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.
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