What Defines the Wallace Line
The Wallace Line, identified by naturalist Alfred Russel Wallace, is an invisible biogeographical boundary. It separates the distinct animal life of Asia from that of Australasia across the Indonesian archipelago. This theoretical demarcation is not a physical barrier, but indicates a dramatic shift in species composition. The line passes through a narrow strait between Borneo and Sulawesi, and also between Bali and Lombok, despite their close proximity.
Species Divided by the Line
A striking contrast in animal life exists on either side of the Wallace Line. To the west, fauna largely resembles that of mainland Asia, characterized by placental mammals like tigers, rhinoceroses, and monkeys. Birds such as pheasants, woodpeckers, and sunbirds are also common. These species reflect a shared evolutionary history with the Asian continent.
Conversely, east of the Wallace Line, animal life shows strong affinities with Australia. This side is home to marsupials like kangaroos, wallabies, and possums. Bird species such as cockatoos, parrots, and birds of paradise dominate. This difference in animal groups highlights the Wallace Line as a biogeographical divider, separating two major evolutionary realms.
The Geological Story Behind the Line
The Wallace Line’s existence is deeply rooted in the region’s geological history. The faunal divide stems from extremely deep ocean trenches that historically separated the landmasses. These deep waters acted as persistent barriers to animal migration, even during glacial maximums when global sea levels were significantly lower. During these ice ages, the shallow Sunda Shelf to the west became exposed land, connecting islands like Borneo, Java, and Sumatra to mainland Asia, allowing Asian species to spread.
At the same time, the Sahul Shelf to the east connected New Guinea and Australia, facilitating Australasian species movement. However, the deep marine basins between these two shelves, known as the Wallacean region, largely remained submerged. This continuous deep-water separation prevented widespread faunal exchange between the Asian and Australasian continents for millions of years. Plate tectonic movements further contributed to the complex island arrangement, maintaining distinct evolutionary paths for the animals on either side.
The Wallacea Region and Related Biogeographical Concepts
While the Wallace Line marks a significant faunal boundary, the transition is not always perfectly abrupt. The region between the Sunda and Sahul shelves is known as Wallacea, a complex zone of islands that represents a unique transitional area. Within Wallacea, species from both Asian and Australasian origins can be found, often coexisting or forming unique hybrid faunas. This mixing occurs because some species were able to cross the deep-water barriers over long periods, or through island hopping.
Other biogeographical lines have also been proposed to refine the understanding of species distribution in this intricate archipelago. Weber’s Line suggests a point where the balance of Asian and Australasian species is roughly equal, often lying slightly to the east of Wallace’s original line. Lydekker’s Line, positioned even further east, marks the approximate western limit of the Australian continental shelf. These additional lines highlight the complex and nuanced patterns of biodiversity in Southeast Asia, demonstrating that while the Wallace Line is a powerful concept, the region itself is a dynamic area of biological overlap.