Introduction

An ecosystem is often defined simply as a community of organisms interacting with each other and their biophysical environment. This definition arose from early conceptions of how the natural world is organized and is elegant in its simplicity because it captures the basic elements of a functioning system (Tansley 1935; Leopold 1939; Lindeman 1942). But those trying to develop a synthetic, empirical understanding of how ecosystems function and how they will respond to environmental change are abundantly aware that there is much inherent complexity implied by this seemingly simple definition. To cope with this complexity, ecologists have traditionally abstracted one part of the definition and elaborated the other. For example, ecosystem ecologists have long assumed that interacting organisms can be simply assigned to different compartments (e.g., producer, primary and secondary consumer, decomposer) and focused on environmental and biophysical aspects that dictate the transformation and flow of materials and energy among various compartments (Lindeman 1942; Odum 1969; Likens et al. 1970). In contrast, community ecologists have downplayed the biophysical aspects of materials and energy transfer and focused on organismal populations (Shelford 1913; Elton 1927; Hutchinson 1957; Paine 1966; MacArthur 1972), their diversity and the myriad interactions (e.g., predation, competition, facilitation) that determine their distribution and abundance (Reiners 1986; DeAngelis 1992).

Modern efforts to integrate organismal and abiotic factors into the study of ecosystems arguably were inspired by Hairston, Slobodkin and Smith’s (HSS) classic paper (Hairston et al. 1960), which sought to merge Lindeman’s trophic dynamic perspective (Lindeman 1942) and MacArthur’s population ecology perspective (MacArthur 1958) to explain why, in the face of putatively abundant herbivores, the world is still largely green rather than denuded by herbivory. HSS made the simple argument that the world is green because predators limit the impact of herbivores on plants. This paper highlighted the ecological significance of indirect effects by viewing the biological component of ecosystems as being comprised of linear food chains where interacting species (who eats whom) determine the flow of materials and energy through the ecosystem (Paine 1988; Cohen et al. 1990).