How the coexistence of species is affected by the presence of multiple resources is a major question in microbial ecology. We experimentally demonstrate that differences in diauxic lags, which occur as species deplete their own environments and adapt their metabolisms, allow slow‐growing microbes to stably coexist with faster‐growing species in multi‐resource environments despite being excluded in single‐resource environments. In our focal example, an Acinetobacter species (Aci2) competitively excludes Pseudomonas aurantiaca (Pa) on alanine and on glutamate. However, they coexist on the combination of both resources. Experiments reveal that Aci2 grows faster but Pa has shorter diauxic lags. We establish a tradeoff between Aci2’s fast growth and Pa’s short lags as their mechanism for coexistence. We model this tradeoff to accurately predict how environmental changes affect community composition. We extend our work by surveying a large set of competitions and observe coexistence nearly four times as frequently when the slow‐grower is the fast‐switcher. Our work illustrates a simple mechanism, based entirely on supplied‐resource growth dynamics, for the emergence of multi‐resource coexistence.

Natural communities are incredibly diverse, and explaining how this biodiversity is even possible remains a central question in ecology. Resource competition is thought to play an important role in community interactions, but understanding diverse communities using resource competition is limited by the competitive exclusion principle: the prediction that only as many species as resources should coexist. Here, we demonstrate that sequential resource utilization, also known as diauxie, under periodic growth and death cycles allows for many more coexisting species than resources, violating the competitive exclusion principle. These violations become possible when fluctuations produce variations in the resource depletion order on each growth cycle. The depletion order varying allows the community to experience different sequences of temporal niches on each growth cycle, with temporal niches being the increasingly depleted environments produced by sequential resource depletions. While community-driven fluctuations and competitive-exclusion violations are rare under constant environmental conditions, we find that with even small environmental fluctuations most communities violate competitive exclusion, with several times as many survivors as resources in some simulations. We explore the competitive interactions within these communities and show that survivors are accurately predicted by temporal niches. We thus demonstrate highly diverse communities as a likely outcome of resource competition with a competitive structure based on ordered resource preferences.

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. This assembly rule predicts that a species that is excluded by another species in pairwise competition cannot survive in a multi-species community with that species. Despite the empirical success of this bottom-up prediction, its mechanistic origin has remained elusive. In this study, we elucidate how this simple pattern in community assembly can emerge from resource competition. Our geometric analysis of a consumer-resource model shows that trio community assembly is always predictable from pairwise outcomes when one species grows faster than another species on every resource. We also identify all possible trio assembly outcomes under three resources and find that only two outcomes violate the assembly rule. Simulations demonstrate that pairwise competitions accurately predict trio assembly with up to 100 resources and the assembly of larger communities containing up to twelve species. We then further demonstrate accurate quantitative prediction of community composition using harmonic mean of pairwise fractions. Finally, we show that cross-feeding between species does not decrease assembly rule prediction accuracy. Our findings highlight that simple community assembly can emerge even in ecosystems with complex underlying dynamics.