Complexity theory predicts that localized interactions between organisms can generate intricate spatial patterns at large spatial scales, through a process called self-organization. We tested this premise in mussel beds, under both field conditions and in laboratory mesocosms.

Mesocosm experiments revealed that interactions between individual mussels generate spatial patterns, despite of the absence of environmental heterogeneity and starting from near-homogeneous initial conditions. The experiments revealed that mussels aggregate to a constant local, within-aggregate, biomass, regardless of four-fold differences in the overall (per m²) biomass of mussels. We conducted field experiments to investigate for the emergent effects of spatial patterning on mussels survival and growth. Isolated mussels were found to have the highest growth, as they face little competition, but were very vulnerable to dislodgement by waves. Mussels living in dense communities were well protected against waves, but their growth was low because of high competition. Mussels living in patterns experienced the best of both worlds: their growth was about equal to that of isolated mussels, while vulnerability to waves was as low as for mussels in dense communities.

Our experiments provide one of the first experimental verifications of the concept of spatial self-organization in an ecological system by demonstrating that a symmetry-breaking instability, resulting from the interactions between mussels, causes the formation of spatial patterns. Moreover, the experiments revealed important emergent properties of pattern formation, as aggregation improves both survival and growth.