Spatial dynamics of multispecies mutualism

Ecosystems function via a network of interactions between the species. Many of these interactions require physical proximity and are therefore affected by the spatial distribution of the species. We explore spatial structure that emerges due to facilitation and mutualism in multispecies communities and investigate its effects on ecosystem dynamics. In addition to one-way and two-way metabolite exchanges, interactions in multispecies community include loops of cyclic cross-feeding and higher order interactions such as collective mutualism. Collective mutualism requires that several species simultaneously contribute to a particular task such as digestion of a complex polysaccharide. We show that collective interactions become increasingly unstable to demographic fluctuations as the number of participating species increases. Such higher order interactions can nevertheless be stabilized by a network of reciprocal cross-feeding. We show how our results connect to the theory of nonequilibrium phase transitions and discuss the implications of our findings for the design of synthetic microbial communities.