Explanations of the pattern of species have traditionally relied on small-scale, local processes occurring in ecological time. Differences in species richness have associated with different mechanisms avoiding competition, such as spatiotemporal heterogeneity (weaker competitors may find a more favourable place or time) or environmental stress (competition is assumed to be less intensive under difficult conditions). More recently, large-scale process have been taken into account, raising such questions as: which plant species may potentially grow in a certain community? Are evolutionary processes and species dispersal responsible for the differences between communities? The species-pool theory attempts to answer these general questions, and information about species pools is needed for the design of experiments where the number of species in a community is manipulated.