Wallace’s Line is a distinct biogeographical boundary that dissects the Malay Archipelago, marking a profound shift in the types of animal life found on either side. It extends through the Lombok Strait, which separates the islands of Bali and Lombok. It then continues northward through the Makassar Strait, between Borneo and Sulawesi, before moving eastward, south of Mindanao, into the Philippine Sea. This line represents a significant divide between Asian and Australasian fauna, illustrating how geographical barriers can influence species distribution.
Alfred Russel Wallace and His Discovery
Alfred Russel Wallace, a 19th-century British naturalist, embarked on an eight-year expedition to the Malay Archipelago from 1854 to 1862. During his travels, Wallace documented the region’s diverse flora and fauna, collecting over 125,000 specimens. His observations revealed an abrupt change in animal species across seemingly narrow stretches of water, leading him to propose an unseen boundary.
Wallace noticed that islands west of this line, like Bali and Borneo, harbored animals resembling those found in mainland Asia. Islands to the east, such as Lombok and Sulawesi, were home to species with distinct Australasian characteristics. This faunal discontinuity led him to theorize about a barrier. Wallace independently conceived the theory of evolution by natural selection, a concept he communicated to Charles Darwin in 1858 from the Indonesian island of Halmahera.
The Distinct Fauna It Separates
Animal life on either side of Wallace’s Line presents a stark contrast. To the west, on islands like Sumatra, Java, and Borneo, the fauna shares strong affinities with mainland Asia. This includes a variety of placental mammals such as tigers, rhinoceroses, and various monkey species. Bird species on the Asian side also show clear connections to continental Asian lineages, with examples like woodpeckers and pheasants being prevalent.
Moving eastward across Wallace’s Line, the animal communities dramatically shift to resemble those found in Australia and New Guinea. This Australasian side is characterized by a prevalence of marsupials, such as various species of wallabies and cuscus, which are largely absent to the west. Distinctive bird groups like cockatoos and cassowaries are also characteristic of this eastern region.
The Geological Basis
The underlying reason for Wallace’s Line lies in the deep geological history and tectonic activity of the region. The line essentially follows a deep-water channel that delineates the edge of two ancient continental shelves. To the west lies the Sunda Shelf, a shallow underwater extension of the Asian continent that connected islands like Sumatra, Java, and Borneo to mainland Southeast Asia during periods of lower sea levels, such as ice ages. This allowed Asian fauna to disperse across these now-submerged land bridges.
Conversely, to the east lies the Sahul Shelf, which connected Australia and New Guinea, enabling the spread of Australasian species. Deep ocean trenches between these two shelves remained submerged even during glacial periods when global sea levels dropped by over 100 meters. These deep-water barriers acted as enduring obstacles to the dispersal of terrestrial animals, effectively maintaining the distinct evolutionary trajectories of species on either side for millions of years. Tectonic plate movements over vast geological timescales further shaped the archipelago, contributing to the formation and persistence of these deep-water channels.
Wallacea and Its Broader Significance
Between Wallace’s Line and another less distinct boundary known as Lydekker’s Line, lies a transitional zone of islands called Wallacea. This unique area, which includes islands like Sulawesi, Lombok, and Timor, was never recently connected by dry land to either the Asian or Australian continental masses. As a result, Wallacea exhibits a fascinating mix of Asian and Australasian species, populated by organisms capable of crossing the deep-water straits that isolate these islands.
Wallace’s Line remains a foundational concept in biogeography, informing scientific understanding of species distribution patterns globally. It provides a clear example of how geographical isolation can drive evolutionary divergence and speciation. The study of this boundary helps researchers understand the historical movements of species and the processes that lead to the incredible biodiversity observed today. The unique and often endemic species found within Wallacea underscore the importance of this region for biodiversity conservation efforts.