Biotic complexity, encompassing horizontal (within trophic levels) and vertical (among trophic levels) complexity, is crucial for ecosystem stability through various interactions. While plant diversity (horizontal complexity) is known to enhance ecosystem stability from both theory and experiments, the role of consumer diversity (vertical complexity) in these relationships remains underexplored. Over a decade-long grassland experiment, we investigated the effects of these dual complexity gradients on the temporal stability of primary producers by systematically manipulating number of plant species and the presence of heterotrophic consumers. A loss of plant diversity reduced plant community stability but increased the population-level stability of the remaining species. Reducing vertical complexity amplified the synchronization among species caused by plant diversity loss but did not affect either community- or population-level stability. These findings emphasize how consumer diversity loss potentially destabilizes plant communities by exacerbating synchronization and underscore the crucial role of trophic complexity in maintaining ecological equilibrium.