Ecologists have long questioned how the wide diversity of species in ecological communities is maintained.  Classic theory shows that stable coexistence requires competitors to differ in their niches, and numerous studies have identified such differences in natural systems.  Still, demonstrating the collective importance of niches for the diversity observed in natural communities has been exceedingly difficult.  In fact, an alternative theory of diversity maintenance, the neutral theory, explains coexistence by the equivalence of competitors rather than niche differences.  We use a novel integration of theory and experimentation to show that niche differences strongly stabilize the dynamics of experimental communities of serpentine annual plants.

Results/Conclusions

Field-parameterized population models predicted that without the demographic influence of niche differences, population growth rates varied by several orders of magnitude between species, sufficient for rapid competitive exclusion.  When these growth rates were experimentally imposed on annual plant communities for two generations, Shannon diversity declined by one third relative to communities stabilized by niche differences.  This difference in diversity arose because niche differences increased the population growth of species when they became rare, as demonstrated in a manipulation of species relative abundance.  Our work provides among the strongest empirical support for the critical role niche differences play in stabilizing species diversity.