Microbial communities demonstrate robustness in stressful environments due to predictable composition shifts

Environmental stress reduces species growth rates, but its impact on the function of microbial communities is less clear. Here, we experimentally demonstrate that increasing salinity stress shifts community composition towards species with higher growth rates. As a result, the mean community growth rate is more robust to increasing stress than the growth of individual species. We show this by propagating natural aquatic communities at multiple salinities and mapping the observed diversity onto the measured salinity performance curves of >80 bacterial isolates. We further validate these results with pairwise species competitions and in metagenomic data of natural communities sampled from estuarine environments. A Lotka-Volterra model including mortality and salinity-dependent growth rates recapitulates the observed robustness of community growth sustained by more abundant faster growers at high salinity. These results extend to other environmental stressors and point to fundamental mechanisms with which communities maintain growth despite deteriorating conditions.