A growing body of literature from a variety of ecosystems shows strong evidence that various components of biodiversity have significant impacts on ecosystem functioning. However, much of this evidence comes from short-term, small-scale experiments in which communities are synthesized from relatively small species pools and conditions are highly controlled. Extrapolation of the results of such experiments to longer time and larger spatial scales¿-those of whole ecosystems¿-is difficult because the experiments do not incorporate natural processes such as recruitment limitation and colonization of new species. In this section of this special feature on the Long-Term Ecological Research (LTER) network, we show how long-term study of planned and accidental changes in species richness and composition suggests that the effects of biodiversity on ecosystem functioning will vary over time and space. More importantly, we also highlight areas of uncertainty that need to be addressed through coordinated, cross-scale and cross-site research.