The Indochinese-Sundaic zoogeographic transition: a description and analysis of terrestrial mammal species distributions

We describe the distributions of mammal species between the Indochinese and Sundaic subregions and examine the traditional view that the two faunas show a transition near the Isthmus of Kra on the Thai-Malay peninsula. Species distributions are described along a 2000-km transect from 20 degrees N (northernmost Thailand) to 1 degrees N (Singapore). For the 325 species of native non-marine mammals occurring along the transect we used published records to provide a database of their distributional records by degree of latitude. Along the transect we found 128 Indochinese species with southern range limits, 121 Sundaic species with northern range limits, four un-assignable endemics and 72 widespread species. In total, 152 southern and 147 northern range limits were identified, and their distribution provides no evidence for a narrow faunal transition near the Isthmus of Kra (10 degrees 30' N) or elsewhere. Range limits of both bats and non-volant mammals cluster in northernmost peninsular Malaysia (5 degrees N) and 800 km further north, where the peninsula joins the continent proper (14 degrees N). The clusters of northern and southern range limits are not concordant but overlap by 100-200 km. Similarly, the range limits of bats and non-volant mammals cluster at slightly different latitudes. There are 30% fewer species and range limits in the central and northern peninsula (between 6 and 13 degrees N), and 35 more widely distributed species have range gaps in this region. In addition, we found 70 fewer species at the southern tip of the peninsula (1 degrees N) than at 3-4 degrees N. The deficiencies of both species and species range limits in the central and northern peninsula are attributed to an area effect caused by repeated sea-level changes. Using a new global glacioeustatic curve developed by Miller and associates we show that there were > 58 rapid sea-level rises of > 40 m in the last 5 Myr that would have resulted in significant faunal compression and local population extirpation in the narrow central and northern parts of the peninsula. This new global sea-level curve appears to account for the observed patterns of the latitudinal diversity of mammal species, the concentration of species range limits north and south of this area, the nature and position of the transition between biogeographical subregions, and possibly the divergence of the faunas themselves during the Neogene. The decline of species diversity at the southern end of the transect is attributed to a peninsula effect similar to that described elsewhere.