Niche Overlap Is Not Enough: Same Overlap, Contrasting Fluctuations

Niche overlap (NO) is a cornerstone of coexistence theory, summarising the strength of competitive coupling among species. Yet NO collapses distinct mechanisms into a single value and may miss key dynamical features. We quantify this limitation by examining temporal correlations in species abundances, a key out-of-equilibrium observable in microbial ecology. Using a MacArthur-type consumer-resource model, we show that communities with identical NO can display opposite dynamical patterns. Within the resource-mediated fluctuation regime studied here, a yield-depletion mismatch (YDM)-the difference between depletion and yield dissimilarities-consistently predicts the sign and magnitude of abundance correlations across analytical approximations, stochastic simulations, and a reanalysis of microbial time series. In contrast, growth-rate correlations are governed by NO. More broadly, our results show that dynamical observables can depend on mechanistic details beyond those summarised by niche overlap.