the habitat is not uniform, the expansion rate of a population will be affected by the spatial distribution of favorable and unfavorable patches (Shigesada and Kawasaki 1997). Environmental variables interact with biological traits of an organism, and this increases the difficulty of predicting invasion rates. For example, the spread of Scotch broom (Cytisus scoparius) from multiple foci was lower in urban areas than in prairies, primarily because survival and reproductive rates were lower in the less favorable urban habitat (Parker 2000).

Predicting the potential of populations of nonindigenous plants and plant pests to grow and spread will depend in part on what can be determined about the dispersal mechanisms and circumstances of organisms in any range, whether native or new. Studies are needed to quantify the role of the various factors— such as wind, birds, natural migrations, and human-mediated actions—that contribute to long-distance transport of individual species. Continued research into and documentation of the spatial and temporal aspects of known invasions will be necessary if we are to improve our understanding of patterns of range expansion and of the mechanisms by which invasions progress.

COMPETITION FOR RESOURCES

Community Diversity and Resources

Community diversity is viewed as the factor that most influences a community’s susceptibility to invasion by nonindigenous species. Different theories (but few experimental studies) support opposing views of the role of diversity in invasion (Levine and D’Antonio 1999, Dukes 2001, Naeem et al. 2000, Stohlgren et al. 1999). Levine and D’Antonio (1999) argue that natural plant communities with high biodiversity tend to be more prone to invasion than their species-poor counterparts because the factors that control native diversity also control diversity among invaders. A supplemental argument is that the “microheterogeneity” of diverse communities provides differences in the spatial context (such as canopy height and rooting depth) that in turn provide more opportunities for immigrant species than are found in monocultures.

Those arguments are in contrast with the concept of “limited resources space”, which suggests that the more species that occupy an area, the more fully resources are used and the more difficult it is for a new species to become established (Tilman 1987, 1988 and references therein, Knops et al. 1999). In addition, there are many cases in which nonindigenous species have become established in species-poor communities.

Indirect interactions among species can also facilitate invasiveness, regardless of diversity. Some species in a community can deter invasion; others can facilitate it (Palmer and Maurer 1997). Interspecific competition initially appeared to be responsible for the declining numbers of a native leafhopper after establishment by a nonindigenous leafhopper (Settle and Wilson 1990). Field and caging



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