Sundaland

Sundaland (also called Sundaica or the Sundaic region) is a biogeographical region of Southeast Asia. It is a large area of land that was above the sea during certain times over the last 2.6 million years, when sea levels were lower.

Today, the region includes Bali, Borneo, Java, and Sumatra in Indonesia, along with many smaller nearby islands. It also includes the Malay Peninsula on Mainland Southeast Asia.

Sundaland is not the same as Sunderland. The name "Sundas" also leads to this topic. For the Greater Sunda Islands, see that page. For the Lesser Sunda Islands, see that page. For the continental shelf of Southeast Asia, see the Sunda Shelf page.

Extent

Sundaland covers the Sunda Shelf, a stable part of Southeast Asia's continental shelf. This shelf was above sea level during glacial periods over the last 2 million years. It is also roughly the same area as the Sunda plate. The edge of the Sunda Shelf is about the 120-meter isobath. Besides the Malay Peninsula and the islands of Borneo, Java, and Sumatra, it includes the Java Sea, the Gulf of Thailand, and parts of the South China Sea. In total, Sundaland covers about 1,800,000 km². The amount of land above water has changed a lot. Today's land is about half of its largest size.

The western and southern edges are marked by the deep waters of the Sunda Trench, one of the deepest places in the world, and the Indian Ocean. The eastern edge is the Wallace Line. Alfred Russel Wallace identified this line as the eastern limit of Asia's land mammal animals. It also separates the Indomalayan and Australasian realms. The islands east of this line are called Wallacea, which is considered part of Australasia. The northern edge is harder to define. A change in plant life at about 9ºN is considered its boundary.

Larger parts of Sundaland were last above water during the last glacial period, from about 110,000 to 12,000 years ago. When sea levels dropped by 30–40 meters or more, land bridges connected Borneo, Java, and Sumatra to the Malay Peninsula and mainland Asia. Sea levels were higher during the late Pliocene, when less land was exposed. During the Last Glacial Maximum, the sea fell by about 120 meters and the entire Sunda Shelf was exposed.

Modern climate

All of Sundaland is in the tropics, and the equator runs through central Sumatra and Borneo. In tropical areas, rain changes the climate from place to place more than temperature does. Most of Sundaland is very wet all year, with over 2,000 millimeters of rain each year. Rain is greater than evapotranspiration all year round. There are no predictable dry seasons like those found in other parts of Southeast Asia.

The warm, shallow seas of the Sunda Shelf average 28 °C or more. They are part of the Western Pacific Warm Pool and help drive the Hadley circulation and the El Niño-Southern Oscillation (ENSO). This is especially true in January, when the seas give off a lot of heat to the air. ENSO also strongly affects Sundaland's climate. Strong positive ENSO events lead to droughts throughout Sundaland and tropical Asia.

Modern ecology

Heavy rain supports thick, green forests across the islands of Sundaland. Farther north, the forests change to forests that lose their leaves in some seasons and to open woodland with grasses. The lowland forests that have not been cut down are known for giant dipterocarp trees and orangutans. When the forest is logged, it changes. Most of the new plants are shrubs and trees that need strong sunlight. Dipterocarps have mast fruiting events, when many trees fruit at the same time at irregular intervals, so animals that eat the fruit cannot eat it all. Higher forests are shorter and are mostly made up of trees in the oak family. Botanists often group Sundaland, the Philippines, Wallacea, and New Guinea into one floristic province called Malesia, because their plants are similar and mostly come from Asia.

Research on squirrels shows that Sundaland, especially Borneo and Sumatra, is an important biodiversity hotspot. Many of its special species began during the Miocene.

During the last glacial period, sea levels were lower, and all of Sundaland was joined to the Asian continent. Today, the islands are home to many Asian mammals, including elephants, monkeys, apes, tigers, tapirs, and rhinoceros. When the land flooded, animals that had lived together were separated. For example, the river threadfin (Polydactylus macrophthalmus, Bleeker 1858) once lived in a river system now called "North Sunda River" or "Molengraaff river." Today it is found in the Kapuas River on Borneo and in the Musi and Batanghari rivers on Sumatra. Selective pressure, which in some cases led to extinction, has affected each island differently, so each island has its own mix of mammals. The largest islands, Borneo and Sumatra, have the most kinds of land mammals.

