Both trophic and non-trophic interactions determine group structure in the Tatoosh mussel bed network

Background/Question/Methods

Trophic and non-trophic interactions are both known to impact ecological dynamics. However, ecological networks are typically built using only one type of interaction (consumption, pollination) at a time. How well does one specific subset of interactions reflect the overall structure of the network? Using the group model sensu Allesina and Pascual 2009, we group species that have similar trophic roles in a network. We extend the group model to account for non-trophic interactions in addition to trophic ones. We build a network containing all trophic and non-trophic interactions occurring in the mussel bed of Tatoosh Island. We contrast the grouping of species found when only trophic relationships are considered with that obtained when considering non-trophic interactions as well.

Results/Conclusions

The best partition found for the trophic Tatoosh web grouped species into 7 groups, including distinct groups for algae, small producers and detritus, grazers, and worms and predatory snails. In contrast, the best partition found for the complete Tatoosh web grouped species into 17 groups. We find that non-trophic links reinforce algae as a distinct functional group in the Tatoosh mussel bed, and other interactions reveal additional functional groups, such as predatory snails, limpets, barnacles, and birds of prey. Taxonomic grouping, while better than expected by chance, does not accurately capture the structure of the complete web. The trophic web grouping matches the complete web grouping much more closely, but still fails to completely capture the complete web structure. This exercise shows that non-trophic interactions are vital for better describing functional roles in networks. Adding non-trophic interactions to food web data will likely lead to a re-assessment of structural and dynamical features of ecological networks.