Yield and Dynamics of Tri-Trophic Food ChainsO. deFeoauthorS.RinaldiauthorStrong relationships between top productivity and dynamic behavior of tri-trophic food chains are pointed out by analyzing the classical Rosenzweig-MacArthur model. On one hand, food chains are subdivided into under-supplied and over-supplied, the first being those in which a marginal increase of nutrient supply to the bottom produces a marginal increase of mean top productivity. On the other hand, a detailed bifurcation analysis proves that dynamics complexity first increases with nutrient supply (from stationary to low-frequency cyclic regime and, finally, to chaos) and then decreases (from chaos to high-frequency cyclic regime). A careful comparison of the two analyses supports the conclusion that food chains cycling at high-frequency are over-supplied, while all others are under-supplied. A straightforward consequence of this result is that maximization of top productivity requires a chaotic regime. This regime turns out to be very often on the edge of a potential catastrophic collapse of food yield. In other words, optimality implies very complex and dangerous dynamics, as stated long ago for di-trophic food chains by Rosenzweig in his famous paper on the paradox of enrichment.1996-07WP-96-075Monograph