Invisible line through Asia which stopped kangaroos, but not goannas and kookaburras

New research sheds light on why you won’t find kangaroos, koalas and other iconic Australian marsupials in Indonesia, despite the fact that there are many animal groups which made the trip to  Asia, including goannas, rodents and kookaburras.

An invisible boundary seems to exist.

Read more: Which megafauna did Indigenous Australians encounter?

It’s called Wallace’s Line – named after 19th Century naturalist Alfred Russell Wallace, co-discoverer with Charles Darwin, of the process of natural selection.

Map of southeast asia and australia with labelled lines
Map showing Wallace’s Line. Credit: Maximilian Dörrbecker (Chumwa) via Wikimedia Commons.

Wallace’s Line separates Australian and Southeast Asian fauna. It was first proposed in 1859 and runs through Indonesia, between Borneo and Sulawesi, and through the 35-kilometre-wide Lombok Strait between Bali and Lombok.

Its enigmatic character has puzzled biologists for generations.

To the west of Wallace’s Line, you find elephants, tigers, rhinoceroses, apes, cats and monkeys. To the east are the Australasian fauna including marsupials and monotremes.

There are, of course, exceptions including macaques and pigs found east of the boundary on Sulawesi.

Now a new paper published in Science provides an explanation for Wallace’s Line. Biologists at the Australian National University (ANU) and ETH Zurich in Switzerland suggest that changing plate tectonics 45 million years ago, and a dramatic shift in Earth’s climate tens of millions of years ago, caused the uneven distribution of creatures across the invisible boundary.

“About 35 million years ago, Australia was located much further south and was connected to Antarctica,” says Dr Alex Skeels from ANU.

“At some point Australia broke away from Antarctica and drifted north, eventually crashing into Asia. That collision gave birth to the volcanic islands that we now know as Indonesia.”  

Tiger on a branch in a broad leafed tree
Sumatran tiger in Indonesia. Credit: Aprison Photography / Moment / Getty Images plus.

“When Australia drifted away from Antarctica, it opened up this area of deep ocean surrounding Antarctica which is now where the Antarctic Circumpolar Current is. This dramatically changed Earth’s climate as a whole; it made the climate much cooler,” Skeels explains.

“Despite this global cooling, the climate on the Indonesian islands, which organisms used as a gateway to hop to Australia, remained relatively warm, wet and tropical. Asian fauna were already well adapted and comfortable with these conditions, so that helped them settle in Australia.”

“This was not the case for the Australian species. They had evolved in a cooler and increasingly drier climate over time and were therefore less successful in gaining a foothold on the tropical islands compared to the creatures migrating from Asia.”

Australian Mammal Of The Year 2023

About 20,000 species of bird, mammal, reptile and amphibian were analysed as part of the study.

“Our findings could also inform predictions for animal migration in the future and help us predict which species may be better versed at adapting to new environments, as changes to Earth’s climate continues to impact global biodiversity patterns,” says Skeels.

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