Abstract
Cumulative effects (CEs) result from the combined effect of multiple activities over space or time. This implies a persistence through time and often a transmittal mechanism through space. Environmental legislation often requires a broader CE assessment in addition to the more direct, project-specific impacts. Current efforts to evaluate and manage CEs are hampered by the conceptual problems of defining the key issues, specifying the appropriate spatial and temporal scales, and determining the numerous interactions and indirect effects. These problems can be greatly alleviated by following an explicit process. The process proposed in this paper includes a scoping phase, an analysis phase, and a planning and management phase, with each phase consisting of two to five discrete but interrelated tasks. Numerous approaches have been developed to assess CEs, and these range from simple checklists to complex, physically based models. The utility of each approach depends on the resource of concern, relative risk to those resources, information available, and time frame for the evaluation. In nearly all cases the assessment and regulation of CEs is severely hampered by the variability in site conditions and management effects, inability to predict secondary or indirect effects, lack of data on recovery rates, difficulty of validating predictive models, and uncertainty of future events. Since any proposed activity could contribute to a wide range of potential CEs at different spatial and temporal scales, a tiered or nested approach should be followed to assess CEs. The difficulty of assessing and predicting CEs also suggests that in many cases the most efficient approach is to focus on minimizing on-site impacts. Under some circumstances adaptive management can also be a viable alternative to detailed CE assessments. Regular monitoring and feedback is critical to the successful management and regulation of CEs.