Coexistence in five perennial plant communities: weak fitness differences, strong stabilizing niche differences, and a large role for recruitment
Contemporary coexistence theory demonstrates that niche differences must be strong enough to overcome relative fitness differences. However, we do not yet know whether stable coexistence in natural communities reflects strong niche differences overcoming weak fitness differences, or weak niche differences stabilizing species of roughly equivalent fitness. Using multispecies population models fit to long-term demographic data from five perennial plant communities in western North America, we estimated the strength of stabilizing niche differences and relative fitness differences. We also investigated the contribution to stabilization made by processes operating at survival, growth, and recruitment stages.
In all five communities, we found evidence for large niche differences and small relative fitness differences. For a total of 17 pairwise species comparisons, a measure of niche differences based on simulations of invasion growth rates ranged from 0.56 to 0.96 with a mean of 0.82, where 0 indicates perfect niche overlap and 1 indicates zero niche overlap. A corresponding measure of relative fitness differences ranged from 1.01 to 3.16 with a mean of 1.60, where 1 indicates identical fitness and a value of 2 indicates a four-fold difference in sensitivity to competition. In all five communities, species interactions operating at the recruitment stage contributed far more to stabilization than interactions affecting growth and survival. Our results indicate that 1) niche differences are typically much stronger than minimally necessary to stabilize coexistence, 2) community structure reflects wide variation in intraspecific density dependence rather than variation in consistently weak interspecific interaction, and 3) processes operating at early life stages play a disproportionately strong role in stabilizing coexistence. These findings have implications for understanding the community-level impacts of global change and help identify priorities for future research.