Why Animals Don’t Cross This Invisible Border
Why Animals Don’t Cross This Invisible Border
The geopolitical boundary between Asia and Oceania lies between Indonesia and the island of New Guinea. But international borders aside, there’s also an invisible line that cuts through the region that defines the zoological differences between the two continents. It’s called the Wallace Line, named for British naturalist Alfred Russel Wallace, who came to the Malay Archipelago in the mid-19th century to conduct biological research. Wallace’s findings proved something previously unknown about animal evolution and how the species differed on either side of this imaginary faunal boundary. Discover the fascinating history of the Wallace Line.
What Is the Wallace Line?
Wallace traveled to the Malay Archipelago in 1854 to collect biological specimens, and the naturalist spent the next eight years exploring the region — which is home to some 25,000 islands, including the nations of Indonesia, Malaysia, and Papua New Guinea. Wallace visited many of the archipelago’s islands during this time, and he noticed something unusual about how the types of animals differed. On many of the westernmost islands, the fauna was more akin to animals native to Asia. Toward the east, the species were more similar to those found in Australia.
As a result of his findings, Wallace proposed a concept in 1859 known as the Wallace Line. This invisible faunal boundary cuts through modern-day Indonesia, intersecting the islands of Bali to the north and Lombok to the south. It originates in the Indian Ocean and extends north toward the Pacific, separating nations including Brunei and the Philippines in the west from places such as Australia and Papua New Guinea to the east.
While Wallace was a contemporary of Charles Darwin, he independently developed his own theory of evolution. Wallace deduced that animals located to the west of this imaginary line — such as Sumatran tigers and rhinoceroses — had likely evolved from animals native to the Asian mainland. Creatures to the east, however, probably evolved from those found in Australia, such as the marsupials native to the island of Sulawesi.
A History of Shifting Terrain
These distinct evolutionary patterns weren’t a recent phenomenon, but rather one stemming from millennia of rising sea levels and shifting terrain. Millions of years ago, most of the western islands of the Malay Archipelago were united as a large mass called the Sunda Shelf, which was connected to mainland Asia. In the east, there was the similarly large Sahul Shelf, containing land that would later form all the various parts of Oceania.
Over the years, rising sea levels plunged parts of the shelves underwater, creating a system of islands where the larger landmasses used to be. Suddenly, various portions of the Sunda Shelf were no longer connected to mainland Asia. Animal species that originated in Asia were now effectively trapped on these smaller landmasses.
In time, those species evolved in their own unique way, while maintaining similar characteristics to other species native to the Asian mainland. A similar phenomenon happened with the Sahul Shelf, which broke up into various parts also due to rising sea levels. The newer landmasses include Australia, the island of New Guinea, and other smaller islands. The Wallace Line cuts right through where the Sunda and Sahul shelves used to be.
Similar Boundaries Around the World
Several of Wallace’s contemporaries noted certain flaws with his proposed boundary, and some researchers made slight modifications. For instance, biologist T.H. Huxley believed the true line should cut west of the Philippines, instead of east, as Wallace initially proposed.
Zoologist Max Wilhelm Carl Weber, meanwhile, suggested that Wallace had placed the line too far west. He proposed a new line farther east, known as the Weber Line. This invisible line extends east around the islands of Lombok, Timor, and Sulawesi.
Naturalist Richard Lydekker proposed the easternmost line of the bunch. Whichever theory is most accurate remains a fascinating biological argument. However, the entire region is now referred to as Wallacea, where the Wallace Line forms its western boundary, and the Lydekker line forms its eastern boundary.
Places To Visit Around the Wallace Line
If you’re an animal lover, traveling throughout the Malay Archipelago — just as Wallace did all those decades ago — is an unforgettable experience. There are thousands of fascinating creatures to see on both sides of the invisible line.
To the west, start with a trip to Borneo to visit the Danum Valley Conservation Area. This 130 million-year-old rainforest is home to many of the orangutans that the island is known for. It’s also worth heading farther west to Sumatra and stopping in Gunung Leuser National Park, one of the largest protected sites in Indonesia. This idyllic mountain expanse is home to many Sumatran tigers, rhinos, and other creatures whose biological ancestors originated in mainland Asia.
To the east of the Wallace Line, consider a stop at Tangkoko Nature Reserve on the island of Sulawesi. Here, you’ll find the rare Sulawesi bear cuscus, a marsupial of Australian descent. Sulawesi is also home to the tarsier, which is the smallest primate on Earth — it’s the size of a human fist. While you’re in the area, take a quick flight over to the nation of Timor-Leste, which offers some of the best whale-watching opportunities in the region. You’ll have the chance to spot blue whales, orcas, and sperm whales, all of which inhabit the surrounding waters.
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