A line drawn on the water
In the mid-19th century, the British naturalist Alfred Russel Wallace spent eight years (1854-1862) traveling through the Malay Archipelago, collecting 125,660 specimens of insects, birds, and mammals. During his journey, he observed a striking biological division. The fauna of Bali was distinctly Asian, while the fauna of Lombok, just 35 kilometers away, was Australasian. This faunal boundary, first proposed in 1859 and named "The Wallace Line" by Thomas Henry Huxley, follows a deep-water channel that separates two massive continental shelves.
The division exists because of ancient geology. To the west lies the Sunda Shelf, a southeastern extension of the Asian continental landmass that connects Borneo, Java, and Sumatra. To the east is the Sahul Shelf, which connects Australia and New Guinea. During the Pleistocene ice ages, sea levels dropped by more than 110 meters, exposing these shelves as massive land bridges and allowing Asian animals to walk as far as Bali. The Lombok Strait, however, is a deep oceanic trench with a minimum depth of 250 meters. This channel remained flooded, forming a permanent water barrier that most terrestrial animals could not cross for millions of years.
Asian tigers and Australian cockatoos
The biological difference across the Wallace Line is sharp. West of the line, in Bali, the animal life is typical of the Indomalayan region. Species include primates like the crab-eating macaque, and historically, the Balinese tiger (Panthera tigris balica), which became extinct in the 1930s. Bird species are also Asian in origin, including woodpeckers, barbets, and trogons.
Crossing the 35km strait to Lombok places you in the Wallacea transitional zone, with fauna that is increasingly Australasian. Here, you find no native tigers, elephants, or primates (with the exception of the adaptable macaque). Instead, the bird life changes dramatically to include species like the sulphur-crested cockatoo and other parrots, which are common in Australia but absent from Bali. While large marsupials like kangaroos are not found on Lombok itself, their relatives inhabit other islands further east, showing the faunal shift. The line is clearest for terrestrial mammals and many bird species, as they are unable to cross even short stretches of open ocean.
The area between Wallace's line and Lydekker's Line (the eastern boundary of this transitional zone) is Wallacea. This group of islands, including Lombok and Sulawesi, contains species from both the Asian and Australian regions, alongside a high number of endemic species that evolved in isolation.