Understanding food-web persistence from local to global scales

Understanding food-web persistence is an important long-term objective of ecology because of its relevance in maintaining biodiversity. To date, many dynamic studies of food-web behaviour-both empirical and theoretical-have focused on smaller sub-webs, called trophic modules, because these modules are more tractable experimentally and analytically than whole food webs. The question remains to what degree studies of trophic modules are relevant to infer the persistence of entire food webs. Four trophic modules have received particular attention in the literature: tri-trophic food chains, omnivory, exploitative competition, and apparent competition. Here, we integrate analysis of these modules' dynamics in isolation with those of whole food webs to directly assess the appropriateness of scaling from modules to food webs. We find that there is not a direct, one-to-one, relationship between the relative persistence of modules in isolation and their effect on persistence of an entire food web. Nevertheless, we observe that those modules which are most commonly found in empirical food webs are those that confer the greatest community persistence. As a consequence, we demonstrate that there may be significant dynamic justifications for empirically-observed food-web structure.