Elsevier

Journal of Sea Research

Volume 48, Issue 2, October 2002, Pages 157-172
Journal of Sea Research

Indirect effects in complex ecosystems: recent progress and future challenges

https://doi.org/10.1016/S1385-1101(02)00149-1Get rights and content

Abstract

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.

Introduction

Throughout biology, there has been a growing realisation of the need to understand the dynamical interactions among multiple biological components, or biocomplexity (Mervis, 1999). Ecosystems are some of the most obvious complex systems in biology. A fundamental cause of ecosystem complexity is indirect effects, effects of one species on another that only arise in the presence of other species (e. g., Andewartha and Birch, 1984). Consequently, understanding and predicting the behaviour of ecosystems depends on our ability to effectively identify and deal with indirect effects. In this paper, I review some of the recent advances in the study of indirect effects, and outline the future challenges facing studies of indirect effects.

Although the possibility of indirect effects has long been recognized Darwin, 1859, Camerano, 1880, Forbes, 1887, Elton, 1927, interest in them has grown recently because of the widespread application of field experimental methods in ecology. When done in the context of complex natural communities, experiments have often yielded unanticipated results compared to predictions based on pairwise interactions (Sih et al., 1985), and these results are usually most easily explained by indirect effects. The increasing interest in indirect effects can be visualised by the patterns generated by an analysis of ecological papers in the Biological Abstracts database which address indirect effects and related concepts (Fig. 1). Over the past 20 years, the number and fraction of ecological papers on indirect effects have been increasing exponentially, suggesting that this topic has become an important one in ecological research.

Indirect effects can have important implications for ecological study. First, their potential existence complicates experimental interpretation, because multiple pathways of causation are possible (e.g., Connell, 1983, Wootton, 1992). Second, they make implementing conservation and management strategies difficult because the effects of a species loss or an environmental perturbation become difficult to predict a priori (Yodzis, 1988; for some compelling empirical examples, see Estes and Palmisano, 1974, Thrupp, 1990). Therefore, a greater understanding of when they arise, which mechanisms are involved, and how their effects can be predicted is highly desirable.

Section snippets

Terminology

The terminology used in discussing indirect effects has been in a state of flux, as investigators have sought to develop the most useful categorisations and distinctions among these complex interactions Vandermeer, 1969, Miller and Kerfoot, 1987, Wootton, 1993, Menge, 1995, Abrams et al., 1996. A distinction has been made between two basic types of simple indirect effects Vandermeer, 1969, Wootton, 1993, Abrams et al., 1996. First, indirect effects can arise by linking two (or more) direct

Recent results and future challenges

Among the increasing number of studies related to indirect effects (Fig. 1), the vast majority have largely been focused on investigating examples in different systems. Establishing many empirical examples is important to give a sense of the generality of indirect effects, and the range of ways in which they arise. Nevertheless, we will need to move beyond simply documenting indirect effects if substantial progress in grappling with indirect effects is to be made. Therefore, rather than

Acknowledgements

This work was partially supported by a grant from the Andrew W. Mellon Foundation.

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