Scientific dimensions of cumulative effects assessment: toward improvements in guidance for practice

Cumulative effects assessment (CEA) became an increasingly important component of environmental impact assessment (EIA; or simply environment assessment (EA)) shortly after formal processes for EIA were established in North America in the 1970s. Despite a growing body of literature addressing science requirements of exemplary EIA and CEA, practice remains contested. Our mission in preparing this review was to provide a critical update on progress in scientific developments associated with CEA and also to guide practitioners to a broad selection of the recent relevant peer-reviewed formal literature on CEA. In addition, we point to ways in which guidance for CEA practice could be improved. The study canvassed widely for refereed papers in journals and edited books as far back as 2000. On the matter of key concepts related to CEA, the paper addresses the definition of other activities to be assessed, establishment of time and space bounds, impact thresholds, methods for impact prediction, and stressor-based versus effect-based approaches. Definitions of cumulative effect are reviewed, with encouragement for continued work to elaborate the concept. Contributions from science to CEA practice are identified as follows: retrospective and prospective investigative protocols; basic ecological knowledge; effects knowledge; tools and methods; ecological grounds for threshold establishment; and analytically competent practitioners. We observe that the plethora of CEA frameworks populating the scientific literature offer practitioners helpful ways to think about the CEA process. CEA methods are then reviewed, with specific emphasis on geographic information systems, scenario-building, thresholds, indicators, simulation, and public engagement. Several case examples of CEA in practice are summarized, with the observation that none of the published case studies arises from work done to support CEA that is part of the regulated EIA process. The paper reflects on the role of CEA in project-specific EIA (or project EA) as well as class EA, strategic EA, and regional EA. CEA is needed in all forms of EA, but it seems to be particularly difficult to implement well in project-specific EIAs. Recommendations for improvements in guidance materials for practitioners address definitions, scenarios, analytical methods, collaborative methods, thresholds, knowledge accumulation, accidents and malfunctions, project scale, and knowledge integration. We conclude that competent CEA is a vital requirement for securing the sustainability of valued ecosystems and their components.