How do Complex Food Webs Persist in Nature?

This chapter outlines the current state of the complexity-stability relationship in food webs, the different approaches used to examine this issue, and the current understanding of the mechanisms that appear to stabilize complex natural food webs and highlights some for the most promising research directions for future focus. MacArthur, who expressed stability as the effect of a species with "abnormal" abundance on the abundance of other species, formalized the complexity-stability paradigm. He considered the amount of choice that the energy has in following the paths up through the food web as a direct measure of stability. This "amount of choice," a static feature of web topology, is more synonymous with what is now called complexity. By defining complexity as the amount of choice energy has when flowing through a food web, it is clear that complexity is a function of both the number of species "S" and the number of trophic links "L." Complex food webs contain many species and many interactions. A classic measure of complexity is connectance, while many different expressions have been used for the number of possible interactions. While a conceptual appreciation of stability in the context of complex food webs appears straightforward, measurement of stability in dynamic systems is more difficult, not least because it is often ill defined. In an excellent review on this topic, researchers grouped definitions of stability into two broad categories-those based on a system's ability to defy change and those based on the system's dynamic stability.