IN trying to understand the structure of ecological communities, ecologists usually pay particular attention to the interactions between pairs, or small groups of species¹. Questions about the ‘shape’ of the food webs within which these species are embedded are much more rarely asked^2–4. For example, what happens when a population feeds at more than one trophic level (omnivory)? In some real food webs there seem to be no omnivores (Fig. 1a)⁵; in others omnivores are common^6,7 (Fig. 1c)⁸. In this note we attack the problem of omnivory using simple, linear Lotka–Volterra models of food webs⁹, and show that certain patterns are much more likely to persist on an evolutionary time scale than others. We then compare the model predictions with real food webs.