Emergence of stable motifs in consumer-resource communities

Priyanga Amarasekare, Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA and Adam T. Clark, Institut für Biologie, Karl-Franzens-Universität Graz, Graz, Austria

Background/Question/Methods
There is strong empirical evidence that ecological communities exhibit recurrent elements (motifs) that characterize other types of biological networks (e.g., transcription, neural, signal transduction). Analyses of network properties in a wide variety of food webs have identified four dominant motifs: exploitative competition, apparent competition, omnivory and tri-trophic chain. Theory suggests that certain motifs are dominant in transcription and neural networks because they constitute the optimal configurations for information transfer that are also robust to perturbations. Extending this idea to ecological communities in which the transfer is of energy rather than information, we can hypothesize that the dominant motifs observed in ecological communities are those that represent configurations that are both feasible and stable: they can emerge under minimal conditions in closed communities, and they are robust to invasions by other species. Here we test this hypothesis using a mechanistic framework that combines the topological features of networks with biologically realistic dynamics of consumer-resource interactions.
Results/Conclusions

We find that, in a closed community in which the total nutrient availability sets the upper limit to the total biomass, the only motifs that can arise de novo are vertical chains of nutrient-plant, nutrient-plant-herbivore, and nutrient-plant-herbivore-predator interactions. Although temporal variation, either through relative non-linearity in functional responses or temporal nutrient partitioning can allow the coexistence of plant species, such coexistence is not robust to invasions by herbivores, just as herbivore coexistence is not robust to innovation by top predators. This result emerges because vertical trophic chains both maximize energy transfer and biomass production in ecological communities. Not only are such motifs stable to perturbations of species' abundances, but they are also robust to species additions and removals. Our findings provide a mechanistic explanation for why tri-trophic chains are overrepresented in real food webs. They suggest that, because they maximize energy transfer, and can emerge and persist under minimal conditions, vertical trophic chains may constitute the fundamental architecture of consumer-resource communities.