The enigma of food chain length: Absence of theoretical evidence for dynamic constraints

The question of what, if anything, limits the length of food chains has been problematic ever since Elton originally asked it decades ago. Here, we reexamine the major evidence for one of the chief hypotheses: dynamical constraints. Reportedly, food chain stability decreases as chain length increases. However, the separate roles of self-damping (where a species shows density dependence independent of the densities of other species in the food chain model) and chain length have not been adequately determined. We examined food webs of varying chain length while controlling for self-damping. In contrast to the prevailing hypothesis, we find a weak but significant increase in stability in longer chains when we hold constant the number of species and the number of self-damping terms. Thus, the conclusion that longer food chains are more dynamically fragile, thereby setting an upper limit to food chain length, has at best very limited theoretical support. Our results indicate that community stability should tend to lengthen, not shorten, food chains. Mechanisms limiting the length of food chains remain to be convincingly demonstrated.