  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.
