Small rainfall changes drive substantial changes in plant coexistence.

Although precipitation patterns have long been known to shape plant distributions, the effect of changing climate on the interactions of species and therefore community composition is far less understood. Here, we explored how changes in precipitation alter competitive dynamics via direct effects on individual species, as well as by the changing strength of competitive interactions between species, using an annual grassland community in California. We grew plants under ambient and reduced precipitation in the field to parameterize a competition model with which we quantified the stabilizing niche and fitness differences that determine species coexistence in each rainfall regime.