Ecological systems are extraordinarily complex. Thus classical approaches to resolve ecosystem functioning have simplified analyses by conceptualizing ecosystems as being organized into trophic level compartments that contain organisms with similar feeding dependencies (e.g., producers, herbivores, carnivores) (Elton, 1927; Lindeman, 1942). Two competing worldviews on the regulation of ecosystem productivity emanated from such a conceptualization of ecosystem structure. The bottom-up view posits that the productivity of each trophic level is essentially limited by the one immediately below it (Lindeman, 1942; Feeny, 1968), while the top-down view recognizes that resource levels influence production, but contends that herbivore populations are mostly limited by predators rather than producer biomass (Hairston et al., 1960). Accordingly, predators can indirectly increase the productivity of a given system by reducing the negative effects of herbivores on plant biomass, resulting in a world that is green with plant material, rather than denuded by herbivory (Paine, 1969; Oksanen et al., 1981). Bottom-up theory countered that the world is green not because of predators, but instead due to variation in plant quality as a result of anti-herbivore defenses or weather patterns (Murdoch, 1966; Ehrlich and Birch, 1967; Scriber and Feeny, 1975; White, 1978; Feeny, 1991; Polis and Strong, 1996). This variation causes much of the “green” world to be inedible to herbivores; thus herbivores are still resource-limited.
The recognition of context-dependence in the degree of top-down or bottom-up control of ecosystems has resulted in gradual changes in how ecosystem functioning is envisioned. For instance, the “exploitation ecosystems” hypothesis (EEH) addresses context-dependence by combining elements of top-down and bottom-up concepts (Oksanen et al., 1981; Oksanen and Oksanen, 2000). At low levels of soil resource availability, plants are not productive enough to support herbivore populations and are thus bottom-up controlled (see Fig. 5.3). At medium levels of soil resources, an ecosystem can support herbivore populations, which in turn control plant productivity, while carnivores enter the ecosystem and control the herbivore population at the highest resource availability, thus releasing plant productivity from herbivore control.