Climate Warming Drives the Breakdown of Plant-Pollinator Mutualisms Despite Phenological Synchrony.

Climate warming threatens biodiversity, driving population declines and endangering species with extinction. The disruption of species interactions and associated loss of ecological function can precede extinction, yet the effects of warming on interactions remain unresolved. Stress-induced changes in traits and phenological mismatch between interacting species are hypothesized to be primary mechanisms by which warming threatens biodiversity. However, we lack experimental evidence of warming-induced interaction disruption, making it difficult to gauge the importance of these trait-based mechanisms of interaction loss. Using experimental plant-pollinator communities, we show that springtime warming weakens the mutualism not by reducing phenological synchrony but by altering multiple physiological and behavioral traits, ultimately depressing both plant and bee fitness. Despite warming-induced phenological shifts in flowering and bee activity, temporal synchrony between partners was maintained. However, warming reduced flower abundance and bee survival, floral rewards, and bee visits to flowers. Our results refocus attention away from phenological disruption, towards physiological and behavioral changes that reduce mutualism strength in a changing climate.