Indirect Effects in Communities and Ecosystems

by

Thomas E. Miller, Casey P. terHorst

• LAST REVIEWED: 08 September 2021
• LAST MODIFIED: 23 May 2012
• DOI: 10.1093/obo/9780199830060-0020

Introduction

The goal of ecology is to understand the distribution and abundance of organisms, generally by quantifying how abiotic conditions and species interactions contribute to population growth. Much ecology focuses on simple pairwise interactions, such as competition and predation; yet, species naturally exist in much more complex systems in which their abundances are determined by webs of species interactions. An important step for ecologists has been to understand how interactions may occur through loops and webs of connected species: it is these interactions that are now loosely collected together into what we call “indirect effects.” Some types of indirect effects are thought not only to widely occur but also to be particularly important for determining both the abundances of individual species and community properties such as diversity and stability. These include trophic cascades, where predators enhance producer growth by feeding on consumer species, and keystone predation, where predators consume dominant competitors, thus allowing inferior competitors to persist. Identifying and quantifying indirect effects has become a major issue in ecology. Although their general importance is well understood, we have little understanding of the relative importance of different types of indirect effects.

General Overviews

“Indirect effect” is a general term referring to a broad variety of species interactions that can occur through chains of direct species interactions, such as predation or interference competition. Indirect effects are integral to foundation concepts of modern ecology, including trophic pyramids (Elton 1927), keystone species (Paine 1969), the green earth hypothesis (Hairston, et al. 1960), and top-down and bottom-up control (see the review in Powers 1992). Indirect effects fascinate ecologists because they can link the population dynamics of species that do not directly interact, as in the classic example of predatory fish affecting phytoplankton abundance by consuming specific sizes of zooplankton (Brooks and Dodson 1965). In some cases, the strength of indirect effects can negate the effects of any direct interactions (Wilbur 1997). The study of indirect effects has been complicated by the diversity of mechanisms through which they occur, which has contributed to an equally confusing variety of terms. Quantifying indirect effects has also proven to be very difficult because of the inherent difficulties in trying to control some species interactions, while quantifying others, in complex webs of species interactions. Still, indirect effects continue to stimulate significant theoretical and experimental work, and comprehensive reviews have discussed their place in modern ecology (Strauss 1991, Wootton 1994). It remains to be seen if indirect effects will persist as a useful unifying concept in the future.

• Brooks, John L., and Stanley I. Dodson. 1965. Predation, body size, and composition of plankton. Science 150.3692: 28–35.

  DOI: 10.1126/science.150.3692.28

  Along with Paine’s intertidal work (see Paine 1969), this study demonstrated how top predators can control the structure of entire ecosystems, in part through indirect interactions across trophic levels and predator control of dominant competitors within trophic levels. In this case, lakes with Alosa pseudoharengus were largely missing large zooplankton, which in turn affected phytoplankton communities. Available online for purchase or by subscription.

• Elton, Charles. 1927. Animal ecology. London: Sidgwick & Jackson.

  Animal Ecology was one of the first and certainly most influential textbooks in ecology. It is quite surprising to read it now and find insightful discussions of many issues in ecology that are still topical today.

• Hairston, Nelson G., Frederick E. Smith, and Lawrence B. Slobodkin. 1960. Community structure, population control and competition. American Naturalist 94.879: 421–425.

  DOI: 10.1086/282146

  This group of ecologists at Michigan produced a very stimulating and controversial paper that asked “What limits each trophic level?”. Based on simple ideas (e.g., the earth is mostly green), the authors concluded that, in general, plants are limited by resources, herbivores are limited by predation, and predators are limited by prey availability. Available online for purchase or by subscription.

• Paine, Robert T. 1969. A note on trophic complexity and community stability. American Naturalist 103:91–93.

  DOI: 10.1086/282586

  Paine wrote a better-known article in 1966 that described three different marine intertidal communities in which he proposed that a top predator controlled the diversity of species in the rest of the community by controlling dominant competitors among its prey. However, he did not use the term “keystone species” to describe this phenomenon until this later article.

• Powers, Mary E. 1992. Top-down and bottom-up forces in food webs: Do plants have primacy? Ecology 73:733–746.

  DOI: 10.2307/1940153

  This is a nice review article summarizing how the top-down ideas of Hairston, Smith, and Slobodkin (Hairston, et al. 1960) engendered and influenced community ecology for the following thirty years. Top-down is specifically described as a view in which the top predators in food chains are food limited; but, at successively lower levels, species are alternately predator, then food limited. It provides a nice description of the development of ideas from Hairston, Smith, and Slobodkin to the work of Fretwell, Oksanen, and others on food web control. Available online for purchase or by subscription.

• Strauss, Sharon Y. 1991. Indirect effects in community ecology: Their definition, study, and importance. Trends in Ecology & Evolution 6:206–210.

  DOI: 10.1016/0169-5347(91)90023-Q

  This was the first general review of indirect effects in the primary literature. It provides an excellent introduction and discussion of terminology and prior research.

• Wilbur, Henry M. 1997. Experimental ecology of food webs: Complex systems in temporary ponds. Ecology 78:2279–2302.

  DOI: 10.1890/0012-9658(1997)078[2279:EEOFWC]2.0.CO;2

  This is the written version of Wilbur’s lecture at the Ecological Society of America meetings in 1996 in which he summarized much of the work in his lab on direct and indirect interactions across trophic levels in pond ecosystems. It illustrated both the role of indirect effects in simple food webs as well as some of the difficulties in quantifying such interactions when there are nonlinearities and temporal stochasticity.

• Wootton, J. Timothy. 1994. The nature and consequences of indirect effects on ecological communities. Annual Review of Ecology and Systematics 25:443–466.

  DOI: 10.1146/annurev.es.25.110194.002303

  This review paper describes five types of indirect effects found in simple communities (i.e., loops of 3–4 species), as well as the current evidence for these effects. Importantly, Wootton called for the development of better techniques for estimating interaction strengths in dynamic systems. Available online for purchase or by subscription.