The Origin of Motif Families in Food Webs

TL;DR

This study identifies two distinct motif families in food webs, linking global hierarchical order, trophic coherence, to local predation patterns and demonstrating a model that can generate and classify these families effectively.

Contribution

It reveals the existence of two motif families in food webs and connects trophic coherence to local predation patterns, supported by a tunable network model.

Findings

Most food webs fall into two motif families.

Trophic coherence predicts food web properties like omnivory.

A network model can generate and classify these motif families.

Abstract

Food webs have been found to exhibit remarkable motif profiles, patterns in the relative prevalences of all possible three-species sub-graphs, and this has been related to ecosystem properties such as stability and robustness. Analysing 46 food webs of various kinds, we find that most food webs fall into one of two distinct motif families. The separation between the families is well predicted by a global measure of hierarchical order in directed networks - trophic coherence. We find that trophic coherence is also a good predictor for the extent of omnivory, defined as the tendency of species to feed on multiple trophic levels. We compare our results to a network assembly model that admits tunable trophic coherence via a single free parameter. The model is able to generate food webs in either of the two families by varying this parameter, and correctly classifies almost all the food webs in our database. This establishes a link between global order and local preying patterns in food webs.