could be important in assessing the potential for establishment if it were possible to reliably quantify the effects of stochasticity on specific immigrant populations.

Abiotic forces (such as climate) and biotic forces (such as the availability of hosts, vectors, pollinators, or mutualists and the presence of competitors, predators, and pathogen antagonists) also affect the establishment of nonindigenous plants, arthropods, and pathogens. The life-history traits that equip particular species to deal with their environment are crucial in determining their survival.

Much of the quantitative information about the interaction between abiotic and biotic forces and the life-history traits that contribute to a species’ establishment has come from monitoring the release of nonindigenous insects for biological control and from after-the-fact studies of species’ invasions. Although research on a particular species or event can permit conclusions to be drawn about the combination of factors that lead to its survival or extinction, it has not been possible to generalize the findings to other events or other species. Little is known about the phenotypic plasticity of potential plant pests in different environments: we have scant knowledge of the range of tolerance of most plants, pathogens, and arthropods; and we have so little information about the stochasticity and amplitude of abiotic and biotic forces in any environment that reasonable predictions of the strength of their impact on a newcomer cannot be made.

Regardless of those limitations, key points have been identified. For example, climate, latitude, and the availability of a host appear to be among the primary determinants of the suitability of a habitat for insects. Consequently, insects from low latitudes might be less likely to become established in northern latitudes than the reverse, because these insects might find climatic conditions intolerable, fail to find a suitable host, enter diapause (a period of dormancy) too late in autumn, or break diapause too early in spring.

Other clues to the establishment by nonindigenous insects are found in research on diet breadth in insects, the temporal availability and spatial distribution of hosts, the similarity of potential invaders as the prey of predators in the new environment, and the diversity of the predator population. Establishment might also be associated with parthenogenesis, long-lived adult stages, high fecundity, and small bodies.

For pathogens, the availability of a genetically compatible host is critical. Knowledge of the regional distribution of pathogens and the frequency of different avirulence genes can be important in predicting the resistance or susceptibility of a plant taxon (that is, variety, race, subspecies, or genotype). If the virulence of a potential invading pathogen is known, the vulnerability of a plant taxon can be predicted rather accurately. Traits that appear to enhance establishment of plant pathogens are a short infection cycle, a high rate of production of infectious units, and a long infectious period.

The abiotic and biotic factors that affect the establishments of plants are less well understood. The most frequent biotic constraint on plants is attack by pathogens. Plants can have diverse characteristics that contribute to survival, such as

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