traits. Cody and Overton (1996) described the reduction in dispersal ability for wind-dispersed seeds of invasive species onto islands in just a few generations in small isolated populations. Carroll and Dingle (1996) indicate that populations of the soapberry bug (Jadera haemotoloma) have evolved differing beak lengths in response to the introduction of new invasive hosts within only 50 years and Singer et al. (1993) have shown rapid evolution in the feeding preferences of the Euphydryas butterfly for the invading herb Plantago lanceolata in only 10 years. Thus, there are many cases of evolution both in invading species and in the species affected by invaders.

As noted earlier in this report, the ability of pathogens to adapt to different plant genotypes has been studied in detail, and the resulting knowledge has helped in forecasting the fate of new microbial genotypes in the environment (Mundt 1995). Fungal, bacterial, and viral plant pathogen populations evolve quickly to overcome resistance genes in hosts. For example, the average useful life of race-specific genes for resistance to fungal rusts of wheat has been estimated to be only 5 years (Mundt 1995).

Population and Community Effects

Invaders can cause reduction in the biological diversity of native species and the size of populations; next to land transformation, they are the most important cause of extinction (Vitousek et al. 1996). After habitat destruction (which affects 81% of imperiled plant species), introduced species contribute more to the imperilment of species (57%) in the United States than the next three causes combined— pollution (7%), overexploitation (10%), and disease (1%) (Wilcove et al. 1998, 2000) (Categories are nonexclusive and do not sum to 100%). Replacement of natives with nonindigenous species is immediate, readily measurable evidence of the impact of invasions.

Extinction could be the most dramatic impact of invasive species. Small populations of natives suffer the highest risk of extinction from various genetic and demographic causes discussed earlier in this report in connection with the same hazards that small immigrant populations experience. Invaders pose a major risk to threatened and endangered species: about 400 of the 958 species that are listed as threatened or endangered under the Endangered Species Act are considered to be at risk primarily because of competition with or predation by nonindigenous species (Wilcove et al. 1998, Stein et al. 2000). Invaders can also interact with habitat transformation and thus exacerbate the threat to biodiversity (Hobbs 2000).

Extinction of native species, although dramatic, actually characterizes relatively few invasions (Simberloff 1981). Reduced population sizes and local extirpation of a species appear more common than global extinction of a species, but changes in population sizes of native species after invasion by nonindigenous species can vary greatly in magnitude and even direction. For example, establish-



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