Abstract
Biotic complexity, encompassing both competitive interactions within trophic levels and consumptive interactions among trophic levels, plays a fundamental role in maintaining ecosystem stability. While theory and experiments have established that plant diversity enhances ecosystem stability, the role of consumers in the diversity–stability relationships remains elusive. In a decade‐long grassland biodiversity experiment, we investigated how heterotrophic consumers (e.g., insects and fungi) interact with plant diversity to affect the temporal stability of plant community biomass. Plant diversity loss reduces community stability due to increased synchronisation among species but enhances the population‐level stability of the remaining plant species. Reducing trophic complexity via pesticide treatments does not directly affect either community‐ or population‐level stability but further amplifies plant species synchronisation. Our findings demonstrate that the loss of arthropod or fungal consumers can destabilise plant communities by exacerbating synchronisation, underscoring the crucial role of trophic complexity in maintaining ecological stability.