Sundaland
Sundaland 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.The extent of the Sunda Shelf is approximately equal to the 120-meter isobath. In addition to the Malay Peninsula and the islands of Borneo, Java, and Sumatra, it includes the Java Sea, the Gulf of Thailand, and portions of the South China Sea. In total, the area of Sundaland is approximately 1,800,000 km2. The area of exposed land in Sundaland has fluctuated considerably during the past recent 2 million years; the modern land area is approximately half of its maximum extent.
The western and southern borders of Sundaland are clearly marked by the deeper waters of the Sunda Trench – some of the deepest in the world – and the Indian Ocean. The eastern boundary of Sundaland is the Wallace Line, identified by Alfred Russel Wallace as the eastern boundary of the range of Asia's land mammal fauna, and thus the boundary of the Indomalayan and Australasian realms. The islands east of the Wallace line are known as Wallacea, a separate biogeographical region that is considered part of Australasia. The Wallace Line corresponds to a deep-water channel that has never been crossed by any land bridges. 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.
Greater portions of Sundaland were most recently exposed during the last glacial period from approximately 110,000 to 12,000 years ago. When the sea level was decreased by 30–40 meters or more, land bridges connected the islands of Borneo, Java, and Sumatra to the Malay Peninsula and mainland Asia. Because the sea level was 30 meters or more lower throughout much of the last 800,000 years, the current status of Borneo, Java, and Sumatra as islands has been a relatively rare occurrence throughout the Pleistocene. 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. Sundaland was partially submerged starting around 18,000 years ago and continuing until about 5000 BC. During the Last Glacial Maximum the sea level fell by approximately 120 meters, and the entire Sunda Shelf was exposed.
Modern climate
All of Sundaland is within the tropics; the equator runs through central Sumatra and Borneo. Like elsewhere in the tropics, rainfall, rather than temperature, is the major determinant of regional variation. Most of Sundaland is classified as perhumid, or everwet, with over 2,000 millimeters of rain annually; rainfall exceeds evapotranspiration throughout the year and there are no predictable dry seasons like elsewhere in Southeast Asia.The warm and shallow seas of the Sunda Shelf 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, particularly in January when it is a major heat source to the atmosphere. ENSO also has a major influence on the climate of Sundaland; strong positive ENSO events result in droughts throughout Sundaland and tropical Asia.
Modern ecology
The high rainfall supports closed canopy evergreen forests throughout the islands of Sundaland, transitioning to deciduous forest and savanna woodland with increasing latitude. The remaining primary lowland forest is known for giant dipterocarp trees and orangutans; after logging, forest structure and community composition change to be dominated by shade intolerant trees and shrubs. 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. Botanists often include Sundaland, the adjacent Philippines, Wallacea and New Guinea in a single floristic province of Malesia, based on similarities in their flora, which is predominantly of Asian origin.During the last glacial period, sea levels were lower and all of Sundaland was an extension of the Asian continent. As a result, the modern islands of Sundaland are home to many Asian mammals including elephants, monkeys, apes, tigers, tapirs, and rhinoceros. The flooding of Sundaland separated species that had once shared the same environment. One example is the river threadfin, which once thrived in a river system now called "North Sunda River" or "Molengraaff river". 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 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. Island area and number of terrestrial mammal species are related, with the largest islands of Sundaland having the highest diversity.
Ecoregions
;Tropical and subtropical moist broadleaf forests- Eastern Java–Bali rain forests
- Eastern Java–Bali montane rain forests.
- Western Java montane rain forests
- Western Java rain forests
- Borneo lowland rain forests
- Borneo montane rain forests
- Borneo peat swamp forests
- Mentawai Islands rain forests
- Peninsular Malaysian montane rain forests
- Peninsular Malaysian peat swamp forests
- Peninsular Malaysian rain forests
- Southwest Borneo freshwater swamp forests
- Sumatran freshwater swamp forests
- Sumatran lowland rain forests
- Sumatran montane rain forests
- Sumatran peat swamp forests
- Sundaland heath forests
;Montane grasslands and shrublands
;Mangroves
- Sunda Shelf mangroves
History
Early research
The name "Sunda" goes back to antiquity, appearing in Ptolemy's Geography, written around 150 AD. 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. Molengraaff also identified ancient, now submerged, drainage systems that drained the area during periods of lower sea levels.
The name "Sundaland" for the peninsular shelf was first proposed by Reinout Willem van Bemmelen in his Geography of Indonesia in 1949, based on his research during World War II. The ancient drainage systems described by Molengraaff were verified and mapped by Tjia in 1980 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 foraminiferaClimate
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. The presence of fossil coral reefs dating to the late Miocene and early Pliocene suggests that, as the Indian monsoon grew more intense, seasonality increased in some portions of Sundaland during these epochs. Palynological evidence from Sumatra suggests that temperatures were cooler during the late Pleistocene; mean annual temperatures at high elevation sites may have been as much as 5 °C cooler than present.Most recent research agrees that Indo-Pacific sea surface temperatures were at most 2-3 °C lower during the Last Glacial Maximum. Snow was found much lower than at present and there is evidence that glaciers existed on Borneo and Sumatra around 10,000 years before present. 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. Others posit that changes in precipitation have been minimal and an increase in land area in the Sunda Shelf alone is not enough to decrease precipitation in the region.
One possible explanation for the lack of agreement on hydrologic change throughout the Quaternary is that there was significant heterogeneity in climate during the Last Glacial Maximum throughout Indonesia. Alternatively, the physical and chemical processes that underlie the method of inferring precipitation from δ18O records may have operated differently in the past. Some authors working primarily with pollen records have also noted the difficulties of using vegetation records to detect changes in precipitation regimes in such a humid environment, as water is not a limiting factor in community assemblage.