Motifs in the assembly of food web networks
Community assembly lies at the intersection of biogeography and community ecology. Local communities are shaped by ecological interactions (e.g., predation, competition, mutualism, parasitism), local environmental conditions, neutral processes, and the regional pools from which they are assembled, which in turn are shaped by biogeographic and evolutionary history. Community assembly theory largely focuses on competition as the driving force behind patterns of taxonomic, trait, and phylogenetic structure of local communities. Trophic interactions (i.e., assembly of food web networks) have received far less attention in community assembly theory and do not neatly fit into the competition-based framework.
One approach to exploring community assembly beyond patterns of competition within a food web network context is using motifs. Motifs are subgraphs of nodes (e.g., species) and links (e.g., predation) whose abundance within a network deviates significantly as compared to a random network topology. Motifs provide a novel framework for exploring community assembly by explicitly including interactions as opposed to inferring them from patterns of taxonomic or phylogenetic composition. Here, we use three node motifs to explore the assembly of food web networks that inhabit the northern pitcher plant, Sarracenia purpurea, across two scales; from the continental scale to the regional scale and from the regional scale to the local scale. We specifically seek to understand if motif representation (i.e., under, over, or random) in networks matches the motif representation in the network they were assembled from.
Motif representation in networks generally matched the motif representation in the network they were assembled from, regardless of motif and null model, for both continental-to-site and site-to-local network assembly. This consistency shows that the assembly process results in local networks that are a structurally representative samples of the network they are assembled from in terms of motif representation. Two aspects of the pitcher plant network that may facilitate this pattern are 1) well-defined trophic levels and 2) high generality (i.e., same prey items) and vulnerability (i.e., same predators) of the species within each trophic level. As a result, many species are interchangeable (i.e., redundant) in terms of motif contribution to local food web networks. To generalize our results, patterns of motif assembly must be explored across different systems. Furthermore, to effectively employ a motif perspective to study community assembly, a theoretical framework detailing potential mechanisms for all possible relationships in motif representation between a regional and local networks is necessary.