Time-varying interactions change coexistence mechanisms

Species interactions are fundamental to maintaining diversity. However, it remains unclear how environmental variability modifies the structure, sign, and strength of species interactions that ultimately affect coexistence mechanisms. Here, we combined structural stability theory with time-varying population models to study the temporal variation of niche and fitness differences over nine years of annual plants in Mediterranean grasslands. Rainfall variation modulated species-specific responses in performance, and changed the magnitude of self-limitation and the variability in the strength of interspecific interactions for both competition and facilitation. These changes implied substantial variation in yearly niche and fitness differences, which ultimately impacted community stability. Additional simulations show that this full biological reorganization due to rainfall variation can maintain diversity by promoting temporal shifts in winners and losers compared to a single climatic year. Our results highlight the need to incorporate time-varying interactions to understand species coexistence under contrasting environmental conditions.