been carried out, and the conclusions on risk are found to depend on critical assumptions or elements of data, for which existing uncertainties can be resolved through research.
Risk assessment can be used for ''early warning"—a determination that an issue is of sufficient concern to place on the agenda so that existing policies, regulation, or legislation can be reconsidered. Advances in scientific understanding or changes in the stresses affecting an ecological system might indicate a potential for adverse changes that were not previously recognized. Recognition of the potential for adverse changes might allow these changes to be avoided through appropriate actions. Risk assessment can facilitate evaluation that permits earlier recognition and enables timely action.
In addition to situations in which timely warning of adverse changes is important, risk-based measures of ecological systems might facilitate continuing management activities to maintain, enhance, or restore the systems. Human-induced and other stresses interact in complex ways to affect ecological systems. Understanding how management policies affect the ability of an ecosystem to withstand or recover from stress will permit more effective management policies to be selected.
Different Risk Assessment Methods Are Suited to Different Risk Assessment Needs
The discussion above on the varied uses of risk assessment implies that there is not only one correct way to do risk assessment. Rather, risk assessment methods should be considered as a collection of tools from which analysts must select for the task at hand. In some cases, the tools must be developed, because the tools needed for a particular risk assessment task do not yet exist.
Risk assessment applications in similar situations might benefit from the same or very similar methodological approaches. Therefore, it will be important for public agencies and private organizations with similar needs for risk assessment to learn from each other's experiences. Both positive and negative experiences with models, databases, statistical procedures, and methods for assessing expert judgment can provide useful lessons for improving risk assessment practice. One lesson