AbstractEcosystems are simultaneously affected by biodiversity loss and climate change, but we know little about how these factors interact. We predicted that climate warming and CO 2‐enrichment should strengthen trophic cascades by reducing the relative efficiency of predation‐resistant herbivores, if herbivore consumption rate trades off with predation resistance. This weakens the insurance effect of herbivore diversity. We tested this prediction using experimental ocean warming and acidification in seagrass mesocosms. Meta‐analyses of published experiments first indicated that consumption rate trades off with predation resistance. The experiment then showed that three common herbivores together controlled macroalgae and facilitated seagrass dominance, regardless of climate change. When the predation‐vulnerable herbivore was excluded in normal conditions, the two resistant herbivores maintained top‐down control. Under warming, however, increased algal growth outstripped control by herbivores and the system became algal‐dominated. Consequently, climate change can reduce the relative efficiency of resistant herbivores and weaken the insurance effect of biodiversity.