How the Wallace line explains the difference in species across continents Premium

Discover the Wallace Line, a biogeographical boundary between Asia and Australia, through the eyes of naturalist Alfred Russel Wallace.

Kangaroos and cockatoos are synonymous with Australia and tigers and orangutans with Asia. Both these continents boast rich biodiversity that is also very unique. A simple yet popular way to understand these ‘separate greatnesses’ has taken the shape of the Wallace line.

In the late 19th century, the English naturalist Alfred Russel Wallace noticed a dramatic shift in the composition of organisms as he moved from Asia to Australia, New Guinea, and other islands nearby. He posited an invisible barrier in the ocean, later called the Wallace line, running between the islands of Bali and Lombok, striking north between Borneo and Sulawesi before curving south of Mindanao. To him this line was like a fence between the different kinds of animals on the two sides.

Wallace and others conducted eight years of fieldwork to carefully plot the line across many kilometres, in the process laying the foundations of modern biogeography: the study of how species are distributed and how they got there.

Over the years, the line has attracted considerable research interest. “The Wallace line … ties partly into the theory of evolution. Nowhere else on the earth do you see such a dramatic shift over such a narrow distance. Organisms are not just scattered randomly,” Jason R. Ali, honorary associate researcher at the Senckenberg Society for Nature Research, Germany, said.

At their closest, the islands of Borneo and Sulawesi are just over 20 km apart yet they support very distinct plants, mammals, and birds. Wallace was more baffled by Sulawesi. It’s one of the largest islands in the archipelago and home to species found nowhere else on the planet, including tarsiers (family Tarsiidae), the lowland anoa (Bubalus depressicornis), and the mountain anoa (Bubalus quarlesi), which are both of Asian origin. Yet Sulawesi is also home to Australian marsupials like the dwarf cuscus (Strigocuscus celebensis).

The island frustrated Wallace, who repeatedly redrew his line because he was unsure whether it belonged to Asia or Australia. He wrote in 1876 that the animals here showed “affinities” to Africa, India, Java, the Maluku Islands, New Guinea, and the Philippines.

Why do Sulawesi have species from both sides of the line while most others didn’t? Wallace had deduced the essential answer all those years ago but it has accrued greater depth with more research over time.