Sundaland - WikiHQ

thumb|400px|[[Sundaland during the Last Glacial Maximum]]

Sundaland (also called Sundaica or the Sundaic region) is a biogeographical region of Southeast Asia corresponding to a larger landmass that was exposed throughout the last 2.6 million years during periods when sea levels were lower. It includes Bali, Borneo, Java, and Sumatra in Indonesia, and their surrounding small islands, as well as the Malay Peninsula on Mainland Southeast Asia.

Extent

The area of Sundaland encompasses the Sunda Shelf, a tectonically stable extension of Southeast Asia's continental shelf that was exposed during glacial periods of the last 2 million years. It is also roughly coterminous with the Sunda plate.

The extent of the Sunda Shelf is approximately equal to the 120-meter isobath. In total, the area of Sundaland is approximately 1,800,000 km2. The northern border of Sundaland is more difficult to define in bathymetric terms; a phytogeographic transition at approximately 9ºN is considered to be the northern boundary. In contrast, the sea level was higher during the late Pliocene, and the exposed area of Sundaland was smaller than what is observed at present. During the Last Glacial Maximum the sea level fell by approximately 120 meters, and the entire Sunda Shelf was exposed.

The warm and shallow seas of the Sunda Shelf (averaging 28 °C or more) are part of the Indo-Pacific Warm Pool/Western Pacific Warm Pool and an important driver of the Hadley circulation and the El Niño-Southern Oscillation (ENSO), particularly in January when it is a major heat source to the atmosphere. Dipterocarps are notable for mast fruiting events, where tree fruiting is synchronized at unpredictable intervals resulting in predator satiation. Higher elevation forests are shorter and dominated by trees in the oak family. The fish is now found in the Kapuas River on the island of Borneo, and in the Musi and Batanghari rivers in Sumatra. Selective pressure (in some cases resulting in extinction) has operated differently on each of the islands of Sundaland, and as a consequence, a different assemblage of mammals is found on each island. However, the current species assemblage on each island is not simply a subset of a universal Sundaland or Asian fauna, as the species that inhabited Sundaland before flooding did not all have ranges encompassing the entire Sunda Shelf. In an 1852 publication, English navigator George Windsor Earl advanced the idea of a "Great Asiatic Bank", based in part on common features of mammals found in Java, Borneo and Sumatra.

Explorers and scientists began measuring and mapping the seas of Southeast Asia in the 1870s, primarily using depth sounding. In 1921 Gustaaf Molengraaff, a Dutch geologist, postulated that the nearly uniform sea depths of the shelf indicated an ancient peneplain that was the result of repeated flooding events as ice caps melted, with the peneplain becoming more perfect with each successive flooding event. and described in greater detail by Emmel and Curray in 1982 complete with river deltas, floodplains and backswamps.

Data types

The climate and ecology of Sundaland throughout the Quaternary has been investigated by analyzing foraminifera<nowiki/>l δ18O and pollen from cores drilled into the ocean bed, δ18O in speleothems from caves, and δ13C and δ15N in bat guano from caves, as well as species distribution models, phylogenetic analysis, and community structure and species richness analysis.

Climate

Perhumid climate has existed in Sundaland since the early Miocene; though there is evidence for several periods of drier conditions, a perhumid core persisted in Borneo.

Most recent research agrees that Indo-Pacific sea surface temperatures were at most 2-3 °C lower during the Last Glacial Maximum. However, debate continues on how precipitation regimes changed throughout the Quaternary. Some authors argue that rainfall decreased with the area of ocean available for evaporation as sea levels fell with ice sheet expansion. and an increase in land area in the Sunda Shelf alone (due to lowered sea level) is not enough to decrease precipitation in the region. Alternatively, the physical and chemical processes that underlie the method of inferring precipitation from δ18O records may have operated differently in the past.

Savanna corridor theory

Dipterocarp trees characteristic of modern Southeast Asian tropical rainforest have been present in Sundaland since before the Last Glacial Maximum. There is also evidence for savanna vegetation, particularly in now submerged areas of Sundaland, throughout the last glacial period. However, researchers disagree on the spatial extent of savanna that was present in Sundaland. There are two opposing theories about the vegetation of Sundaland, particularly during the last glacial period: (1) that there was a continuous savanna corridor connecting modern mainland Asia to the islands of Java and Borneo, and (2) that the vegetation of Sundaland was instead dominated by tropical rainforest, with only small, discontinuous patches of savanna vegetation. Morley and Flenley (1987) and Heaney (1991) were the first to postulate the existence of a continuous corridor of savanna vegetation through the center of Sundaland (from the modern Malay Peninsula to Borneo) during the last glacial period, based on palynological evidence.

In contrast, other authors argue that Sundaland was primarily covered by tropical rainforest. Soil type, rather than long-term existence of a savanna corridor, has also been posited as an explanation for species distribution differences within Sundaland; Slik et al. (2011) suggest that the sandy soils of the now submerged seabed are a more likely dispersal barrier.

Paleofauna

Before Sundaland emerged during the late Pliocene and early Pleistocene (~2.4 million years ago), there were no mammals on Java. As sea level lowered, species such as the dwarf elephantoid Sinomastodon bumiajuensis colonized Sundaland from mainland Asia. Later fauna included tigers, Sumatran rhinoceros, and Indian elephant, which were found throughout Sundaland; smaller animals were also able to disperse across the region.

The population migrations were most likely to have been driven by climate change — the effects of the drowning of an ancient continent. Rising sea levels in three massive pulses may have caused flooding and the submerging of the Sunda continent, creating the Java and South China Seas and the thousands of islands that make up Indonesia and the Philippines today. The changing sea levels would have caused these humans to move away from their coastal homes and culture, and farther inland throughout southeast Asia. This forced migration would have caused these humans to adapt to the new forest and mountainous environments, developing farms and domestication, and becoming the predecessors to future human populations in these regions.

Stephen Oppenheimer locates the origin of the Austronesians in Sundaland and its upper regions. From the standpoint of historical linguistics, the home of the Austronesian languages is the main island of Taiwan, also known by its unofficial Portuguese name of Formosa; on this island the deepest divisions in Austronesian are found, among the families of the native Formosan languages.

References

Selected faunal references in Borneo

Abdullah MT. 2003. Biogeography and variation of Cynopterus brachyotis in Southeast Asia. PhD thesis. The University of Queensland, St Lucia, Australia.
Corbet, GB, Hill JE. 1992. The mammals of the Indomalayan region: a systematic review. Oxford University Press, Oxford.
Hall LS, Gordon G. Grigg, Craig Moritz, Besar Ketol, Isa Sait, Wahab Marni, Abdullah MT. 2004. Biogeography of fruit bats in Southeast Asia. Sarawak Museum Journal LX(81):191–284.
Karim, C., A.A. Tuen, Abdullah MT. 2004. Mammals. Sarawak Museum Journal Special Issue No. 6. 80: 221–234.
Wilson DE, Reeder DM. 2005. Mammal species of the world. Smithsonian Institution Press, Washington DC.

External links

Review of Oppenheimer's Eden in the East, about Sundaland