decision rules, and other predictive tools used in ecological risk assessment still are needed. Although the committee refers to the process of improvement as "validation," we recognize that none of the approaches in question can be proved fully valid in the sense of perfectly predicting natural ecosystem behavior under all circumstances. The purpose of validation is to improve the credibility and reliability of predictive methods. Validation must be viewed as an iterative process in which predictions are tested, models are refined, and then new predictions are tested.
At least three kinds of studies can contribute to validation: improved measurements of specific quantities and tests of assumptions, experimental testing of models under reasonably realistic conditions (e.g., ponds or enclosures), and monitoring of ecological effects of dams, power plants, or other projects to determine the accordance between actual effects and effects predicted before construction or operation of the dams. Each kind of study has its own advantages and disadvantages, and all three should be included in validation programs.
This committee is not the first to note the need for validation studies. Similar recommendations can also be found in at least two previous NRC reports (NRC, 1981; NRC, 1986). In spite of virtually unanimous support within the scientific community for this activity, the resources currently being expended for improvement of predictive tools are much smaller than those devoted to repetitive assessments and routine monitoring of compliance with permit requirements. The importance of enhanced validation programs needs to be recognized by all regulatory and resource management agencies.
Valuation and cost-benefit analyses are recognized as integral components of the risk management process. Such analyses contribute to the regulations that provide the context for risk assessments and to the eventual risk management decisions. Cost-benefit analyses are major parts of the planning and ranking process within and between agencies. Ecological cost-and-benefit analyses have gained acceptance where individual behavior can be used to directly reflect economic preferences, e.g., recreational use and associated travel-cost analysis (Yang et al., 1984; DesVouges and Skahen, 1985). These analyses have also been
applied with some success where people's direct use of a resource was the specific issue, e.g., dam construction vs. maintenance of the natural river channel. Methods also have been developed for monetizing ecological values beyond those associated with the use of a resource, but the uncertainties associated with applications of those methods are often quite high.
Ecological values are sometimes described by resource economists as services provided by the environment to humans. Such economists categorize economic values into two segments termed "use" and "nonuse" values. As noted above, reasonably reliable techniques are available for determining use values (e.g., land valuation and recreational use) from the actual behavior of resource users. Several methodologies have been developed to date for estimating nonuse values. For example, contingent valuation uses public surveys to elicit statements of how much an individual hypothetically would be willing to pay for improvements (or to prevent reductions) in the quantity or quality of natural resources. It requires people to assign subjectively economic values for environmental goods. Recent empirical research indicates that the results vary depending on the way the assessments are elicited (Opaluch and Segerson, 1989; Grigalunas and Opaluch, 1991; Hausman, 1991; Rosenthal and Nelson, 1992), and the resulting values must be interpreted with care.
Clearly, a considerable need remains for increased communication and clarification between ecologists and economists to improve the use of valuation methods in ecologic risk management decisions. There is already a substantial literature on the economic value of wetland ecosystems (Scodari, 1990). Valuation of other kinds of ecosystems is being actively discussed (e.g., Orians, 1990), but generally accepted principles for ecosystem valuation do not yet exist.