Explaining the causes of the zoogeographic transition around the Isthmus of Kra: using bats as a case study

Aim The causes of a zoogeographic divide in peninsular Thailand around the Isthmus of Kra have not been adequately resolved. We explored climatic, historical and geological perspectives to gain insights into factors that may have contributed to the development and maintenance of this zoogeographic transition, and to determine whether a faunal transition occurs for bats. Location Southeast Asia, focusing on the Thai Peninsula. Methods Spatial principal components analysis was used to determine the relationship between climate and species distribution patterns. We studied bats (order Chiroptera) because of their ability to bypass small-scale geophysical barriers. Spatial data on bat species distributions on the Thai Peninsula were analysed in relation to multivariate measures of climate to determine the possible influence of climatic zonation on distribution patterns. We assessed the effects of the interaction of climatic zonation with the highly dynamic environmental conditions the area has undergone in relation to species distribution patterns. Results A zoogeographic transition was found, with 44 species (out of 127) restricted to the north of the Isthmus of Kra and 29 restricted to the south, although there were relatively few abrupt changes in distribution at the exact position of the isthmus. Northern and southern species were associated with specific climatic conditions. Major transitions in the distribution of bat species exist at 6-6.5 degrees N and 13-13.5 degrees N, with a smaller peak at 11.0 degrees N. These major peaks fall in the same areas as the borders of climatic zones, and the 6-6.5 degrees N peak falls in the same area as a floristic divide (the Kangar-Pattani Line). Main conclusions On the mainland, climatic zones cause gradual changes in species distributions. However, in addition to climatic factors, repeated changes in the breadth of the Sunda Shelf during recent glacial cycles may have caused locally high extinction rates at narrow points on the peninsula, exacerbating transitions in species distribution patterns along the region, in the context of a peninsula effect that reduces opportunities for recolonization.