Ecoregions

Tropical and subtropical moist broadleaf forests

• Eastern Java–Bali rain forests (Java, Bali)
• Eastern Java–Bali montane rain forests (Java, Bali).
• Western Java montane rain forests (Java)
• Western Java rain forests (Java)
• Borneo lowland rain forests (Borneo)
• Borneo montane rain forests (Borneo)
• Borneo peat swamp forests (Borneo)
• Mentawai Islands rain forests (Mentawai Islands)
• Peninsular Malaysian montane rain forests (Malay Peninsula)
• Peninsular Malaysian peat swamp forests (Malay Peninsula)
• Peninsular Malaysian rain forests (Anambas Islands, Malay Peninsula)
• Southwest Borneo freshwater swamp forests (Borneo)
• Sumatran freshwater swamp forests (Sumatra)
• Sumatran lowland rain forests (Sumatra, Nias, Bangka Island)
• Sumatran montane rain forests (Sumatra)
• Sumatran peat swamp forests (Sumatra)
• Sundaland heath forests (Indonesia)

Tropical and subtropical coniferous forests

• Sumatran tropical pine forests (Sumatra)

Montane grasslands and shrublands

• Kinabalu montane alpine meadows (Borneo)

Mangroves

• Sunda Shelf mangroves (Borneo, Sumatra, Riau Islands)

History

The name "Sunda" is very old. It appears in Ptolemy's Geography, written around 150 AD. In 1852, English navigator George Windsor Earl suggested a "Great Asiatic Bank." One reason was that mammals on Java, Borneo and Sumatra looked alike. In the 1870s, explorers and scientists began mapping the seas of Southeast Asia, mostly by using depth sounding. In 1921, Dutch geologist Gustaaf Molengraaff suggested that the flat sea floor was an ancient peneplain. He thought repeated floods, as ice caps melted, had shaped it. He also found ancient, now submerged, drainage systems that once carried water across the area when sea levels were lower. In 1949, Reinout Willem van Bemmelen first proposed the name "Sundaland" for this underwater shelf. His work was based on research he did during World War II. Later, in 1980 and 1982, these old drainage systems were studied in more detail. Scientists found river deltas, floodplains and backswamps.

Scientists have learned about the climate and plants of Sundaland over a long time. They studied foraminifera, δ¹⁸O and pollen from cores taken from the ocean floor. They also looked at speleothems from caves, and δ¹³C and δ¹⁵N in bat droppings. The area has been very wet since the early Miocene. Some drier periods happened, but Borneo stayed very wet. As the Indian monsoon grew stronger, seasons became easier to notice in some places. This happened in the late Miocene and early Pliocene. Pollen found in Sumatra suggests that temperatures were cooler in the late Pleistocene. During the Last Glacial Maximum, sea surface temperatures were at most 2 to 3 °C lower than today. Glaciers existed on Borneo and Sumatra. Scientists still debate how rainfall changed over time. Sundaland, especially Borneo, has been rich in biodiversity since the early Miocene. This is because plants and animals moved in again and again, and groups were sometimes separated by vicariance events. Dipterocarp trees have been here since before the Last Glacial Maximum. Savanna plants also grew during the last glacial period. Researchers do not agree on how much savanna there was. Some think a long savanna corridor connected mainland Asia to Java and Borneo. If so, animals and early humans could have moved between Sundaland and the Indochinese region. This may have helped new species form through allopatric speciation and geodispersal. Others believe rainforest covered most of Sundaland. Before Sundaland appeared in the late Pliocene and early Pleistocene, about 2.4 million years ago, Java had no mammals. As sea levels fell, animals such as Sinomastodon bumiajuensis, a relative of the dwarf elephant, reached Sundaland from mainland Asia.

Human migrations

The most widely accepted idea is that the ancestors of today's Austronesian peoples moved south from the East Asian mainland to Taiwan. From there, they spread to the rest of Maritime Southeast Asia. Another idea, the "Out of Sundaland" theory, suggests that Austronesian languages began in Sundaland, the area that is now underwater. Very few experts agree with this idea. Most researchers accept the Out of Taiwan model, though not necessarily the Express Train Out of Taiwan model.

Rising sea levels likely pushed people to move. A study from Leeds University, published in Molecular Biology and Evolution, looked at mitochondrial DNA. It suggested that people moved from the Philippine Islands as far north as Taiwan within the last 10,000 years. Scientists think climate change and the flooding of an ancient land helped cause these migrations. This flooding also helped form the Java and South China Seas and the thousands of islands that make up Indonesia and the Philippines today. Stephen Oppenheimer places the origin of the Austronesians in Sundaland. Historical linguistics, however, points to Taiwan as the home of the Austronesian languages.