Fig. 1. The biogeographical units used as basis for the species-area relationships. Each unique combination of one of the 14 natural biome types of the IMAGE model and the biogeographic realms of Olson et al. (2001) is defined as an unit, e.g., all boreal forest in the Paleartic realm. The realms "Oceania" and "Antartic" have not been considered separately in the analysis, leaving a total of six realms.