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Predicting Invasions of Nonindigenous Plants and Plant Pests
Recommendation 9. The release of nonindigenous organisms for biological control presents an opportunity to collect detailed demographic data on immigrant populations from the moment of their introduction. A substantial effort should be made to document the fate of these organisms, including the efficacy of the introduced organism on the target pest and on nontarget species, as a guide to the performance of unintended releases and as a mechanism to improve risk assessment in deliberate introductions of nonindigenous species.
Understanding and quantifying the forces that cause populations to become extinct, especially by natural forces in a new range, will prove central to building our understanding of the invasion process (Harper 1982, Mack et al. 2000). The comparatively little that is known in quantitative terms about invasions has been learned by following the rare event of a nonindigenous species that actually develops an invasion. We also need comprehensive knowledge of the circumstances under which almost all immigrants go extinct—whether in transit, at the point of entry, or as adventive species in a new range (Mack 2000)—or those that decline numerically after having seemingly attained persistence (Simpson 1984). Releases of organisms for biological control are, in effect, experiments that assess the performance of species in their native range and simultaneously in potential new ranges. Following the fate of these populations could be invaluable in that the genetic composition, precise native range, size of the founder population, and time of release are all probably known (Grevstad 1999a,b).
Recommendation 10. Plants native to the United States that are growing in other countries, such as in botanical gardens and arboretums, should be monitored to determine the species to which they are susceptible and to evaluate the potential for these species to arrive in the United States. The severity of the damage to native U.S. plants by pathogens, arthropods, and other taxa, and the abiotic and biotic forces that contribute actively to the damage, should be documented.
Expert judgment could be improved substantially through a deliberate and expanded effort to assess species that attack U.S. crops and native plants in ranges outside the United States. Cereal crops, cotton, soybeans, some pines, and numerous ornamental trees, shrubs, and herbaceous plants are grown worldwide and are exposed to pests not currently found in the United States. Reports of the organisms that attack these plants are invaluable when coupled with some estimate of the potential for the harmful species to arrive in the United States (Hewitt and Chiarappa 1977). For instance, the potential hazard of introduction of the Russian wheat aphid and plum pox was understood, on the basis of foreign experience, before these species arrived in the United States. But too many species of plant pathogens and phytophagous arthropods occur outside the United States to evaluate the potential damage from all of them. The list of species that deserve careful attention would be substantially reduced and focused by monitor-