resident competitor (Crawley 1986). Immigrants should have a greater probability of establishment if the rate of resource availability is adequate or the number of competitors is low enough to permit the immigrant population to be maintained. Crawley (1986) proposed that when insect guilds are structured by interference competition, invasion by a larger or fiercer species is more likely. Conversely, a small species could more likely become established within a guild structured by exploitation interference in which it can reduce the resource supply rate to the point where larger, resident species can no longer be supported.
Although those predictions are intuitively appealing, it is difficult to find situations where competition with native residents has directly affected establishment of nonindigenous insects. Levins and Heatwole (1973) introduced a Drosophila species, an ant, a snail, frogs, and lizards to a small Puerto Rican island. The Drosophila species, the frogs, and the snail went extinct rapidly because of severe weather. The introduced ant and lizards survived for a while but eventually went extinct. The authors attributed extinction of the ant to competition with aggressive native fire ants and extinction of the lizards to competition and predation by resident species.
Introductions of herbivorous insects for the biological control of nonindigenous plants often provide little information, because interspecific competition is, by careful prerelease evaluation, minimal in such situations. Some have noted that the populations of entomophagous species for biological control introduced earliest are more likely to establish than populations introduced later (Tallamy 1983, Ehler and Hall 1982). In some cases, that outcome could reflect competitive exclusion of later introductions by previously established species. A more probable explanation, however, is that the order of species’ introductions is determined by the expectations of those importing the species; that is, biological control practitioners attempt to establish the species that are most likely to be rapidly successful (Keller 1984, Simberloff 1989).
How and whether competitive abilities influence establishment of nonindigenous insects have attracted much speculation, particularly with respect to asymmetry in the establishment of insects between two regions or countries. Vermeij (1991) proposed that the prevalence of asymmetrical exchanges of species between regions could be related to differences in the competitive abilities of immigrants originating in different donor ranges. European insects, for example, have disproportionately invaded other regions (Crosby 1986, di Castri 1989, Niemelä and Mattson 1996, Simberloff 1989). Niemelä and Mattson (1996) speculated that climatic and anthropogenic disturbances in Europe shaped selection for suites of traits likely to enhance the survival of insect species in the fragmented and impoverished European forests. A lower ratio of phytophagous insect species to plant species compared with the ratio on other continents might have also intensified interspecific competition in Europe. European plant-feeders that arrive in a new range, therefore, might be inherently strong competitors. That hypothesis would obviously be difficult to test empirically, and the explana-