Predictable shifts in microbial species composition lead to community-wide robustness to environmental stress.

Environmental stress reduces species growth rates, but how it impacts microbial community function is less clear. Here we experimentally demonstrate that increasing salinity stress shifts community composition towards species with higher growth rates. This shift leads the mean community growth rate to be more robust to increasing stress than the growth of individual species. We demonstrate this by propagating natural aquatic communities at multiple salinities and mapping the observed diversity onto the measured salinity performance curves of more than 80 bacterial isolates. We validated these results with pairwise in vitro species competitions and using 16S data of microbial communities from estuarine environments. A generalized 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 could be extended to other environmental stressors, pointing to a fundamental mechanism with which communities maintain growth despite deteriorating conditions.