Seed dispersal is more limiting to native grassland diversity than competition or seed predation

1. Competition has historically been viewed as the predominant process affecting plant community structure. In particular, it is often assumed that the dominant resident species is the superior competitor and therefore has large impacts on plant community diversity. This assumption, however, is seldom tested. As well, there are a variety of other processes such as dispersal limitation and seed predation that can influence community structure, although the relative importance of these processes in relation to competition from resident species is still unclear. 2. We examined how interspecific competition, dispersal limitation and post-dispersal seed predation by mice individually and interactively influenced the richness and diversity of local plant assemblages in grasslands of western Montana. 3. We added seeds of 20 mostly locally uncommon native species to subplots in and out of larger rodent exclosure plots. Using a 'species-blind' approach, we manipulated competition from resident plants in these subplots by removing the same amount of cover of one dominant species or several common species. 4. The species richness and diversity of local assemblages was higher in subplots with seed addition than without. Across all levels of the other treatments, average richness for subplots without competitor removal was lower than for competitor removal treatments. However, the removal of one locally dominant species had similar effects to removal of several common species. In contrast, preventing seed predation by mice did not have significant effects on richness. There were no significant interactions between the treatments. 5. Synthesis. Our results reveal a hierarchy of filters that determine local community structure. Many regionally rare species were dispersal limited and established after the seed addition, regardless of release from competition or the presence of seed predators. Subsequently, we found competitive equivalence between dominant and common species.