Species-pool functional diversity as a driver of community assembly

Jonathan A. Myers, Maya D. Samuels-Fair, Benjamin C. Chase and Erin O'Connell, Tyson Research Center, Washington University in St. Louis, Eureka, MO, Jonathan A. Myers, Joseph A. LaManna, Christopher P. Catano, Maya D. Samuels-Fair, Benjamin C. Chase and Erin O'Connell, Department of Biology, Washington University in St. Louis, St. Louis, MO, Marko J. Spasojevic, Department of Evolution, Ecology, and Organismal Biology, University of California Riverside, Riverside, CA, Joseph A. LaManna, Department of Biological Sciences, Marquette University, Milwaukee, WI, Christopher P. Catano, Department of Plant Biology, Michigan State University, East Lansing, MI

Background/Question/Methods
Trait-based ecology and the species-pool concept each provide a path towards understanding community assembly and biodiversity responses to environmental change. However, most species-pool frameworks do not explicitly consider how spatial and temporal variation in the functional-trait diversity of regional species pools, known as species-pool functional diversity, influences community-assembly processes and emergent patterns of biodiversity. Theory suggests that high species-pool functional diversity increases the importance of deterministic community assembly by increasing niche selection across environmental gradients. In contrast, low species-pool functional diversity can increase the importance of stochastic community assembly by increasing historical contingencies in the colonization and establishment of species with similar traits, or by increasing ecological drift when trait similarity reduces fitness differences among species. We explored these questions in a landscape-scale, forest-ecology experiment in the Missouri Ozarks, USA. To test the hypothesis that species-pool functional diversity influences community assembly, we randomly assembled 300 local tree-seedling communities from seed using two experimental species pools that differed in the functional similarity of six leaf and seed traits. To test the hypothesis that species-pool functional diversity influences biodiversity responses to environmental change, we replicated these treatments across three pairs of large (2.5 ha) unburned landscapes and landscapes managed with prescribed fire. In each seedling community, we measured species composition, growth, and mortality over three years.
Results/Conclusions
Our results support the hypothesis that high species-pool functional diversity increases the importance of deterministic community assembly. Seedling communities assembled from a more functionally-diverse species pool were more dissimilar in trait composition (higher functional beta-diversity). Null-model analyses indicated that higher functional beta-diversity reflected greater non-random recruitment of seedlings with respect to species’ traits. Functional beta-diversity of all six traits combined mirrored patterns for two leaf traits related to seedling performance across soil-resource gradients (leaf nitrogen & specific leaf area). Moreover, high species-pool functional diversity increased functional beta-diversity more in burned relative to unburned landscapes, likely due to stronger recruitment limitation in undisturbed sites. These findings suggest that species-pool functional diversity mediates community assembly in two important ways. First, high species-pool functional diversity increases opportunities for niche selection across environmental gradients, leading to divergence in trait composition among communities that differ in environmental conditions. Second, communities assembled from more functionally-diverse species pools are more likely to exhibit divergent responses of biodiversity to disturbance. We highlight implications of species-pool functional diversity for comparative studies of community assembly and restoration outcomes in ecosystems altered by environmental change.