17 June 2013 1 of 5




Alfred Russel Wallace (1823-1913) was a 19th Century contemporary of Charles Darwin, a naturalist who conducted fieldwork in South America and what was then the Dutch East Indies, modern day Indonesia. He was also the man who nudged Darwin into making his theory of evolution public - without Wallace we may never have heard of Darwin and Evolution Through Natural Selection, but that is another story.

It was while he was on his second quest, an eight-year collecting and observating expedition down the length of the Malay Archipelago, that he noticed a change in the fauna as he moved from west to east (birds and butterflies mostly but it applies to other organisms too). This recognition of different faunas was most apparent to Wallace as he made the short (35 km) sea journey from Bali to the neighbouring island of Lombok, the next island in the Lesser Sunda chain, where suddenly Asian birds were replaced by Australasian birds. This puzzled Wallace greatly since he had only travelled a short distance, far shorter than the gaps between the ornithologically similar islands of Borneo, Sumatra, Java, and Bali. Although he may not have known it at the time the change in the fauna was the result not of a horizontal distance but of a vertical distance: the depth of the sea-bed.

Borneo, Sumatra, Java and Bali lie on a shallow-water shelf, the Sunda Shelf, that links them to the Southeast Asian mainland. At times of lowered sea levels (which occurred for more than 40,000 years in the last two million due to ice ages binding water in glaciers) has permitted considerable migration of species and biological exchange. A similar shelf, the Sahul Shelf, links Australia and New Guinea and accounts for the similarities in their faunas. But in between are numerous islands that either lie on small shelves of their own or rise up from the depths as oceanic mountains and volcanoes. Sulawesi (formerly Celebes) is isolated from Borneo and New Guinea in this way, and Lombok and the other islands of the Lesser Sundas are separated from Bali by a narrow but deep marine trench. Sulawesi and the Lesser Sunda and neighbouring Maluku (Moluccan) Islands are neither Asian nor Australasian, they belong to their own zoogeographic region that is separated from Bali and the rest of the Sunda region by Wallace's Line, an invsible line of zoogeographic differentiation defined by Wallace in 1859. Today this small region is named Wallacea in honour of the man who first recognised their uniquenss and who became known as the "Father of Biogeography". These islands contain elements of the Asian and Australasian faunas but also their own endemic species, found nowhere else.

Bali and Lombok are only 35 km apart but the deep water trench between them effectively places them
on opposite of the invisible but extremely important Wallace's Line

We had planned to do a jungle trek for reptiles and amphibians on Bali but the lateness of our arrival and the transportation issues we experienced on arrival meant this could not be accomplished. No matter, Bali is certainly part of the Sunda region of Southeast Asia with many similarities to the Malaysian Peninsula where we had already done one jungle trek. Now we planned to visit Lombok on the other side of Wallace's Line.

Anybody interested in Wallace's Line and Island Biogeography is directed to the following titles:
Wallace, Alfred Russel 1869 The Malay Archipelago
MacArthur, Robert H. & Edward O. Wilson 1967 The Theory of Island Biogeography
Quammen, David 1996 The Song of the Dodo (this last is a very readable account for non-scientists)

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