ORIGINAL ARTICLE Spatial congruence of ecological transition at the regional scale in South Africa Berndt J. van Rensburg 1 , Patricia Koleff 2 , Kevin J. Gaston 2 and Steven L. Chown 3 * INTRODUCTION The composition of species assemblages changes in complex ways across space. In some areas the numbers of species gained as a transition or gradient is crossed might be equalled by the number of those lost, whilst elsewhere there may be strong asymmetries in such gains and losses (Gaston, 2000; Bell, 2001). The result is a landscape comprising a spectrum of patterns of transition in species assemblages, from smooth gradients to sharp discontinuities (Cody, 1975; Wilson & Shmida, 1984; Desrochers & Fortin, 2000; Lennon et al., 2001; Rahbek & Graves, 2001). Sharp discontinuities, or regions of considerable species turnover, have long fascinated biologists. At large scales, they have been used to define the spatial boundaries of biogeo- graphical regions, which in turn have formed and continue to form a significant, though often contentious, basis for understanding the evolutionary history of life on earth (see e.g. Cox, 2001; Morrone, 2002). At smaller scales, sharp discontinuities are often used to demonstrate regional faunal 1 Department of Zoology & Entomology, University of Pretoria, Pretoria, South Africa, 2 Biodiversity and Macroecology Group, Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK and 3 Spatial Physiological and Conservation Ecology Group, Department of Zoology, University of Stellenbosch, Matieland, South Africa *Correspondence: Steven L. Chown, SPACE Group, Department of Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. E-mail: slchown@sun.ac.za ABSTRACT Aim To determine whether patterns of avian species turnover reflect either biome or climate transitions at a regional scale, and whether anthropogenic landscape transformation affects those patterns. Location South Africa and Lesotho. Methods Biome and land transformation data were used to identify sets of transition areas, and avian species occurrence data were used to measure species turnover rates (b-diversity). Spatial congruence between areas of biome transition, areas of high vegetation heterogeneity, high climatic heterogeneity, and high b-diversity was assessed using random draw techniques. Spatial overlap in anthropogenically transformed areas, areas of high climatic heterogeneity and high b-diversity areas was also assessed. Results Biome transition areas had greater vegetation heterogeneity, climatic heterogeneity, and b-diversity than expected by chance. For the land transformation transition areas, this was only true for land transformation heterogeneity values and for one of the b-diversity measures. Avian presence/ absence data clearly separated the biome types but not the land transformation types. Main conclusions Biome edges have elevated climatic and vegetation heterogeneity. More importantly, elevated b-diversity in the avifauna is clearly reflected in the heterogeneous biome transition areas. Thus, there is spatial congruence in biome transition areas (identified on vegetation and climatic grounds) and avian turnover patterns. However, there is no congruence between avian turnover and land transformation transition areas. This suggests that biogeographical patterns can be recovered using modern data despite landscape transformation. Keywords Avian species turnover, b-diversity, biome, heterogeneity, spatial congruence. Journal of Biogeography (J. Biogeogr.) (2004) 31, 843–854 ª 2004 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 843