with publication of the highly influential symposium volume The Genetics of Colonizing Species, edited by Baker and Stebbins (1965). Contributors to that volume provided much insight in identifying ecological and genetic characteristics most commonly found in invading species.
Baker (1965, 1974) compiled a list of traits found in the “ideal weed” (which is roughly analogous to the plant invaders dealt with here), including rapid development to reproductive maturity, high reproductive capacity, small and easily dispersed seeds, broad environmental tolerance, and high phenotypic plasticity. As Mack (1996b) has pointed out, the potential value of such characteristics to an invader are readily perceived, and many of the world’s plant invaders do indeed possess some of them (reviewed in Brown and Marshall 1986). However, many species with no record of producing an invasion also possess many of the traits listed by Baker. In addition, the failure of surveys among floras to find support for “ideal weed” characteristics among invaders have led to challenges to the strength of earlier generalizations about the traits associated with plant invaders (Noble 1989, Perrins et al 1992a,b, Williamson 1996). Weeds that are invasive but have none of the traits attributed to invasiveness include Watsonia bulbillifera (wild watsonia) and Homeria miniata (two-leaf cape tulip) in Australia (Parsons and Cuthbertson 1992) and Gunnera tinctoria in Ireland (Clement and Foster 1994). Baker’s list was also heavily biased toward traits commonly found in weeds of agricultural and ruderal habitats. Those habitats make up only a portion of the diverse environments invaded by plants; not surprisingly, diverse colonizing strategies are associated with invaders.
The shortcomings of attempts to use traits as a predictive tool have fueled skepticism about whether it would ever be possible to predict successfully which species would become invaders (Crawley 1987, Williamson and Fitter 1996). Traits commonly found among invaders are often assumed to contribute to these species’ invasions, although this assumption is rarely demonstrated by appropriate ecological and demographic studies (Schierenbeck et al. 1994, Rejmanek and Richardson 1996). The design of such investigations is formidable because isolating features of an organism’s life history that contribute substantially to fitness is difficult. Ideally, an investigator would be able to specify the combination of traits that cause invasiveness. However, evolutionary divergence is often so great that most life-history traits differ to some extent among even related taxa, and this divergence makes inferences difficult about the key features responsible for invasiveness. The less closely related the taxa, the more acute the problem becomes; however, even species comparisons involving congeners often reveal large differences in ecology and life history.
The difficulties of making predictions among broad taxonomic groups were recently demonstrated by Goodwin et al. (1999). They examined predictive capability on the basis of three plant traits related putatively to invasiveness: life form (annual or perennial), stem height, and flowering period. They then compared invasive and noninvasive European plant species in their new range in New