Length and evolutionary stability of food chains

Abstract

Grazing food chains in natural ecosystems tend to be relatively short, usually involving no more than four or five trophic levels ^1–3. Cohen⁴ describes 19 food webs with a total of 102 top predators. Recent independent analyses of these data by Cohen⁴ and Pimm (see ref. 5) show that most top predators occupy the third trophic level. However, as Slobodkin³ has observed, “It is possible to conceive of predators that are small enough and ferocious enough to attempt to live off the top of a trophic pyramid, and thereby become a seventh, or even an eighth, trophic level. It is by no means clear why this situation has not occurred.” Our answer, which we discuss here, incorporates energy transfer calculations introduced by Hutchinson¹ and Odum and Odum² within the framework of the evolutionarily stable strategy (ESS) developed by Maynard-Smith and Price ^6,7. We argue that food chains should collapse to the shortest possible length compatible with biochemical and physiological constraints. This minimal length is frequently three, and is largely independent of primary productivity.