Risk is the product of the likelihood of an event or process and its consequences As a result of the joint consideration of likelihood and outcomes, events with low to moderate likelihood but serious consequences can carry high risk, whereas events with high likelihood but insignificant consequences often carry little, if any, risk. Although predicting the likelihood of invasion by nonindigenous species has much appeal, from a practical perspective it is risk that must be evaluated. Risk incorporates the economic and environmental outcomes of invasions, and these outcomes are of primary importance to society.

Further rationalization for engaging in risk assessment rather than prediction of invasiveness itself arises from the uncertainty that will accompany most predictions of invasiveness compared with assessments of risk. Uncertainty refers to the degree of confidence in a prediction or assessment (Kammen and Hassenzahl 1999). If only the likelihood of a nonindigenous species’ becoming established in a new environment is considered, it is difficult to apply the level of uncertainty in this prediction fully in decision-making. But if the consequences of an invasion are considered, uncertainty can be incorporated into the evaluation of risk. Greater caution might be used in response to the uncertainty about events and processes that can have serious consequences. Thus, with respect to the practical application of predicting biological invasions, it is the risk posed by invasion by a particular species that should be evaluated.

Several systems have been devised for evaluating the likelihood that a nonindigenous species will become invasive, and they all share a qualitative structure. They assess the likelihood that a nonindigenous species will arrive, become established, and proliferate and spread with a set of species traits and environmental characteristics. In that regard, they are similar to the predictive structure shown in Figure 6-1. Usually, the characteristics are described in a dichotomous fashion (for example, present or absent), and the likelihood measures are categorical (for example, high, medium, or low). The systems also rely on the qualitative assessment of consequences, usually by indicating whether consequences will occur. A collective measure of consequences is determined by summing the expected occurrences of individual consequences (such as crop loss, decreased market, or direct impact on endangered species). The estimate of likelihood of establishment and expectation of consequences are combined to identify a categorical estimate of risk.

Two specific pest risk-assessment procedures are portrayed in Figures 6-2 and 6-3. Figure 6-2 depicts a generic system used by the Animal and Plant Health Inspection Service (USDA/APHIS 1997). The likelihood of pest introduction is based on the estimated quantity of imported material and “pest opportunity”, which is a synthesis of the arrival and establishment stages of the invasion process. It is somewhat incongruous that pest opportunity is measured as a sum of scores that have a probabilistic basis. Because these scores represent subjectively determined probabilities, their joint consideration should reflect the multiplication of the chains of independent probabilities to determine their joint likelihoods. The

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