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Predicting Invasions of Nonindigenous Plants and Plant Pests
that are being deliberately introduced, the correct identification cannot be entrusted totally to the applicant. The past performance of any congeners of the species is also considered in the evaluation. Having a congener with a record as a weed elsewhere does not automatically cause a species to be prohibited entry, but it does prompt further assessment. Use of records of congeneric weeds prompts further investigation at several points in the flow diagram.
An important feature of the WA system is that final approval of entry of a species is not usually automatic, especially for species for which there is no record. There are in effect several layers of examination and evaluation that each species must pass through before import approval is given. This multilayer system contains benefits for both the public and the applicant for species entry. Only a species that appears on a federal or WA prohibited list is automatically excluded from further evaluation. A species that is already naturalized in the state may be allowed entry (that is, entry of additional populations), although there is provision that these introductions may be denied. As a result, blocking their entry can reduce the amount of genetic variation in a species in the state. Although a record of a species as a “significant weed” (one demonstrated in the scientific literature to have an impact in agriculture or natural ecosystems) does not automatically prohibit a species from entry into the state, it does prompt a full weed assessment. Such assessments are more detailed examinations of the record of behavior of the species than simply whether it appears as a weed anywhere. A decision can be made after this assessment to prohibit or permit the species’ entry. One apparent anomaly concerns the history of established domestic and international trade in the species; it is not clear to what degree a species with such a trade history would be permitted entry.
The strength of this system appears to depend on the comprehensiveness of the database maintained on nonindigenous species; that is, expert judgment here is based largely on known behavior of the species outside WA (Randall and Stuart 2000).
of reducing the likelihood of events in the chain. Although this method is quantitative, it often requires substantial subjectivity in assigning the probability distributions used to represent events along a pathway. Ideally, empirical data would be used to select the distributions, but such data are often not available.
As currently used, the method considers only the arrival and initial survival of a nonindigenous species, processes that usually are much better understood than the proliferation and spread phase of an invasion. Quantitative scenario analysis should be expanded to include all aspects of the invasion process, especially when consequences are high. The array of information suggested by the committee in Table 6-1 could be incorporated in scenario analysis to reflect a more comprehensive and realistic perspective on the risk posed by the introduction of pests and of organisms intended as biological control agents.