The Species Pool and Its Relation to Species Richness: Evidence from Estonian Plant Communities

Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.