establishment of a founder population except at very low population densities (such as six pairs or fewer), when long-lived species would be expected to have a lower extinction rate. In two other studies, insects with small bodies and high r were found to be more likely to establish than insects with large bodies (Crawley 1986, Lawton and Brown 1986).

Limitations in prediction arise, however, if estimates of r are used to predict the likelihood of establishment for a given arthropod species. One problem is that establishment can be linked to other life-history strategies. For example, in Crawley’s (1986) review of biological control introductions, insects with long-lived adult stages were found more likely to establish. Adult longevity presumably enabled oviposition to occur over a protracted period, increasing the probability that the nonindigenous arthropod would encounter suitable conditions for establishment. In addition, r is often considered on a relative or qualitative basis, and it is not clear how large it would need to be to enhance the probability of establishment. Furthermore, it is difficult to disassociate r from other traits, such as reproduction strategy, dispersal, and interactions with predators or other taxa in a new habitat.

KEY FINDINGS

The numerous factors identified in this chapter form a basis for predicting the establishment of a nonindigenous plant or plant pest. The degree of uncertainty in our ability to measure these factors depends on whether the identity of the immigrant is known, whether important information about its life history is available, and whether the circumstances of its introduction have been accurately assessed.

Stochastic Forces
  • The likelihood of establishment of nonindigenous plants and plant pests depends in part on the number of organisms that are introduced and the frequency of the introductions. Nonindigenous plants and plant pests typically arrive in small numbers and are vulnerable to demographic, environmental, and other stochastic forces that drive small populations to extinction.

  • The chance of extinction due to demographic stochasticity is a function of the number of immigrants, their reproductive rate, and, if sexually reproducing, their success in finding mates. Populations of plants and arthropods of fewer than 50 individuals are highly vulnerable to extinction.

  • Genetic bottlenecks, small size of the founder population, and strong directional selection on immigration can reduce the probability of establishment. Inbreeding in low-density populations can reduce the fitness of progeny. There are cases, however, where reduced heterozygosity resulting from a genetic bottleneck has enhanced the success of a nonindigenous species. Effects of such



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