Food Chains and Food Webs

Abstract

Food webs make up all of ecological nature. Because of their complex nature, food webs are usually simplified for analysis by ecologists. Some simplifications yield simple unbranched food chains, others yield interconnected chains, and yet others yield more complex food webs (which are still much less complex than all species and links found in the web in nature). Every food web and chain has as its backbone, transfer of energy to consumers from producers; this is the bottom-up force. Some but not all webs and chains have opposite top-down effects, which attenuate and change bottom-up forces. Top-down effects are best understood in light of how they change bottom-up effects in food chains and webs. Decomposition is a food web process that often lacks top-down forces because the organisms that break down dead plant and animal material usually do not affect its supply rate. Perhaps the best-known example of strong top-down effects is the trophic cascade in lakes, while a well-researched example of the absence of top-down effects is to be found in the food webs of the aphids living in trees. The lion’s share of net primary productivity on Earth becomes detritus. Detritivory can contribute substantially to the structure and dynamics of ocean food webs, as seen in arctic near-shore ecosystems and in those of the vast abyssal plains on continental margins in the oceans. The complex interplay of top-down and bottom-up forces is only beginning to be revealed by ecological science in the open ocean at a time when heavy fishing is beginning to alter these systems.