Miller, T.E. and C.P. terHorst. 2012

Miller, T.E. and C.P. terHorst. 2012. Indirect effects in communities and ecosystems. Oxford Bibliographies in Ecology. Ed. David Gibson. New York: Oxford University Press. link

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.

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