Limits to trophic levels and omnivory in complex food webs: Theory and data

Richard J. Williams, Neo D. Martinez

Research output: Contribution to journalArticlepeer-review

231 Scopus citations


While trophic levels have found broad application throughout ecology, they are also in much contention on analytical and empirical grounds. Here, we use a new generation of data and theory to examine long-standing questions about trophic-level limits and degrees of omnivory. The data include food webs of the Chesapeake Bay, U.S.A., the island of Saint Martin, a U.K. grassland, and a Florida seagrass community, which appear to be the most trophically complete food webs available in the primary literature due to their inclusion of autotrophs and empirically derived estimates of the relative energetic contributions of each trophic link. We show that most (54%) of the 212 species in the four food webs can be unambiguously assigned to a discrete trophic level. Omnivory among the remaining species appears to be quite limited, as judged by the standard deviation of omnivores' energy-weighted food-chain lengths. This allows simple algorithms based on binary food webs without energetic details to yield surprisingly accurate estimates of species' trophic and omnivory levels. While maximum trophic levels may plausibly exceed historically asserted limits, our analyses contradict both recent empirical claims that these limits are exceeded and recent theoretical claims that rampant omnivory eliminates the scientific utility of the trophic-level concept.

Original languageEnglish (US)
Pages (from-to)458-468
Number of pages11
JournalAmerican Naturalist
Issue number3
StatePublished - Mar 2004


  • Food chains
  • Omnivory
  • Trophic level

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics


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