What are the roles of local interactions and dispersal in community assembly in California grasslands across a soil nitrogen gradient?

摘要

Across the globe, the environment is changing at unprecedented rates in multiple directions due to changes in climate, land use, disturbance regimes, nitrogen (N) deposition and invasive species. In order to understand how these factors are affecting ecosystems, there has been a renewed interest in determining the contribution of local interactions, abiotic conditions, and regional processes in determining species abundance and diversity at multiple scales. Furthermore, in order to successfully restore native ecosystems, it is crucial to understand how local and regional processes affect community assembly. My dissertation aims to explore the role of local interactions and dispersal on the community assembly of California annual grassland communities along a soil N gradient.;Results from this dissertation show that annual species within a community have different recruitment functions due to differences in the relative importance of seed limitation and establishment limitation. Furthermore, the recruitment functions scaled up to community level processes to affect local scale diversity patterns. I found that beta-diversity exhibited a complex pattern of increases and decreases over time in California annual grassland communities. After two growing seasons beta-diversity was higher in a low N soil environment due to increased heterogeneity in soil N and slowed competitive exclusion. Multiple processes were acting throughout the growing season that influenced species abundance, alpha- and beta-diversity, and these processes differed based on the soil N environment. In addition, I found that local interactions, environmental conditions and dispersal structure experimental California annual grasslands communities. As predicted, initial community composition, local interactions, environmental conditions and dispersal act simultaneously to determine local species abundances and diversity at multiple scales.;Future studies in natural areas with intact vegetation are needed to better understand the community's response to N enrichment, but my dissertation work shows that in order to understand and predict community assembly in California grasslands, local interactions and dispersal need to be considered. In addition, results from this dissertation suggest that in order to restore California grasslands, restoration ecologists need to consider the effect of local and regional scale processes on diversity at multiple scales.