From Populations to Ecosystems: Theoretical Foundations for a New Ecological Synthesis

A food web describes the network of trophic interactions between species, i.e., who eats whom, in an ecosystem. Since trophic interactions are both the vehicle of energy and material transfers and one of the most significant ways in which species interact, they have always lain at the confluence of community and ecosystem ecology. But they have been approached from different perspectives in different traditions. The energetic view articulated by Lindeman (1942) and developed by ecosystem ecology during the following decades views food webs as networks of pathways for the flow of energy in ecosystems, from its capture by autotrophs in the process of photosynthesis to its ultimate dissipation by heterotrophic respiration. A different approach, rooted in community ecology, was initiated by Elton (1927) and developed by May (1973), Pimm (1982), and many others. This approach focuses on the dynamical constraints that arise from species interactions and emphasizes the fact that too much interaction (whether in the form of a large number of species, a large connectance among these species, or a high mean interaction strength) destabilizes complex ecological systems, including food webs. Food webs have also been studied from a topological perspective: the pattern of trophic interactions in a food web is nonrandomly related to species traits, in particular, body size, which led to the development of size-based models of food-web structure such as the cascade and niche models (Cohen et al. 1990; Williams and Martinez 2000). Perhaps the approach that lies most closely to the interface between community and ecosystem ecology is that based on the trophic cascade concept (Carpenter et al. 1985). Hairston et al. (1960) hypothesized that carnivores control herbivores, thereby releasing plants from control by herbivores, in most ecosystems. This simple idea led to a flurry of studies on the community- or ecosystem-level consequences (though mostly the biomass of the various trophic levels) of the top-down control exerted by higher trophic levels on lower trophic levels.

-79-