tion is almost certainly confounded by differences in the number of opportunities for transport (Simberloff 1989). However, the much larger number of European forest insects established in North America than the reverse—despite historical movement of plant material, lumber, and other products from North America to Europe (Niemelä and Mattson 1996)—is consistent with such a pattern. Simberloff (1989) addressed the general assumption that islands are more frequently invaded by mainland species than the reverse because mainland species are superior competitors. He pointed out that the available data, which are primarily from agricultural systems, do not support that assumption. The relatively frequent invasions of islands by mainland species could instead reflect the greater abundance of mainland species or the greater frequency of opportunities for invasion of islands by mainland species. Moreover, although competition had been documented for some guilds of insects, such as ants (Suarez et al. 1999) and pine phloem-feeders (Light et al. 1983, Poland and Borden 1998), evidence of interspecific competition among foliage-feeding and sap-feeding insects is scarce (Denno et al. 1995, Strong et al. 1984).

Plant competitors can locally extirpate an immigrant population of plants. The evidence has been indirect–the abandonment of a commercial planting because competition by native plants was so severe and pervasive that cultivation alone was insufficient to foster plant establishment. Several native vines apparently prevent the establishment of introduced tree species in the Solomon Islands (Neil 1984). Competition for light probably produces many more cases of such biotic constraint among nonindigenous species, although no specific search for examples has yet been attempted. Closed-canopy forest communities in the United States, dominated by angiosperms or conifers, have much lower numbers of naturalized species than the same sites once the canopy is removed. For example, although the naturalized and adventive flora of New England probably exceeds 800 species (Seymour 1982), few (for example, Ailanthus altissima) are naturalized in New England forests. The rest occur only in undisturbed open communities and in forest habitats in which the canopy has been removed, such as sites logged or long held in cultivation. Although shade would not provide a strong barrier to nonindigenous species that are shade-tolerant, shade-tolerant species either have been introduced infrequently or, more likely, other physical or biotic factors have so far constrained their establishment. Nevertheless, nonindigenous shade-tolerant species constitute a functional category of species whose careful monitoring appears warranted upon their introduction in the United States.

Another difficulty in addressing how competition affects establishment arises because we cannot necessarily distinguish between effects of natural enemies and of competitors when a nonindigenous species arrives. An immigrant species and a native prey species can exhibit “apparent competition” through a shared enemy— a concept similar to the competition for enemy-free space described by Jeffries and Lawton (1984). The outcome of such competition can depend on r, the intrinsic rate of increase of the immigrant, and the rate at which the nonindigenous

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