The British naturalist Alfred Russel Wallace proposed one of the most important concepts in the study of how geography affects animal distribution. His observations in the Indonesian Archipelago revealed a remarkable division in animal life, a boundary that today bears his name: Wallace’s Line. This invisible separation delineates the species of Asia from those of Australasia, creating a striking pattern in global biodiversity.
Defining the Invisible Boundary
The existence of Wallace’s Line is a direct consequence of ancient geological history and deep ocean trenches. The line snakes through the Malay Archipelago, running between the islands of Bali and Lombok to the south, and northward through the Makassar Strait between Borneo and Sulawesi. West of this boundary lies the Sunda Shelf, a shallow underwater extension of the Southeast Asian continental plate that includes Sumatra, Java, and Borneo. East of the Line is the Sahul Shelf, a similar extension of the Australian continental plate that connects Australia and New Guinea.
The separation between these two shelves is a deep-water channel, sometimes referred to as the Wallace Trench, which is the physical mechanism behind the Line. During the Pleistocene ice ages, global sea levels dropped by as much as 120 meters, exposing vast expanses of the Sunda and Sahul Shelves as dry land. These exposed areas formed continuous land bridges that allowed Asian fauna to migrate freely into what is now Indonesia, and Australasian fauna to expand westward from the Australian mainland. The deep channel separating the shelves never fully drained, maintaining a persistent marine barrier that prevented species from crossing between the two continental regions.
The Biogeographical Divide
The most compelling evidence for Wallace’s Line is the dramatic difference in animal types found on either side of the boundary. To the west, the islands of the Sunda Shelf, such as Borneo and Java, are dominated by fauna with Asian origins, including placental mammals. Species like tigers, rhinoceroses, and various primate groups, such as gibbons and orangutans, are characteristic of this region. These animals migrated across the former land bridges from the Asian mainland when sea levels were low.
Crossing the narrow Lombok Strait, a distance of only about 35 kilometers, reveals an entirely different biological world influenced by the Australian continent. On the islands east of the Line, placental mammals are largely absent, replaced by marsupials and unique bird groups. For instance, the sugar glider-like cuscus, a type of tree-dwelling marsupial, and the striking, crested cockatoos are abundant here, but they are not found on the Asian side.
The contrast is noticeable when comparing neighboring islands, such as Borneo, which hosts Asian fauna, with Sulawesi, just across the Strait. Sulawesi species are a mixture of Asian and Australian forms, including unique endemic mammals like the babirusa and the Sulawesi macaque. This distinct split among mammals and birds provided Wallace with the empirical proof that a powerful geographical force must have separated the evolutionary histories of the two regions.
Wallace’s Journey and Discovery
The conceptualization of Wallace’s Line arose from the fieldwork of Alfred Russel Wallace during his eight-year expedition through the Malay Archipelago, spanning from 1854 to 1862. Wallace traveled thousands of miles across the islands, meticulously collecting and cataloging over 125,000 specimens of insects, birds, and mammals. His scientific methodology involved systematically recording the distribution of life forms on each island he visited.
The realization of the Line dawned on him as he traveled eastward, noting the sharp decline in typical Asian species and the sudden appearance of Australasian ones. He was particularly struck by the difference in bird life and mammals when he sailed between Bali and Lombok, observing that the faunal contrast was far greater than the difference in climate or physical geography. This observation that two islands separated by a small strait could harbor completely different animal populations was the foundation of his discovery. Wallace published his findings in 1863, detailing the biogeographical separation he had identified.
Beyond Wallace’s Line
While Wallace’s Line represents an important boundary, modern biogeography views the transition between the Asian and Australasian realms as a complex zone rather than a single, absolute division. The islands situated between Wallace’s Line and another boundary farther east, Lydekker’s Line, form a transitional region known as Wallacea. This area, which includes Sulawesi, the Moluccas, and the Lesser Sunda Islands, was never connected to either the Asian or Australian continental shelves.
Wallacea is characterized by a unique mix of species that have dispersed from both the west and the east, alongside a high number of species that evolved in isolation on these oceanic islands. Within this transitional zone, another line, Weber’s Line, has been proposed to denote the point where the influence of Asian species and Australian species is roughly equal. Lydekker’s Line, the eastern limit of Wallacea, marks the edge of the Australian continental shelf, completing the picture of a broader, three-part biogeographical system.