Estimating trophic position in marine and estuarine food webs

Structural or binary approaches, based on presence-absence of feeding links, are the most common method of assembling food webs and form the basis of the most well explored food web models. Binary approaches to assembling feeding links are often criticized as being less powerful and accurate than flow-based methods. To test this assumption we compared binary estimates of trophic position with estimates based on stable isotope values of nitrogen (δ15N). For 366 species from eight marine and estuarine food webs we compared trophic position estimates based on binary (presence-absence) feeding links with estimates based on the stable isotope of nitrogen (δ15N). For a subset of 127 fish species in four of the webs we further compared trophic position estimates based on gut content analysis using a flow-based algorithm using data from FishBase.org with binary and δ15N estimates. Across all species and webs binary estimates of trophic position were strongly correlated (R = 0.644) with δ15N estimates. On average binary estimates differed from baseline corrected δ15N estimates by 2.33% for mean trophic position and 6.57% for maximum trophic position. On average the difference between binary δ15N estimates was 0.14 of a trophic level. For the subset of 127 fish species binary estimates performed similarly or more accurately in predicting δ15N values than the flow-based estimates. Binary approaches to assembling feeding links are often criticized as being less powerful and accurate than flow-based methods. Our results show a high concordance between binary and δ15N estimates of trophic position as well as showing that in some cases binary estimates are better predictors of δ15N than flow-based estimates, reaffirming the robustness of the structural approach to assembling food webs. Additional cross-validation studies in other ecosystems are necessary to determine whether our results can be generalized to terrestrial and freshwater ecosystems.

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