Indirect effects in complex ecosystems: recent progress and future challenges

Indirect effects are fundamental to the biocomplexity of ecological systems, and provide severe challenges to predicting the impacts of environmental change. Interest in indirect effects has expanded exponentially over the past 20 years. Indirect effects arise when direct interactions mediated by changes in density share a species,'when species change the interaction between individuals of other species, or both. Past work, including many studies of marine food webs, has primarily documented the existence of indirect effects and how particular examples arise. Future challenges include (1) developing methods for the systematic detection of indirect effects, (2) documenting the functions describing different classes of interaction modifications, (3) exploring methods to predict indirect effects, including measurement of interaction strengths among species, (4) integrating time-scale differences into the theory of indirect effects, and (5) linking indirect effects and environmental variability. Ecologists are not alone in dealing with complex systems. Consequently, progress may be facilitated by exploring approaches developed in other scientific disciplines oriented toward complex systems. (C) 2002 Elsevier Science B.V. All rights reserved.