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What is it? -

The Wallace Line is a boundary that cuts through the archipelago nation of Indonesia and separates the fauna of Southeast Asia and Australasia. The boundary may be visibly imaginary, but it represents a stark division in how species are distributed between the two regions.

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Identification -

The division between species was first identified by Alfred Russel Wallace, a naturalist and explorer, in the 19th century, although it wasn’t given its name until later.

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Discovery -

Wallace explored the Malay Archipelago in the mid-1800s, and while studying species distribution he noticed a distinct shift in the types of animals on either side of the narrow Lombok Strait (between the islands of Bali and Lombok).

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Non-existent -

The first thing Wallace noticed were the birds. As he traveled east from Bali to Lombok, Wallace saw that certain species that were plentiful in Bali (like the coppersmith barbet) didn’t exist in Lombok.

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Location -

The Wallace Line runs between the islands of Bali and Lombok, and extends north between Borneo and Sulawesi. The islands are, at most, 22 miles (36 km) apart, but the line divides them into two regions: the Asian side to the west, and the Australasian side to the east.

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Biogeographical importance -

The Wallace Line holds great importance for biogeography, as it marks the point where the species of Asia and Australasia meet. Despite the proximity of the islands, their flora and fauna remain distinctly different. But why is that?

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Geological origins -

The world beneath our feet sits on puzzle pieces known as tectonic plates, which are always in motion. The origins of the Wallace Line can be traced back to the movements of these tectonic plates millions of years ago.

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Continental shelves -

All the landmasses west of the Wallace Line sit on the Sunda Shelf, while everything to the east (including Australia) sits on the Sahul Shelf.

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Drifting apart -

Long ago, the movements of these shelves caused the Asian and Australasian landmasses to drift slowly apart, which created deep ocean trenches that have acted as natural barriers to species migration for 50 million years.

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Lower sea levels -

During the Pleistocene Era approximately 2.5 million years ago, sea levels were much lower, and many land connections between islands were exposed. This allowed animals to migrate further, but they still could not cross between Asia and Australia.

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Ocean trenches -

The Wallace Line corresponds closely with deep ocean trenches that have historically prevented land bridges from forming. These trenches, which are incredibly deep, have kept terrestrial species from being able to cross between Asia and Australasia.

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To the west -

On the western side of the Wallace Line, species are primarily of Asian origin, such as tigers, elephants, and primates.

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To the east -

The eastern side of the line, in Australasia, marsupials (like kangaroos) and monotremes (like the platypus) are more prevalent.

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Ocean currents -

Ocean currents have also contributed to the separation of species across the Wallace Line. The strong currents in the straits between islands make it difficult for aquatic species and seeds carried by water to cross between the two regions.

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Birds -

Even though birds are capable of flight, many species also do not cross the Wallace Line. Birds often avoid crossing open water, since vegetation allows them to be protected and they are unlikely to fly out into open space.

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Bats - Interestingly, bats are among the few terrestrial mammals capable of crossing the Wallace Line due to their ability to fly long distances, and some species of bats can be found on both sides of the line.

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Flora -

Unlike the fauna of the region, flora aren’t strictly divided along the Wallace Line. This is because they colonize new areas in different ways than animals.

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Exception -

There is only one genus of plants that does not cross over the Wallace Line, which is the Australasian Eucalyptus.

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Marine species -

Marine species also don’t adhere to the Wallace Line. The area between the Sahul continental shelf and Wallace Line is known as the coral triangle, which is home to the most diverse marine environment in the world.

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Theory of evolution -

Although Charles Darwin is often considered to be the person who 'discovered' evolution, Alfred Russel Wallace was the one who independently conceived the theory of evolution. His studies in Indonesia formed the basis of the theory.

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In history -

But Alfred Russel Wallace was not the first person to notice the extreme differences in the species of these two regions. In fact, questions such as these have been traced back to 1521, when Venetian explorer Antonio Pigafetta noticed the same differences.

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Shaping the future -

Other naturalists, including G.W. Earl, also made similar observations. All of this prior work done by previous professionals played a pivotal role in shaping Wallace’s theories.

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A worthy name -

The name for the Wallace Line was only coined in 1968, almost a decade after Alfred Russel Wallace ventured through Indonesia. The name was put forward by T.H. Huxley, who also put the line to the west of the Philippines (despite Wallace’s initial hesitation).

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Weber’s Line -

Some naturalists have preferred to use Weber’s Line to show the stark difference between species. Weber’s Line was proposed by German-Dutch zoologist Max Wilhelm Carl Weber and is situated further east than the Wallace Line.

Image credit: Hennemann, Conle, & Suzuki (2015)

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Lydekker's Line -

Lydekker's Line, which was put forward by naturalist and geologist Richard Lydekker, puts the divide even further to the east. The line runs close to the island of New Guinea, almost 1,000 miles (1,600 km) east of the Wallace Line.

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Modern research -

Ongoing genetic studies, fossil discoveries, and climate models are deepening our understanding of how this boundary has shaped life in Southeast Asia and Australasia.

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The lasting legacy -

Alfred Russel Wallace’s work on the Wallace Line left an enduring legacy in natural sciences. His insights helped to establish him as the father of biogeography (how living things are distributed). His work remains central to our understanding of how life is structured across the planet.

Sources: (Interesting Engineering) (The Financial Express) (Britannica)

See also: Signs of evolution you can find on your body

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