The evolutionary processes leading to the development of new species, their extinction, immigration and emigration may be influenced by both historical and contemporary mechanisms.
For instance, falling sea levels and climate during the last glacial period (about 18,000 to 10,000 years before present) had developed a savanna-like habitat in the "exposed" land area that connects the current landmasses in Sundaland (the Malay Peninsula, Sumatera, Java and Borneo). Such massive changes are predicted to have played a key role in influencing faunal composition and geographic distribution of species in the Southeast Asia region.
The current pattern of bird fauna distribution is predicted to fit the proposed landbridges or dispersal routes that appear due to the physical splitting of once continuous area by the processes of continental drift and changes in sea levels during the Last Glacial Maximum (LGM).
A study head by Asociate Professor Dr Mustafa Abdul Rahman of Universiti Malaysia Sarawak (UNIMAS), was conducted to compare the patterns of bird's species distribution in the Wallacea, Sunda and Sahul Shelf (the subregions and island of Thailand, Peninsular Malaysia, Sumatra, Borneo, Java, Bali, Lombok, Sulawesi, the Philippines and New Guinea).
Data on bird fauna in Southeast Asia, as well as New Guinea, was taken from documented field guides. A total of 88 families (with 479 genera and 1718 species) of resident breeding birds were collated (migrant species excluded). This accounts for about 17.6% of the total number of bird species in the world, making this region one of the richest bird's species regions in the world.
Analysis of species similarity among the 10 subregions/island in the study supports the model of biogeographic divergence based on historical changes in sea levels. At each of the three subgroups, the distribution of land and water birds revealed a major transition line that fitted with the glacial model.
New Guinea and the Philippines showed little species resemblance with the rest of the subregions/islands in this study. This finding agrees with the geographic position of New Guinea lying on the continental shelf of Australia and sharing much of its fauna with that island-continent.
The Philippines, on the other hand, was never joined to the Sunda Shelf's islands and mainland Asia during the last glacial period, because of the presence of deep water channels (>200m deep) that separate Philippines from the region. Even when sea level was predicted to have dropped as much as 160 m during the glacial periods, that still failed to connect the Philippines (except Palawan) archipelago with Borneo and the rest of the region.
Their findings suggest that bird's species diversification took place across a range of glacial stages, with ancient diversification between families taking place at a time of lower sea level. Diversification within families occurs at a time when Sumatra moved away from mainland Asia to form a group with Borneo. Diversification within species related closely to mainland Asia.
The pattern of distribution suggests that the descendant that crossed over from mainland Asia or remained in Sumatra during the glacial and inter-glacial period may have perished and became extinct, especially those families that have few species and shows vulnerability to environmental changes and human intervention.
While the findings support the glacial model of Southeast Asia's geographical distribution of biodiversity over space and time, it does not agree with the proposed faunal boundaries of Wallace’s line that separates the zoogeographical regions of Asia and Australasia -- the finding does not point to the major faunal transition between Bali and Lombok as well as between Borneo and Sulawesi. Instead their findings at family level agree with Mayr's line which cuts at the west of Lombok, allowing Lombok to be grouped with Java and Bali.
The bird species composition in Bali shows more resemblance to Java and Borneo while Lombok shares more of its genera and species with Sulawesi; and the bird compositions of Borneo and Sulawesi have little resemblance even at the family level even though the two islands are separated by a 50 km wide Straits of Makasar.
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