meaningful way. More important, weather is the stochastic expression of climate, and the persistence of organisms, whether native or introduced, is much influenced by this variation about the mean characteristics of any climate (Mack 1995).

CLIMEX is one example of the models developed to predict the potential ranges of introduced species (Sutherst et al. 1999). The model in its revised forms has been used repeatedly to predict the geographic distributions of plants, microbial pathogens, and arthropods under both current conditions and global



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