grant population to develop an invasive population (that is, the multiplication of probabilities). As with the first system, the overall risk reflects the combined influences of the likelihood of introduction and consequences of introduction.

Because risk-assessment systems use the same structure as that depicted in Figure 6-1 to assess the likelihood of introduction, they share the limitations identified for a scientifically based prediction of invasion. Foremost among the limitations is that risk-assessment systems require subjective determination of characteristics of nonindigenous species and the environments into which they might be introduced, and they identify the risk of introduction by subjectively placing species and environmental characteristics into likelihood categories. If the characteristics for assessing the likelihood of introduction shown in Figure6-3 are used as an example, it is not clear how one would determine whether an organism has a high capacity for increase, what the capacity is for dispersing, or what constitutes an extended period for colonizing new hosts. Those determinations are made subjectively and are thus susceptible to bias. Furthermore, none



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