The Wallace Line: Faunal Boundary

Our customers often teach us new things. With great explorers and adventures commissioning globes.. this post is inspired by one addition that has been requested to a bespoke globe just today…

The Wallace Line or Wallace’s Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace that separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia. West of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origin is present. Wallace noticed this clear division during his travels through the East Indies in the 19th century.

The line runs through Indonesia, between Borneo and Sulawesi (Celebes), and through the Lombok Strait between Bali and Lombok. The distance between Bali and Lombok is small, about 35 kilometres (22 mi). The distributions of many bird species observe the line, since many birds do not cross even the smallest stretches of open ocean water. Some bats have distributions that cross the line, but other mammals are generally limited to one side or the other; an exception is the crab-eating macaque. Other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably consistent. Flora do not follow the Wallace Line to the same extent as fauna.

The Wallace Line is the most distinctive biogeographic border on the planet.

A stylised painting of Wallace in the jungle, with his notebook and two birds of paradies in front of him, nicely dressed, in the background his jungle hut.

A stylised painting of Wallace in the jungle, with his notebook and two birds of paradies in front of him, nicely dressed, in the background his jungle hut.

The 'Wallace Frog', a so called 'flying frog', rather a sliding frog, jumping from a higher point, e.g. a tree, to another plant several meters away, what saves him the effort and danger to climb down, walk over and climb up again on the other side. A species in transfer, using it's webs for gliding. Sketches by A.R. Wallace, the two left in his notebook, the one right is an illustration in 'The Malay Archipelago' (1969)

The ‘Wallace Frog’, a so called ‘flying frog’, rather a sliding frog, jumping from a higher point, e.g. a tree, to another plant several meters away, what saves him the effort and danger to climb down, walk over and climb up again on the other side. A species in transfer, using it’s webs for gliding. Sketches by A.R. Wallace, the two left in his notebook, the one right is an illustration in ‘The Malay Archipelago’ (1969)

In addition to the Wallace Line there are two other biogeographic lines further southeast. The Lydekker Line separates Australia and New Guinea from the islands northwest of them. The differences have their cause in the last ice age. 70,000 to 40,000 years ago the sea level was about 50 to 125 meters lower than it is now; considerable parts of the world’s oceanic waters were frozen and piled up at the arctic poles who were much larger than they are now. Therefore the great Sunda islands Sumatra, Java, Bali and Borneo were connected with the Southeast Asian mainland, while Papua New Guinea (Irian Jaya) was connected with Australia. Both land blocks hosted different kinds of species, who were later, when the sea level lowered and the populations were isolated on a number of different islands, evolving in certain, local ways, adapting to the changing environments.

That explains why there are tigers on Sumatra and still were on Java and Bali in Wallace’s time, who developed over thousands of years of isolation then slight differences. It’s the same thing with the orangutans on Sumatra and Borneo, and there are many, many more examples for the separation of species and their evolution after being cut off from the mainland. There was for instance the immigration of homo erectus to Java (the ‘Java Man‘), early humans who used the landbridge in an earlier ice age than the last one.

Between the two big former land blocks of Australia (Sahul) and Southeast Asia (Sunda) are a number of islands who didn’t belong to one of these blocks but were isolated since much, much longer, as Sulawesi for instance. They are surrounded by a sea deeper than 90m and there developed other certain species, very particular ones. Immigration barely happened, only when small groups of individuals survived being drifted accidentally from the mainlands to the islands. This group of islands is called ‘Wallacea’, bordered by the Wallace Line and the Lydekker Line.

Map of the 'Malay Archipelago' showing position of the Wallace Line. Copyright G. W. Beccaloni. Map of the 'Malay Archipelago' showing position of the Wallace Line. Copyright G. W. Beccaloni.

Map of the ‘Malay Archipelago’ showing position of the Wallace Line. Copyright G. W. Beccaloni.

More on Wallace, life, work, ideas: asienreisender.de/wallace.html

Official website & biography: wallacefund.info/content/biography-wallace

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