Commentary: New insights on a classic topic: The biogeography of Southeast-Asian mammals

Abstract

ISSN 1948-6596 news and update and Allen, J.R.M. (eds.) Past and Future Rapid Environmental Changes: The Spatial and Evolu- tionary Responses of Terrestrial Biota, pp. 73- 80. Springer-Verlag, Berlin. Lee Hannah Conservation International Center for Applied Biodiversity Science, Arlington, VA, USA and University of California at Santa Barbara, USA e-mail: l.hannah@conservation.org http://www. bren.ucsb. edu/people/Faculty/ lee_hannah.htm Edited by Joaquin Hortal commentary New insights on a classic topic: The biogeography of Southeast- Asian mammals Investigating the fauna of Malesia, the insular re- gion between Indochina and Australia, has a long history in biogeography. In relating the distribu- tion of animals to geographic settings of the pre- sent and the past, A.R. Wallace (1869) has not only defined our discipline but also provided data and ideas that substantially aided understanding organic evolution (e.g., Davies 2008). Given the depth of investigation of the Malesian region by Wallace and subsequent researchers until today (see references in Woodruff and Turner 2009), it is surprising that the delineation of the region to- wards the north-west, i.e. between ‘Sundaland’ and the Indochinese continental region, and the causes for this faunal transition, remained out of focus for so long. In their recent article, Woodruff and Turner (2009) have used data on mammal distributions to investigate the faunal transition between conti- nental Southeast Asia and the Thai-Malay penin- sula (Fig. 1), building upon earlier analyses of the group (e.g. Woodruff 2003, Hughes et al. 2003). Woodruff and Turner show that there is not, in contradiction to common presumption and earlier results (e.g., Cranbrook 1981, Hughes et al. 2003, de Bruyn et al. 2005), a well-defined Indochinese- Sundaic faunal boundary, but rather a ca. 800 km gap between a southern fauna of the Malay pen- insula (south of 5oN) and the continental fauna (north of 14oN). In this region species richness is reduced, and even many widespread species have gaps in their distributions. The authors discuss sea level changes as an agent that may explain this pattern. In the following I will first appraise two methodological and conceptual features – namely, ‘simplistically’ relying on raw distributional data, and the consideration of ancient shorelines during times when sea levels were higher than today. I will then raise the issue of historical versus envi- ronmental explanations in biogeography, in rela- tion to Woodruff and Turners’ fine work. Woodruff and Turner relied on actual pres- ence records to analyze the limits of species’ dis- tribution. The nature of their study region, with a clear north-south orientation, allowed using latitu- dinal range limits as a simple measure of distribu- tion. For all but the best-sampled regions and taxa of Europe and North America, such data are surely biased by undersampling – species may occur well outside recorded limits, but have not yet been found. A large number of methods, ranging from more or less clearly defined ‘expert range assess- ments’ to numerical models of species’ ecological niches (e.g., Elith et al. 2006), have been proposed to correct for such biases. However, such esti- mates of ‘true’ ranges are unable, at present, to account for the effects of dispersal barriers or lo- cal extinctions (e.g., Munguia et al. 2008), but re- quire a priori assumptions on distributional limits caused by historical factors. Any gain by correcting for undersampling would therefore be punished by an increased circularity of arguments when the aim is to define historically caused boundaries of distribution. Decisions on using range estimates or raw data must be weighted in light of the investi- gated system (i.e., degree of undersampling) and the questions asked, and in Woodruff and Turner’s study this is clearly in favour of the ‘old- © 2009 the authors; journal compilation © 2009 The International Biogeography Society — frontiers of biogeography 1.1, 2009