The Wallace Line is an invisible boundary running through the Indonesian archipelago that marks a significant difference in the distribution of animal species between Asia and Australia. It is a foundational concept in biogeography, which studies how and why species are distributed across the planet. This line reveals a deep evolutionary and geological history that explains the dramatic contrasts in the region’s wildlife. Understanding the Wallace Line provides insight into how continental movements, ancient sea levels, and geographic isolation shape the diversity of life on Earth.
The Discovery and Geography of Wallace’s Line
The existence of this faunal boundary was first documented by the British naturalist Alfred Russel Wallace during his expedition through the Malay Archipelago (1854–1862). Wallace collected many specimens and meticulously recorded their locations, noticing a striking and sudden change in the types of animals present as he traveled eastward through the islands.
The line’s path is defined by a series of narrow but deep straits within the Indonesian islands. It runs north to south, passing between Bali and Lombok, a distance of only about 35 kilometers at its closest point. Further north, the line continues through the deep Makassar Strait, separating Borneo to the west from Sulawesi (formerly Celebes) to the east. This arrangement highlights the abruptness of the faunal shift, where two islands separated by a short stretch of water host entirely different evolutionary lineages.
A Biological Divide Australasia Meets Asia
The significance of the Wallace Line lies in its separation of two major biological realms: the Oriental (Asian) realm to the west and the Australasian realm to the east. The Asian side is dominated by placental mammals, characterized by a long gestation period. Examples of these large Asian species found west of the line include tigers, rhinoceroses, elephants, and various species of apes.
In contrast, the islands immediately east of the line host fauna related to Australia and New Guinea, dominated by marsupials and monotremes. Marsupials, such as kangaroos and wallabies, are characteristic of this region. Although large placental mammals are absent on the Australian side, the region is rich in unique birdlife, including cockatoos and birds of paradise. This division demonstrates a fundamental difference in evolutionary history, where millions of years of separation led to the divergence of two distinct animal groups.
The Deep Water Barrier How Geology Shaped the Line
The Wallace Line exists as a sharp biological boundary due to the region’s unique geological history and bathymetry, or underwater topography. The line corresponds directly to the edge of two major continental shelves. To the west lies the Sunda Shelf, a shallow extension connecting islands like Borneo, Java, and Sumatra to mainland Asia. To the east is the Sahul Shelf, which connects Australia and New Guinea.
During the Pleistocene epoch, global sea levels dropped significantly due to multiple Ice Ages. This drop exposed large areas of the continental shelves as dry land, forming two vast landmasses known as Sundaland and Sahul. This allowed Asian species to migrate across Sundaland and Australasian species to move across Sahul. However, the deep ocean trench that marks the Wallace Line, particularly the Makassar Strait, remained submerged even at the lowest sea levels.
This deep water channel acted as a persistent oceanic barrier for terrestrial animals, preventing the mixing of Asian and Australian faunas for millions of years. This enduring separation allowed species on either side to evolve in isolation, creating the distinct evolutionary paths observed today.
Refining the Boundary Wallacea and Related Biogeographical Lines
In modern biogeography, the Wallace Line is understood not as a single break, but as the western edge of a complex transition zone known as Wallacea. This intermediate region, situated between the Sunda and Sahul shelves, comprises islands like Sulawesi, Lombok, and the Moluccas, which were never fully connected to either Asia or Australia.
The fauna of Wallacea is a unique blend, featuring species of both Oriental and Australasian origin, alongside a high number of endemic species. This complex mixing led later scientists to propose additional lines to better describe the faunal shifts. Weber’s Line runs east of the Wallace Line and represents the point where the balance of Asian and Australian species influence is roughly equal.
Another demarcation, Lydekker’s Line, marks the eastern boundary of Wallacea, separating the transition zone from the fully Australian fauna of the Sahul Shelf. These subsequent lines demonstrate that while Wallace’s initial observation remains foundational, the region is best understood as a broad, geologically complex interface.