variation, and differential migration rates could modify the asymmetry of host and pathogen evolutionary rates (Koltz and Shykoff 1998).

Selection pressures encountered during the invasion of natural habitats are likely to be much more complicated than those which occur in agroecosystems. In particular, selection is likely to be less predictable and perhaps less directional. Diversifying selection would then be expected to maintain variation in populations, and the rapid evolution of specialized races of limited ecological tolerance would be less likely at least over short periods. Predicting the course of evolution in such biotically complex situations will be more difficult than in agricultural habitats, where growers strive for environmental homogeneity through the production of a uniform set of growing conditions for the crop.

KEY FINDINGS

  • Some persistent nonindigenous species undergo a transition and become invasive. However, compared with the factors that govern arrival and persistence, there is considerable uncertainty regarding the biotic and abiotic factors that produce an invading population.

  • The transition from established (or persistent) to invasive does not have a single explanation, but appears to be caused by the interaction of chance events in the new range, the biological attributes of the species, and the recipient community’s composition.

  • The history of a species’ invasion in another location or country is often a useful predictor of its behavior in a new area.

  • Many of the characteristics that determine establishment also are important in determining an invading population. For instance, a high reproductive or growth capacity and the availability of suitable habitats, resources, or hosts are important. It is not always known why a species with a moderate rate of population growth makes the transition to a very high rate of growth.

  • Dispersal is clearly important in facilitating the transition to an invasion. Relevant dispersal components include the number of propagules available for movement, the opportunity for and mechanisms of dispersal, the distances that propagules are transported, and the spatial pattern of the dispersed propagules.

  • Although some invasions advance as a continuous wave front, most advance spatially by establishment of some minimal number of widely separated foci. Establishment of multiple foci appears to be very important in triggering an invasion, regardless of the taxonomic group.

  • Habitats and communities differ in their vulnerability to the entry of potentially invasive species. For example, arid or otherwise treeless communities appear more vulnerable to airborne plant dispersal than communities with a continuous forest canopy. In contrast, forest communities might be more vulnerable to animal-dispersed seeds and fleshy fruits. Sometimes, nonindigenous species



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