The biogeographical relations of the frogs and snakes of Sundaland

Aim We seek to relate the present distributions of frogs and snakes of Sundaland and the known geological history of the region. Location From the Isthmus of Kra to Java and Sulawesi. Methods We relate the known ecological requirements of frogs and snakes to their geographical distributions and information on geological history. Results Microhabitat requirements for larvae of various groups of frogs are strong predictors of the breadth of their geographical distributions. At the species level, the frog faunas of the Malay Peninsula and Sumatra are the most similar. The Sulawesi frog fauna, mainly derived from Sundaland lineages, shows almost no similarity to the other frog faunas at the species level. The ecological zones occupied by snake species show association with the breadth of their geographical distributions There are only minor differences among similarity ratios for the Malay Peninsula-Sumatran, Malay Peninsula-Borneo, and the Borneo-Sumatra pairs. The Sulawesi snake fauna has distinctly lower similarity with the faunas of the other areas. The similarity ratios between faunas are larger for snakes than for frogs. This difference between the two groups reflects the difference between them in ability to cross salt water barriers, frogs being extremely vulnerable to saline water. Also snakes may establish founder populations more easily as a single gravid female or one carrying stored sperm may introduce a clutch into previously unoccupied territory. Conclusions A few species of frogs and snakes probably reached their present, almost ubiquitous distributions in Sundaland within the last few millenia or even more recently. Other widely distributed species may have been able to disperse among land masses within Sundaland until 10,000-17,000 yr BP; the frogs of this category are common in environments that almost certainly characterized the exposed area of the South China Sea. The distributions of other frogs common to the Malay Peninsula, Sumatra and Borneo probably antedate the Pleistocene, as their larval development requires hilly topography which was not generally available on the Pleistocene-exposed bed of the South China Sea. Many of the endemic species of frogs and snakes probably owe their origins to events of the Miocene or earlier. Several genera of frogs and one genus of snakes have undergone extensive speciation. and display considerable sympatry and elevational stratification of species, suggesting their present distributions are the result of events as old as the Eocene. We have cast these conclusions in the form of hypotheses that can be tested mainly by the use of molecular genetics, but in some cases, by additional field sampling.