tion about the capacity of these organisms for population growth and spread and improve the ability to predict invasiveness.

Quantifying a species’ performance in environments it has not before encountered can only be accomplished through experimentation (Hairston 1989, Mack 1996a). Our inadequate knowledge of the early stages in an invasion demonstrates the need for experimentation based on carefully constructed hypotheses about a species in specific ranges under measured environmental conditions. For example, detailed demographic information among populations and cohorts occurring in different environments is essential if we are to predict the outcome of these critical periods in the course of an invasion (such as Mack and Pyke 1983, 1984; Grevstad 1999a,b). Such a protocol already exists in the USDA Agricultural Research Service plant germplasm introduction and testing laboratories. These laboratories evaluate new accessions, including pasture and range species, for their likelihood to contribute to U.S. agriculture. Their goal is to identify species that will grow profitably in the United States—that is, free of predators, pathogens and competitors (USDA 1984). Species deemed free of such hazards should however be reviewed carefully before release because many features that are prized in land reclamation, rangeland management and landscape horticulture are the same features that could enhance these species’ persistence outside cultivation in the United States (McArthur et al. 1990). A geographically broad network of experimental gardens to identify species that could readily spread and persist upon release should be established. Extending these ideas to arthropods and pathogens, including those under consideration for deliberate introduction, would be valuable, although practical considerations associated with experiments in field settings may be more difficult to overcome. Well-designed studies that obtain information on life history traits, interactions with native species and population level parameters in a realistic environmental context could provide valuable information about the potential for population growth and spread of an arthropod or pathogen in a new habitat, and increase our overall understanding and ability to predict invasiveness.

RISK ASSESSMENT

From our discussion thus far, it is clear that we do not have a comprehensive taxonomically based ability to predict with sufficiently high accuracy whether, when, or how a nonindigenous species will become established in a new range. Others have reached the same conclusion (Mack et al. 2000, Williamson 1996). However, our knowledge of the factors and processes that mediate invasion can be used in a related endeavor: risk assessment. Risk assessment can provide valuable insights into both the likelihood of an invasion and its consequences, and these insights can prove useful for forming management strategies and policies.



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