Environmental warming alters food-web structure and ecosystem function

We know little about how ecosystems of different complexity will respond to global warming(1-5). Microcosms permit experimental control over species c omposition and rates of environmental change. Here we show using microcosm experiments that extinction risk in warming environments depends on trophic position but remains unaffected by biodiversity. Warmed communities dispro portionately lose top predators and herbivores, and become increasingly dom inated by autotrophs and bacterivores. Changes in the relative distribution of organisms among trophically defined functional groups lead to differenc es in ecosystem function beyond those expected from temperature-dependent p hysiological rates. Diverse communities retain more species than depauperat e ones, as predicted by the insurance hypothesis, which suggests that high biodiversity buffers against the effects of environmental variation because tolerant species are more likely to be found(6,7). Studies of single troph ic levels clearly show that warming can affect the distribution and abundan ce of species(2,4,5), but complex responses generated in entire food webs g reatly complicate inferences based on single functional groups.