Climate variability disrupts microbial mutualism-driven population persistence.

Understanding how species interactions impact population dynamics and long-term persistence over broad temporal and spatial scales is crucial for predicting species distributions and responses to global change. Here we investigate how microbial mutualisms can promote long-term and range-wide population persistence of plants, particularly by ameliorating drought stress. We integrate range-wide field surveys of ~90 grass host populations spanning 13 years with demographic modelling based on 6-year common garden experiments conducted across the host range. We found that mutualistic fungal endophytes (genus Epichloë) promote population-level persistence and growth of their native host grass (Bromus laevipes) across its distribution, with non-mutualistic populations four times more likely to go locally extinct. However, endophyte prevalence declined eightfold more in historically mutualistic populations that experienced high climate variability. This demonstrates that mutualisms can underpin population persistence and buffer hosts against environmental stress but may themselves be vulnerable to global change, with concerning implications for long-term population viability and, ultimately, species distributions under an increasingly uncertain climate.