Food-web topology: Universal scaling in food-web structure?

Arising from: D. Garlaschelli, D. Caldarelli & L. Pietronero Nature 423, 165–168 (2003); Garlaschelli, Caldarelli & Pietronero reply. The statistical analysis of empirical food webs seeks to discover patterns in their structure. Garlaschelli et al.1 describe food webs as transportation networks and show that the empirical webs used in their study have universal scaling exponents. Here we analyse 17 of the most comprehensive food webs — including the nine used by the authors1 — but find no evidence for this universality. We also argue that the exponents that are observed are not a signature of food-web architecture but are a general property of networks that have few trophic levels, irrespective of their structure. We conclude that the short range of empirical exponents occurs because food webs contain only a few trophic levels and therefore that it does not add to our understanding of food-web topology.