A deep-water invisible barrier separates the wildlife of Asia from Australia
In the narrow Lombok Strait, just 35 kilometers of ocean separates two entirely different biological worlds. On one side, elephants and apes roam; on the other, marsupials dominate. This invisible boundary, known as the Wallace Line, reveals how ancient sea levels and tectonic shifts shaped the Earth's biodiversity.
The Wallace Line is a faunal boundary first identified in 1859 by British naturalist Alfred Russel Wallace. It marks the edge of the biogeographic transition between the Asian (Oriental) realm and 'Wallacea,' a zone between Asia and Australia. To the west of the line, organisms are closely related to Asian species; to the east, a mixture of Asian and Australian origins exists, including many marsupials.
The mechanism behind this division is geological. The line follows deep-water channels that separate the Sunda Shelf, which connects Sumatra, Java, and Borneo to mainland Asia, from the Sahul Shelf, which links Australia to New Guinea. During the Pleistocene, even when ocean levels dropped by as much as 120 meters, these deep-water barriers remained, preventing most land mammals from migrating between the two continental shelves.
While the line is a remarkably accurate predictor for many groups, it is not an absolute wall. Certain animals, such as bats, can cross the boundary, whereas larger terrestrial mammals generally cannot. Interestingly, the line's exact placement has been debated. While Wallace originally proposed a boundary east of Bali, biologist Thomas Henry Huxley later redrew it to the west of the Philippines in 1868 after observing distinct bird populations there.
Modern science continues to refine this map. Recent studies, such as those by Ali et al. in 2020, suggest that the distribution of land mammals on Christmas Island may require a re-conformation of the line, potentially moving the island from the Oriental side to the Australasian side.
Source: Wallace Line
