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The Wallace Line, named after the naturalist Alfred Russel Wallace who explored the islands between 1854 and 1862 runs between Bali and Lombok, extending north through the Makassar Strait between Kalimantan (Borneo) and Sulawesi. On the western side of this line the animals are predominately of Asian origin (tigers, rhinoceros etc.). On the eastern side of the Wallace line the animals are of Australian descent with a lot of endemic species.
Another Naturalist, Weber was interested in how far Australian animals and plants spread into the Eurasian area. He noticed that he could draw a line between Sulawesi and Irian Jaya (= Indonesian part of Papua New Guinea) and between Timor and Australia. To the west, the fauna is more than 50 percent Oriental, while to the east of the line, fauna is more than 50 percent Australian in origin.
Today biogeographers think of the area between these lines more as a zone of transition. This zone, encapsulating Sulawesi, Nusa Tenggara and Maluku, is called Wallacea. With animals and plants from both regions to draw upon, as well as a huge number of endemic species (= restricted to a particular geographic region and found nowhere else in the world), Wallacea is specially interesting for naturalists and divers alike.
Map of Wallacea with Wallace's, Weber's and Lydekker's line (print version - weather map)
Nusa Tenggara is a good example to show what happens, if animals disperse over a chain of islands. Nusa Tenggara belongs to Wallacea and is the Indonesian name for the over 500 islands east of Bali, running from Lombok in the west to Timor in the east. Nusa Tenggara stretches over 1300 kilometer and lies just a few degrees south of the equator. The northern islands (Lombok, Sumbawa, Flores to Alor) are volcanic, the southern islands (Sumba, Savu, Roti and Timor) are uplifted coral limestone and sediment. There are over 40 volcanoes with half of them still active.
Nusa Tenggara is divided from Bali by the Lombok Strait. Alfred Russel Wallace was here between 1854 and 1862 and while collecting birds he noticed, that while Bali shares nearly all its birds with Java (we now know, that it is about 97%), Lombok and Bali have much less birds in common (only 50%).
Why is there such a distinct separation between the bird populations of Bali and Lombok which lies only about 25 km across the strait? During the ice age Bali was connected by a land corridor to Java. There was still a channel between Bali and Lombok. Some birds, which are good dipersers and had no problem to cross the gap, other bird stayed in Bali and never reached Lombok. As these bidrs dispersed further east across Nusa Tenggara they became more distinct from the mother population in Java. In Nusa Tenggara and the Maluku (Moluccs) there are 562 species of birds recorded, 144 of these are endemic (= found nowhere else). Timor, the island to the very east has the highest number of endemic species of any of the other islands of Nusa Tenggara.
Bathymetric map of Bali, Lombok, Lombok strait and Sumbawa during the Pleistocene times (palaeogeographical reconstruction after Kitchener 1990): Dark green = dry land connecting the islands - print version
Each further gap in the necklace like chain of islands which is Nusa Tenggara was a further obstacle to overcome. For example the Sape Strait between Sumbawa and Komodo or the Ombai Strait between Alor and Timor. Thus the species of birds that could successfully cross all these gaps dwindled and the birds staying in a particular habitat changed and adjusted and became endemic on that island.
Map of Nusa Teggara
An other example is Sulawesi (Celebes). In the west it is divided from Borneo and thus from the Asian mainland by the narrow but deep Makassar Strait. Even during the ice age with its low sea level Sulawesi was never actually connected to Borneo. Of the known Sulawesi fauna 62% of mammal, 27% of the bird 62% of reptile and 76% of amphibian species are endemic! For example there are marsupials (related to Australian kangaroos and possums) on the eastern islands, but they are not seen in Borneo which lies west of Sulawesi. These animals reached Sulawesi by hopping across landbridges during the ice age coming from Australia.
Map with migration routes between Australasia and Asia through the Wallacean islands during sea level changes due to the ice age (70'000 to 40'000 years ago) after Birdsell 1977. Dark green = dry land connecting the islands. print version
Personally I am always astonished at the number of species you can find on a single dive. Indonesia is said to contain 10 to 15 percent of the world's coral reefs. It is often assumed, that you have to travel to far away places for the best diving, but isolation often also means an impoverished fauna, because for the animals it is also difficult to reach these places! The islands of Sulawesi, Mollucas, Bali and Nusa Tenggara are ideally positioned in the Indonesian Throughflow (Arus Lintas Indonesia), a massive flow of water that passes from the Pacific to the Indian Ocean and deposits planktonic larvae in the waters of Wallacea. This results in a very high diversity of species. It is said that the photographer Rudie Kuiter catalogued in Maumere Bay (Flores) alone over 1200 species of fish including some new to science.
Map of the Indonesian Throughflow, current patterns in February (print version - weather map)
In 2002 researchers identified global priority areas for coral reef conservation and prepared a list with the world's top 10 coral reef hotspots. These are areas rich in marine species which are found only in small area. Therefore they are highly vulnerable to extinction.
The Wallacea hotspot encompasses some 346'782 km2 and covers Nusa Tenggara (Lombok, Sumbawa, Komodo, Flores, Sumba, Savu, Roti and Timor), the Mollucas and Sulawesi. Wallacea is divided from an other hotspot, Sundaland, by the Wallace's Line. The marine life in this region is astonishingly rich. The major threats to this hotspot are pollution from land-based sources, sediment pollution from logging and mining, intensive destructive fishing (dynamite fishing) and live reef fish trade (for the aquarium trade and for restaurants in South East Asia).
More information on hotspots - Hotspot Wallacea - Report of the IUCN
The naturalist Alfred Russell Wallace is a very interesting historic person. Born in 1823 in England he actually earned his living by hunting and collecting wildlife for museum collections. He first visited the Amazon but in 1854, after a disastrous fire that destroyed his whole collection he started out from Singapore and spent a total of 8 years in Borneo, Sulawesi, Nusa Tenggara, the Moluccs and the Aru islands (Irian Jaya). He recounted this interesting journey and the scientific conclusions he reached during his travels in his book "The Malay Archipelago". I can only recommend this book, it is a thoroughly readable account of his travels and gives you a lot of insight into this region.
Already in 1858 Alfred Russel Wallace had written a thesis about the evolution of species "On the Tendency of Varieties to Depart Indefinitely From the Original Type". He outlined in a clear manner the origin of species by natural selection, explaining the tendency of species to diverge from a common ancestor. Wallace was a contemporary of Charles Darwin and he sent his paper for appraisal to him. At that time Darwin had already formulated his theory on the origin of species but hadn't published any papers. The two now made a joint public announcement, however with Wallace still away in distant Indonesia, it was Darwin's name which became later connected with the evolutionary theory.
More information about Sir Alfred Russel Wallace / Information about the Wallace line.
Books: The Malay Archepelago by Alfred Russel Wallace - complete online version by Papua web / On the zoological geography of the Malay archipelago / On the Physical Geography of the Malay Archipelago / list of all books and articles of Alfred Russel Wallace / The song of the Dodo by David Quammen
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Dive sites around the Daymaniyat Islands, Kharabah, Fahal in Oman (maps, descriptions) - print version |
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Galapagos islands: center Galapagos, north Galapagos, southeast Galapagos, west Galapagos |
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. Copyright Teresa Zubi (write to me)
