Introduction | The Wallace Line
| Geological History | The
Ring of Fire | References | Credits
Introduction:
Southeast Asia is an area of extraordinarily high biodiversity. This high diversity
in land and sea is a result of three major factors: the overlap of independently
evolved species ranges, high rates of local speciation, and differentially high
survival among temporally and spatially heterogeneous habitats (McManus 1985).
This has resulted in perhaps the highest degree of biodiversity in the world,
especially in the archipelagos of Indonesia and the Philippines. One major indication
of this is the large number of tropical habitats in the area; over 30% of the
world's coral reefs are found in southeast Asia. The source of this biodiversity
can be found in the geological formation of the South China Sea.
The plant and animal life of Asia and Australia are separated by a biogeographic boundary, referred to as the Wallace Line. It was named after Alfred Russel Wallace, who first discovered the phenomenon during his travels through the Malay Peninsula and Indonesian archipelago in the mid-nineteenth century. Travelling between Bali and Lombok, two islands separated by only 25 kilometers (15 miles), Wallace discovered the native fauna of each island were completely different. Bali had animals common to the Asian continent: monkeys, tapirs, and woodpeckers. However, he found none of these in Lombok. On Lombok, he found animals similar to those in Australia: tree kangaroos, cockatoos, and brush-turkeys. In general, the areas west of the Wallace line are dominated by placental mammals, east of the line are almost entirely marsupials and birds.
The striking contrast between the two biogeographic areas are described in detail by Wallace in one of his journal entries.
"The Australian and Indian regions of Zoology are very strongly contrasted. In one the Marsupial order constitutes the great mass of the mammalia,-in the other not a solitary marsupial animal exists. Marsupials of at least two genera (Cuscus and Belideus) are found all over the Moluccas and in Celebes; but none have been detected in the adjacent islands of Java and Borneo. Of all the varied forms of Quadrumana, Carnivora, Insectivora, and Ruminantia which abound in the western half of the Archipelago, the only genera found in the Moluccas are Paradoxurus and Cervus. The Sciuridæ, so numerous in the western islands, are represented in Celebes by only two or three species, while not one is found further east.
"Birds furnish equally remarkable illustrations. The Australian region is the richest in the world in Parrots; the Asiatic is (of tropical regions) the poorest. Three entire families of the Psittacine order are peculiar to the former region, and two of them, the Cockatoos and the Lories, extend up to its extreme limits, without a solitary species passing into the Indian islands of the Archipelago. The genus Palæornis is, on the other hand, confined with equal strictness to the Indian region. In the Rasorial order, the Phasianidæ are Indian, the Megapodiidæ Australian; but in this case one species of each family just passes the limits into the adjacent region. The genus Tropidorhynchus, highly characteristic of the Australian region, and everywhere abundant as well in the Moluccas and New Guinea as in Australia, is quite unknown in Java and Borneo. On the other hand, the entire families of Bucconidæ, Trogonidæ and Phyllornithidæ, and the genera Pericrocotus, Picnonotus, Trichophorus, Ixos, in fact, almost all the vast family of Thrushes and a host of other genera, cease abruptly at the eastern side of Borneo, Java, and Bali.
"All these groups are common birds in the great Indian islands; they abound everywhere; they are the characteristic features of the ornithology; and it is most striking to a naturalist, on passing the narrow straits of Macassar and Lombock, suddenly to miss them entirely, together with the Quadrumana and Felidæ, the Insectivora and Rodentia, whose varied species people the forests of Sumatra, Java, and Borneo." (Alfred R. Wallace, On the Zoological Geography of the Malay Archipelago. 1859)
Geological
History:
These distinct bioregions occur in such proximity as a result of millions of
years of evolution and plate tectonics. Australia was one part of Gondwana a
massive peninsula of the supercontinent Pangaea. It consisted of most of the
southern continents and was separated from the northern continents by the Tethys
sea for over 500 millions years. Over that time, pieces have broken off and
everything has drifted to the north, closing the gap to Eurasia. India arrived
with such force that it formed the Himalayas.
Australia broke
from Antarctica less that 100 million years ago. Some ocean islands were trapped
between Australia and Asia, others were created by the force of the continent's
meeting. Many of the islands of Southeast Asia have existed for less than 3
million year. This has not been sufficient time for animals to spread throughout
the archipelago. Therefore, there is a distinct line between the two bioregions,
the Wallace line.
Over the next 50 million years, Australia continued to move closer to Asia and
caused intense stress and pressure along the fault lines. This pressure caused
many earthquakes and volcanoes which formed and changed the islands that exist
today. The area is known as the "Ring of Fire" because of the extraordinarily
high occurrence of volcanoes.
A very thorough description of the plate tectonic process illustrating the geological formation of the South China Sea can be found in Oosterzee (1997) Where Worlds Collide: The Wallace Line.
The break-up of Pangaea and the collision of India and Asia are well described in an animation from Christopher Scotese's PALEOMAP Project, "The Collision of India and Asia (90 mya - Present)" at http://www.scotese.com/indianim.html.
The "Ring
of Fire" of volcanoes and earthquakes, created where the two tectonic
plates are colliding, is illustrated in a National Geographic map (March 1998).
Credits
This site was created and edited by Miranda Hillyard and David Rosenberg as
part of the Middlebury College Winter Term collaborative research course ID
069 for the South China Sea WWW Virtual Library.