Predator loss amplifies the disruptive impacts of heatwaves

Heatwaves are intensifying under climate change, often triggering the catastrophic loss of predators. Still, it remains unclear whether their ecological impacts on ecosystems arise from direct thermal stress or from altered food web dynamics following predator loss. Using experimental tri-trophic fresh water food webs, we independently manipulated heatwave intensity (none, moderate or severe) and predator persistence (fish removed during the onset of heatwaves versus fish persisted during heatwaves) to disentangle these pathways. We show that predator loss, not heat alone, is the primary driver of community restructuring following a heatwave. Predator loss produced strong trophic cascades, and communities became dominated by large Daphniidae consumers that were less diverse, less temporally stable and more synchronized. These changes coincided with declines in primary producers despite increased nutrient availability from decomposing fish. By contrast, direct thermal effects were modest and depended strongly on trophic structure. When predators persisted, heatwaves intensified top-down pressure, producing declines in predator-sensitive consumers. When predators were lost, heatwaves instead magnified trophic disruptions by releasing vulnerable consumers from top-down control, generating substantial and persistent shifts in community structure. Ecosystems that retain predators may therefore be more resilient to heatwaves, underscoring predator conservation as a key strategy for maintaining ecosystem stability under accelerating climate extremes.