also promote temporal synchronization of mate-finding or dispersal (Wood et al. 1990).

Natural Enemies

Typically, nonindigenous plants, insects, and pathogens that are accidentally introduced into a new range arrive without the complement of natural enemies— such as predators, parasitoids, parasites, antagonists, and pathogens—with which they interacted in their native range. Whether native species can expand their diet to include the immigrants and whether the newly encountered enemies will prevent establishment of the nonindigenous species are important questions.

Establishment will be less likely if the intrinsic rate of increase (r) of the immigrant is small, if the immigrant is particularly vulnerable to attack by resident enemies, or if populations of resident enemies are large (Lawton and Brown 1986). The likelihood that native enemies will exclude a nonindigenous species presumably depends on the numbers and densities of the potential predator and pathogen species, their feeding preferences, and the total number of native prey species that are of higher preference than the immigrants (Crawley 1986). All together, the more similar an immigrant species is to a native species, the more vulnerable it will be to native enemies, because the rate of attack should be high (Lawton 1990).

The likelihood that predators or other enemies in the new range will prevent establishment of a nonindigenous insect could also be influenced by the structure of the predatory “guild” in the community. Pimm (1989) suggested that when predation exerts a strong influence on prey species, an immigrant species would have more difficulty becoming established in a community with a single predator than one with several predatory species. He based that suggestion on observations of a close correlation between predator and prey species in communities with various numbers of species (Jeffries and Lawton 1984) and the assumption that the correlation reflects the influence of predators on numbers of prey, as well as the reverse.

Evidence of the ability of indigenous species to prevent establishment of nonindigenous insects in its new range is scarce, and the topic deserves much further research. Much of the evidence stems from postrelease evaluations of herbivorous insects introduced for biological control. Goeden and Louda (1976) assessed impacts of resident predators, parasitoids, and pathogens on nonindigenous herbivorous insects introduced for biological control in 23 projects. Effects of resident natural enemies varied widely; there was no discernable impact on the introduced species in five to nine projects, there was some adverse effect on the introduced species in 12 to 17 projects, and establishment was prevented in two projects.

In postrelease evaluations of biological control releases of insects, impacts of predators were generally greater than impacts of parasitoids or pathogens. Goeden



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