Ecologists have long been aware that animals are not mere passive recipients of the nutritious bounty that plants provide them, that they can shape the ecosystems around them through food webs and by altering the nature of the habitat around them. However, until recently such thinking has rarely been scaled up to our understanding of how the function of whole biomes is shaped by animals. Our models of the global biosphere still entirely ignore animals. In this lecture I explore a variety of ways in which large animals can influence ecosystem structure, biomass, fire regimes and even climate, drawing on evidence from the Pleistocene to modern times, and drawing on ongoing experiments and “rewilding” projects around the world. An appreciation of animal influence on ecosystem function leads to awareness of how many even apparently low disturbance ecosystems may carry the legacy of past extinctions. In a world of increasing human pressure and shifting climates, animals can have an important role in maintaining resilient ecosystems.