FOOD-WEB COMPLEXITY AND COMMUNITY DYNAMICS

Food webs in nature have multiple, reticulate connections between a di versity of consumers and resources. Such complexity affects web dynami cs: it first spreads the direct effects of consumption and productivit y throughout the web rather than focusing them at particular ''trophic levels.'' Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital chan nels, other habitats, life-history omnivory, and even trophic mutualis m. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lo wer than if donor-controlled resources were absent. We propose that su ch donor-controlled and ''multichannel'' omnivory is a general feature of consumer control and central to food web dynamics. This observatio n is contrary to the normal practice of inferring dynamics by simplify ing webs into a few linear ''trophic levels,'' as per ''green world'' theories. Such theories do not accommodate common and dynamically impo rtant features of real webs such as the ubiquity of donor control and the importance and dynamics of detritus, omnivory, resources crossing habitats, life history, nutrients (as opposed to energy), pathogens, r esource defenses, and trophic symbioses. We conclude that trophic casc ades and top-down community regulation as envisioned by trophic-level theories are relatively uncommon in nature.