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Predicting Invasions of Nonindigenous Plants and Plant Pests
accepted (Vitousek 1990, Parker et al. 1999, Simberloff and Von Holle 1999, Goodell et al. 2000). One is that, in the aggregate, impacts (however defined) are likely to increase with the number, geographic range, and abundance of the introduced species. Some support for that prediction comes from well-studied natural enemies released for biological control, in which suppression of target plant species increases with the number of control species established (Hoffmann and Moran 1998). A second is that the more geographically isolated the biota of a region is, the more vulnerable it is to invasion. The latter generalization is based largely on the conventional wisdom that oceanic islands are more easily invaded than mainland areas and that native species on oceanic islands are somehow more vulnerable than those on the mainland (Elton 1958, Carlquist 1965, Wilson 1965). The explanation for that vulnerability is that the descendants of the native species on islands experienced new colonization events infrequently and might have evolved in the absence of competitors, predators, or pathogens (Carlquist 1974). However, Simberloff (1995) finds little support for the conventional wisdom that islands are more easily invaded than mainland areas or that ecological effects of invasions are greater on islands than in mainland situations. Failed immigrations are often unrecorded, and claims that an introduced species has displaced a native one are often based on correlated population changes rather than experiment or detailed field observations. Species transfers from mainland to islands appear to be far more common than transfers from islands to mainland. In Simberloff’s view, the data are inadequate to support conclusions about the invasibility or fragility of islands and mainland. A third generalization is that invaders likely to have impacts are those which create changes in disturbance regimes for which native species are ill prepared (Mack 1989): native species are especially likely to be adversely affected by the arrival of new immigrants because nothing in their evolution would likely have been comparable with the interactions suddenly faced.
Those generalizations do not identify specific potential invaders because they ignore mechanisms; the mechanisms of an invasion that threatens native species on islands are as varied as the characteristics of the invaders. For example, an invader might duplicate a functional role already played by resident species but be competitively superior in this role (Mooney and Cleland 2001), reducing the natives’ role and threatening their existence. Or an invader could perform novel functions in its new community, as when a mammalian predator invades an oceanic island that lacks native mammalian predators (Elton 1958) or a nitrogen-fixing plant invades a region of nitrogen-poor soil (Vitousek et al. 1987, Vitousek 1990).
Although the mechanisms of invasions of an isolated biota are too varied and too poorly characterized to be applied elsewhere, the identification of the vulnerability of an isolated biota is, from a management perspective, a reason at least to take special precautions to keep nonindigenous species from proliferating there. The determination of measures for all aspects of the impact of invasive species