Human activities have resulted in rising temperatures and the introduction or extirpation of top predators worldwide. Both processes generate cascading impacts throughout food webs and can jeopardize important ecosystem services. We examined the impact of fish stocking on communities and ecosystems in California mountain lakes across an elevation (temperature and dissolved organic carbon) gradient to determine how trophic cascades and ecosystem function vary with climate. Here, we show that the impact of fish on the pelagic consumer-to-producer biomass ratio strengthened at low elevation, while invertebrate community composition and benthic ecosystem rates (periphyton production and litter decomposition) were most influenced by predators at high elevation. A warming climate may therefore alter the stability of lake ecosystems by shifting the strength of top-down control by introduced predators over food web structure and function.