The relative importance of dispersal- and trait-mediated processes in determining community composition

Understanding the interplay between dispersal and trait-based processes is becoming more urgent as local and regional species pools are becoming increasingly altered by human activity. We hypothesized that early stages of community assembly (arrival and establishment) are mediated by dispersal, while later stages (growth and reproductive output) are trait-mediated processes. We tested the contribution of dispersal and trait-deterministic processes in experimental California annual grassland communities by varying the initial abundance of species (with equal representation of forbs, grasses, and N-fixing legumes) through seed addition and following species abundances through time. These manipulations were conducted across three nitrogen levels to determine potential shifts in the importance of dispersal and species traits across abiotic environments. Strong dependency of establishment and final abundance on initial sowing abundance indicated the importance of dispersal limitation while consistent shifts in final abundance of species indicated the importance of trait-based deterministic processes. The contribution of initial sowing abundance to final abundance differed among species and functional groups. N-fixing legume final abundance was not as strongly influenced by initial abundance compared to the grass and forb functional groups. Indicating that species traits may play a more important role for N-fixing species. Initial seeding abundance did not correlate with end of season seed production suggesting species traits are more important in determining reproductive success. These results suggest restoration efforts should focus on overcoming dispersal limitation across environments and focus on manipulating the environment (i.e. adding carbon to reduce soil nitrogen levels) in high nitrogen environments.