Resource competition explains simplicity in microbial community assembly

Predicting the composition and diversity of communities is a central goal in ecology. While community assembly is considered hard to predict, laboratory microcosms often follow a simple assembly rule based on the outcome of pairwise competitions. Despite the empirical success of this bottom-up prediction, its mechanistic origin has remained elusive. In this study, we elucidate how resource competition can lead to simple patterns in community assembly. Our geometric analysis of a consumer-resource model shows that trio assembly is always predictable when some species grow faster than another species on every resource. We also identify all trio assembly outcomes under three resources and find that only two outcomes violate the assembly rule. Simulations demonstrate that pairwise competitions often accurately predict trio assembly with up to 50 resources. We further show that this predictability holds for communities larger than trio and with cross-feeding between species. Our findings highlight that simple community assembly can emerge even in ecosystems with complex underlying dynamics.