Concern about the loss of Earth's biological diversity sparked two decades of research of unprecedented intensity, intellectual excitement, and societal relevance. This research shows that biodiversity is among the most important factors determining how ecosystems function. In particular, the loss of biodiversity decreases the productivity, stability, and efficiency of terrestrial, freshwater, and marine ecosystems. These research findings come at a time of rapidly increasing threats to global biodiversity resulting from agricultural land clearing, climate change, and pollution caused by globally accelerating demand for food and energy. The world faces the grand, multifaceted challenge of meeting global demand for food and energy while preserving Earth's biodiversity and the long-term sustainability of both global societies and the ecosystems upon which all life depends. The solutions to this challenge will require major advances in, and syntheses among, the environmental and social sciences.