Emergence of stable motifs in consumer-resource communities

Understanding how and why complex communities can be stable has preoccupied ecologists for over a century.  Data show that real communities tend to exhibit characteristic motifs and topologies.  Despite a large body of theory investigating both ecological (niche partitioning) and evolutionary (speciation and extinction) mechanisms, a general explanation for why particular motifs are more common than others remains elusive.  Here we develop a mechanistic framework that investigates the set of possible motifs that can emerge under minimal conditions of a nutrient-limited system with no external inputs, and no spatial heterogeneity.  Focusing on consumer-resource communities structured by competition and predation, we find that the emergent motifs under these minimal conditions are vertical trophic chains that maximize energy transfer and biomass production. Not only are such motifs stable to perturbations of species9 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.