climate-change scenarios (Sutherst et al. 1989, 1998, 1999). Its most extensive applications have been useful for predicting the fate of immigrant species that are candidates for release in biological control (by identifying possible sites to collect biocontrol species and sites where they might be released and persist), and in predicting the fate and new ranges of introduced pests (Sutherst 1991a,b).

In its basic composition, CLIMEX contains an open-ended meteorological database of about 2500 locations that span the world. Meteorological data include average monthly maximal and minimal temperatures, relative humidity, and precipitation. The model interpolates monthly average climatic data into weekly values. CLIMEX has two main operating modes: “Match Climates” and “Compare Locations”. The “Match Climates” mode compares meteorological conditions among locations without reference to species. Either a specific meteorological variable (such as minimal winter temperature) or a suite of meteorological variables can be compared between locations for a specific period or the entire year. The model allows users to search the meteorological database in asking “Does Location X [a potential new locale] have a climate similar to that of Location Y [a species’ native or other locale] under preset standards of similarity?” That basic task is common to many climate-matching software programs (Kriticos and Randall 2001).

The “Compare Locations” function is more applicable to the common need in predicting future invaders: it is used to predict potential distributions of species under current climatic regimes. Predictions are based on the climate in a species’ current range, on experimentally determined tolerances of a species with respect to key environmental characteristics (for example, Vickery 1974), and on life-history information, such as senescence or diapause. Information on a species is used to find other locales in which the species potentially could persist.

A major limitation in the use of climate-matching (to compare a current range with a potential range) as a predictor of a species’ potential new range(s) lies in the assumption that climate is the main, if not the only, determinant of a species’ distribution. Distributions are also strongly influenced by the biotic component of any environment (Crawley 1992, Mack 1996a) and by chance dispersal; that is, a species might be absent from a locale (even in the general native range) through the vagaries of species dispersal rather than through environmental limitation (Sutherst and Maywald 1985, Davis et al. 1998). Furthermore, this system does not attempt to measure the impact of a species in its new range.

In addition, some species’ distributions are comparatively uniform or at least quite narrowly defined: the climatic range that they can tolerate is not reflected in their native or current new distributions. For example, Sorghum halepense (Johnson grass) has extended its range into southern Canada, thereby far exceeding the climatic range that would have been predicted for it by considering its native subtropical range (Warwick et al. 1984). Such complications diminish or

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