Sundaland

Sundaland, also called Sundaica or the Sundaic region, is a special area in Southeast Asia that was once a bigger piece of land. This land was exposed when sea levels were lower during the last 2.6 million years. It includes places like Bali, Borneo, Java, and Sumatra in Indonesia, along with nearby small islands. It also includes the Malay Peninsula on Mainland Southeast Asia. This area is important for studying plants and animals because many unique species live there.

Extent

Sundaland is a special area in Southeast Asia that was connected to land when sea levels were lower during the last 2 million years. It includes places like Borneo, Java, Sumatra, and the Malay Peninsula, along with nearby waters such as the Java Sea and the Gulf of Thailand.

When glaciers covered large areas of ice, sea levels dropped, and more land became visible. During these times, islands like Borneo, Java, and Sumatra were connected to each other and to mainland Asia by land bridges. Over time, as sea levels rose, these areas became separate islands. The borders of Sundaland are marked by deep ocean areas and a line called the Wallace Line, which separates it from another region called Wallacea.

Modern climate

All of Sundaland lies near the equator, running through central Sumatra and Borneo. In these tropical areas, rain is more important than temperature for deciding the climate. Most of Sundaland gets more than 2,000 millimeters of rain each year, with no real dry seasons.

The warm, shallow seas of the Sunda Shelf, which stay above 28 °C, help shape weather patterns far away. They play a big role in global air movements and can affect weather during special times of year, bringing dry spells to Sundaland and much of tropical Asia.

Modern ecology

The islands of Sundaland get a lot of rain, which helps forests grow with tall trees all year round. In some places, these forests become shorter and have fewer trees as you move farther north. The forests that have not been cut down are special because they have very tall trees and orangutans. When forests are cut down, the types of plants and animals change.

Long ago, during an icy time called the last glacial period, the sea level was lower, and all of Sundaland was connected to Asia. This is why many animals from Asia, like elephants, monkeys, apes, tigers, tapirs, and rhinoceroses, live there today. When the sea levels rose, it split up the animals that used to live together. Each island now has its own mix of animals because of this change. The biggest islands, Borneo and Sumatra, have the most different kinds of animals.

History

The name "Sunda" has been used since ancient times, appearing in old writings from around 150 AD. In the 1800s, explorers started studying the seas around Southeast Asia. A Dutch scientist in 1921 suggested that the shallow seas were once land that flooded many times as ice melted.

The term "Sundaland" was first used in 1949 for the underwater land that connects parts of Southeast Asia. Scientists have studied the climate and plants of this area by looking at tiny sea creatures, pollen, and other clues from the ocean floor and caves.

Sundaland has been a key place for animals and plants to evolve since the early Miocene period. The islands of Borneo, Java, and Sumatra have protected many species during cold periods in the past million years.

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 foraminiferal 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. 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 (approximately 1,000 meters lower) 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 (due to lowered sea level) 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.

Ecology

Sundaland, and in particular Borneo, has been an evolutionary hotspot for biodiversity since the early Miocene due to repeated immigration and vicariance events. The modern islands of Borneo, Java, and Sumatra have served as refugia for the flora and fauna of Sundaland during multiple glacial periods in the last million years, and are serving the same role at present.

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.

The presence of a savanna corridor—even if fragmented—would have allowed for savanna-dwelling fauna (as well as early humans) to disperse between Sundaland and the Indochinese biogeographic region; emergence of a savanna corridor during glacial periods and subsequent disappearance during interglacial periods would have facilitated speciation through both vicariance (allopatric speciation) and geodispersal. 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. Using the modern distribution of primates, termites, rodents, and other species, other researchers infer that the extent of tropical forest contracted—replaced by savanna and open forest —during the last glacial period. Vegetation models using data from climate simulations show varying degrees of forest contraction; Bird et al. (2005) noted that although no single model predicts a continuous savanna corridor through Sundaland, many do predict open vegetation between modern Java and southern Borneo. Combined with other evidence, they suggest that a 50–150 kilometer wide savanna corridor ran down the Malay Peninsula, through Sumatra and Java, and across to Borneo. Additionally, Wurster et al. (2010) analyzed stable carbon isotope composition in bat guano deposits in Sundaland and found strong evidence for the expansion of savanna in Sundaland. Similarly, stable isotope composition of fossil mammal teeth supports the existence of the savanna corridor.

In contrast, other authors argue that Sundaland was primarily covered by tropical rainforest. Using species distribution models, Raes et al. (2014) suggest that Dipterocarp rainforest persisted throughout the last glacial period. Others have observed that the submerged rivers of the Sunda Shelf have obvious, incised meanders, which would have been maintained by trees on river banks. Pollen records from sediment cores around Sundaland are contradictory; for example, cores from highland sites suggest that forest cover persisted throughout the last glacial period, but other cores from the region show pollen from savanna-woodland species increasing through glacial periods. And in contrast to previous findings, Wurster et al. (2017) again used stable carbon isotope analysis of bat guano, but found that at some sites rainforest cover was maintained through much of the last glacial period. 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.

Human migrations

People moved from East Asia to places like Taiwan and then to Maritime Southeast Asia. Some think an area called Sundaland may have been where their languages began, but most experts agree they came from Taiwan.

Studies show that people moved when sea levels rose, changing the land and forcing them to find new homes. These changes happened because of climate change, and people had to adapt to new places and ways of living.