Stochastic Forces in Microbial Community Assembly: Founding Community Size Governs Divergent Ecological Trajectories

Biological community dynamics arise from both deterministic and stochastic processes. While species' responses to environmental factors define attractors of community structure, stochasticity, particularly during early assembly, can redirect ecological trajectories. However, quantifying such roles of stochasticity in community assembly has remained challenging. We tracked community assembly in two multi-replicated experimental systems, each with four levels of founding community size, analysing > 3000 samples across four time points. Stronger initial stochasticity led to greater divergence of both population- and community-level consequences. Strikingly, conspicuous differentiation into alternative trajectories of community assembly occurred when the absolute number of founding prokaryotic cells was less than the order of 10⁴. Thus, quantitative differences in stochasticity produced qualitative differences in community fate. These results demonstrate that early stochastic events can have enduring impacts on ecological dynamics. Deeper quantitative insights into stochasticity will reorganise our views on biological invasions, agroecosystem microbiome management, and therapeutics of human-associated microbiomes.