Structured interactions explain the absence of keystone species in synthetic marine microcosms

From oceans to guts, ecosystems are held together by complex interspecies interactions, and can experience dramatic impacts when certain "keystone" species are removed. Yet, both the prevalence of keystone species and the ecological factors that favor their emergence remain unknown. Here, by experimentally assembling complex marine microcosms in a range of controlled environmental conditions, we find that dramatic impacts (extinctions or blooms of other species) are exceedingly rare upon removal of a species, consistent with no species being a keystone. By mathematically modeling communities with interspecies interactions, we show how progressively structured interactions, as opposed to independent interactions, can systematically reduce the incidence of keystone species. Finally, using statistical learning, we infer the interspecies interactions in our laboratory microcosms and test this theoretical prediction, finding that the interactions are indeed strongly structured. Specifically, species with large abundance experience strong interactions from others, but interact weakly with them in return. Our results suggest that structured interactions may be prevalent in communities, and connect them with the apparent absence of keystone species with dramatic impacts.