Dominant species have long been appreciated for their role in determining ecosystem attributes such as vegetation structure, successional patterns, soil characteristics, hydrology, and productivity. Exotic species may reach such high densities that they become community dominants, and it is in this role that exotics pose the greatest threat to native ecosystems. Four commonly observed patterns related to species dominance and their implications for understanding exotic invasions are discussed: the importance of scale in relationships between dominance and diversity, the positive correlation between local abundance and geographic range, the effects of dominants on ecosystem processes, and the mass effect of seed production in determining dominance. Understanding determinants of dominance will help us to better understand community invasibility.

Additional index words: Diversity, ecosystem effects, geographic range, mass effect.