Environmental heterogeneity promotes regional coexistence through shifts in local competitive dominance

Understanding plant competitive outcomes, driven by niche and fitness differences, requires exploring these differences across simultaneous changes in abiotic and biotic conditions, such as soil properties, mutualisms and natural enemies. Here, we combined coexistence theory with detailed field observations in an annual grassland over two consecutive years and in silico simulations to assess changes in competitive outcomes, from coexistence to competitive exclusion, and from exclusion to priority effects. Our analysis shows that competitive exclusion is the most common outcome but with marked changes in the identity of the superior competitor under abiotic and biotic variation, highlighting the potential for regional coexistence. Herbivores were identified as the factor with the largest effect on the sensitivity of species pairs to shifts between coexistence and exclusion, although the effect size reflects sensitivity rather than the direction of change. Additionally, herbivores strongly influence the identity of the superior competitor, which can promote coexistence at larger spatial scales. Local coexistence was rarely predicted in our system. Finally, priority effects were mostly observed in the absence of a deterministic driver, as expected. Overall, our results suggest that while local coexistence is limited, species diversity can be maintained across larger spatial scales through shifts in competitive dominance mediated by multiple abiotic and biotic drivers.