Ecological communities are characterized by a wide diversity of species with important functional, economic, and aesthetic value. Ecologists have long questioned how this diversity is maintained. Classic theory shows that stable coexistence requires competitors to differ in their niches, motivating numerous investigations of ecological differences presumed to maintain diversity. That niche differences are key to coexistence, however, has recently been challenged by the neutral theory, which explains coexistence by the equivalence of competitors. The ensuing controversy has motivated calls for better understanding the collective importance of niche differences for the diversity observed in ecological communities. Dr. Levine will present a novel integration of theory and experimentation to show that niche differences collectively stabilize the dynamics of experimental communities of serpentine annual plants. The study used field-parameterized population models to develop a null expectation for community dynamics without the stabilizing effects of niche differences. The population growth rates predicted by this null model varied by several orders of magnitude between species, sufficient for rapid competitive exclusion. Moreover, after four generations of community change in the field, Shannon diversity was over 100% greater in communities stabilized by niche differences relative to those exhibiting dynamics predicted by the null model. Last, in an experiment manipulating species’ relative abundance, population growth rates increased when species became rare - the demographic signature of niche differences. This research provides strong evidence that species differences play a critical role in stabilizing species diversity.