How to determine a regional species pool: a study in two Swedish regions

The species pool hypothesis has been proposed as one of the possible explan ations for the local species richness of plant communities. For testing and validating this theory, it is of crucial importance to determine the dimen sion of the regional pool, which is the reservoir of species that are poten tially able to exist in a community. The main purpose of this study was to develop and rest different methods for the determination of the regional sp ecies pool. Two regions in Sweden served as study areas, Oland and Uppland. In both regions, three different vegetation types were treated: dry calcar eous grasslands, coastal meadows and deciduous forests. For the determinati on of the regional pool two main groups of methods are proposed: 1) six eco logical approaches, based on Ellenberg species indicator values, and 2) two phytosociological approaches, based on the occurrence of species in differ ent syntaxa in the framework of the Braun-Blanquet system. The different sc reening methods were tested using Sorensen's index expressing the similarit y between the community species pool and the regional species pool. Two typ es of error were recognized which may result in low index values. For the six ecological methods Sorensen's index values were below 50%. The methods differed considerably from each other in accuracy, due to large dif ferences in errors of both types. The phytosociological methods resulted in higher similarity values of up to almost 70%. The two approaches differed in error type but gave similar results.