The coincidence of rarity and richness and the potential signature of history in centres of endemism

Walter Jetz,* Carsten Rahbek and Robert K. Colwell Ecology and Evolutionary Biology Department, Princeton University, Princeton, NJ, USA Biology Department, University of New Mexico, Albuquerque, NM, USA Zoological Museum, University of Copenhagen, Copenhagen, Denmark Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, USA *Correspondence and present address as of December 2004: Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093, USA. E-mail: [email protected] Abstract We investigate the relative importance of stochastic and environmental/topographic effects on the occurrence of avian centres of endemism, evaluating their potential historical importance for broad-scale patterns in species richness across Sub-Saharan Africa. Because species-rich areas are more likely to be centres of endemism by chance alone, we test two null models: Model 1 calculates expected patterns of endemism using a random draw from the occurrence records of the continental assemblage, whereas Model 2 additionally implements the potential role of geometric constraints. Since Model 1 yields better quantitative predictions we use it to identify centres of endemism controlled for richness. Altitudinal range and low seasonality emerge as core environmental predictors for these areas, which contain unusually high species richness compared to other parts of sub-Saharan Africa, even when controlled for environmental differences. This result supports the idea that centres of endemism may represent areas of special evolutionary history, probably as centres of diversification.