Bali and Lombok are separated only by the Lombok Strait, which is about 24 kilometres wide. The distance is short enough to cross within a few hours, yet it separates two remarkably different groups of wildlife. This contrast was first documented by British naturalist Alfred Russel Wallace when he travelled from Bali to Lombok in 1856.
While in Bali, Wallace encountered birds such as barbets, fruit-thrushes, and woodpeckers, which share similarities with Asian fauna. But the scene changed when he arrived in Lombok. There, he found cockatoos, honeyeaters, and brush-turkeys, birds more closely associated with Australian fauna.
The contrast was striking. How could two islands separated by just a few dozen kilometres have such different animal populations? Wallace went on to map the boundary between these two faunal groups, which became known as the Wallace Line.
A Line That Marks a Divide in Wildlife
In his account, Wallace described the Lombok Strait as being about 15 miles, or 24 kilometres, wide. He even wrote that a person could travel “in two hours from one great division of the earth to another”.
Modern measurements from the INSTANT ocean-current research programme put the strait's width at around 35 kilometres, with its underwater sill reaching a depth of approximately 300 metres. Although the figures differ, both measurements show that Bali and Lombok are separated by a relatively narrow stretch of water.
So, what exactly is the Wallace Line? It is not a physical barrier that prevents animals from Asia from entering Australia, or vice versa. Rather, it is a marker scientists use to identify changes in the types of animals found across different regions.
To the west of the line, wildlife shares more similarities with Asian fauna. To the east, animals are more closely associated with Australian fauna.
The line we know today was also not established in a single version accepted by all scientists. After Wallace published his line in 1863, other scientists proposed different boundaries to explain the distribution of wildlife. Thomas Huxley, for instance, proposed another line in 1868. Wallace himself later revised the route around Sulawesi in 1910.
These differing proposals reflect the fact that animal distribution in nature does not always stop at one precise boundary. Scientists eventually recognised a transitional region between Asian and Australian fauna. This region has been known as Wallacea since 1924.
In this sense, the Wallace Line is better understood as a marker of changes in wildlife distribution rather than an absolute dividing line.
How Deep Waters Can Limit the Movement of Animals
To understand why Bali and Lombok have different wildlife, we need to look at what the landmasses of the Indonesian archipelago looked like in the past. Tens of thousands of years ago, sea levels were much lower than they are today. Large volumes of seawater were locked up in vast ice sheets across the planet.
Research published in the Proceedings of the National Academy of Sciences estimates that global sea levels fell to around 134 metres below their present level between 29,000 and 21,000 years ago. As the seas retreated, shallow underwater areas became exposed as land.
One example is the Sunda Shelf, the shallow seabed around Sumatra, Java, and Kalimantan that was once connected to mainland Asia, as described in the Journal of Biogeography.
This newly exposed land provided routes for terrestrial animals to move across the region. Animals from Asia could reach islands in the western part of the Indonesian archipelago without crossing the sea. However, this land connection did not extend as far as Lombok.
The reason lies in the depth of the Lombok Strait. Its underwater sill reaches approximately 300 metres below sea level. Even when sea levels fell by around 120 to 134 metres, the strait would still have remained about 166 to 180 metres deep. It never dried up or became a land bridge that terrestrial animals could cross.
This helps explain why Bali and Lombok developed different histories of wildlife distribution. Bali was connected to the Asian landmass when sea levels fell, while Lombok remained separated by deep water. Terrestrial animals that could not cross the sea therefore had fewer opportunities to move between the two islands.
Still, the depth of the Lombok Strait alone cannot explain all the differences in wildlife across the Indonesian archipelago. The geological formation of the islands, environmental changes, and millions of years of evolution also shaped how animals spread across the region.
Ultimately, the Wallace Line did not create a boundary that animals were unable to cross. Instead, it helps us understand the region's long natural history. Between Bali and Lombok, a seemingly narrow strait shows how two neighbouring islands can have remarkably different histories of life.
