Many studies have documented niche differences between species, but the degree to which these differences stabilize coexistence remains poorly understood, leading to persistent questions about the potential for niche differences to explain the high species diversity of natural communities. The neutral theory of biodiversity has renewed interest in this problem by proposing that fitness similarities are more important than niche differences for maintaining diversity. We used long-term data on four sagebrush steppe plant species to build demographically mechanistic models of community dynamics, and then perturbed model parameters to remove the stabilizing effects of niche differences.
Results/Conclusions
With stabilizing mechanisms operating, all species showed positive growth rates when rare, satisfying a classic coexistence criteria, and three of four species persisted indefinitely in stochastic simulations. Fluctuation-independent mechanisms contributed more than temporal variability to diversity maintenance and operated more strongly on recruitment than growth or survival. As expected in the absence of all stabilizing niche differences, three of the four species had negative invasion growth rates and went extinct in stochastic simulations. However, differences in the invasion growth rates of the three grasses were small and competitive exclusion by a single dominant grass required 300-400 years, indicating similarity in fitness. Our analysis shows an excess of niche differences: stabilizing mechanisms were not only strong enough to maintain diversity in this system but were much stronger than necessary to overcome the fitness differences. These results suggest that patterns of species diversity cannot be understood without explicit consideration of the niche differences that neutral theory ignores.