Links between biodiversity and ecosystem function provide compelling reasons for conserving maximal numbers of species in ecosystems(1-6). Here we describe a previously unrecognized effect of biodiversity on ecosystem predictability, where predictability is inversely related to temporal and spatial variation in ecosystem properties, By manipulating biodiversity in aquatic microbial communities, we show that one process, ecosystem respiration, becomes more predictable as biodiversity increases, Analysis of similar patterns extracted from other studies(2,3,6) indicates that biodiversity also enhances predictability in terrestrial ecosystems, Biodiversity can also affect average levels of ecosystem performance, but the extent to which different species make unique or redundant contributions to ecosystem processes remains controversial(3,7-10). Nonlinear effects of biodiversity on the decomposition of particulate organic matter and resistance of communities to invasion indicate that different species have redundant functions in our system, The consequences of biodiversity are also not restricted to early successional situations as described in previous studies(1-4,6), because strong effects persist even after ecosystems develop for periods corresponding to 40-80 generations of dominant organisms.