Biodiversity may represent a form of biological insurance against the loss or poor performance of selected species. If this is the case, then communities with larger numbers of species should be more predictable with respect to properties such as local biomass. That is, larger numbers of species should enhance ecosystem reliability, where reliability refers to the probability that a system will provide a consistent level of performance over a given unit of time. The validity of this hypothesis has important ecological, management and economic implications given the large-scale substitution of diverse natural ecosystems with less diverse managed systems. No experimental evidence, however, has supported this hypothesis. To test this hypothesis we established replicated microbial microcosms with varying numbers of species per functional group. We found that as the number of species per functional group increased, replicate communities were more consistent in biomass and density measures. These results suggest that redundancy (in the sense of having multiple species per functional group^,,) is a valuable commodity, and that the provision of adequate redundancy may be one reason for preserving biodiversity.