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November 2020   |   Volume 22 No. 1

Towing the Line

An iconic boundary line drawn up in 1800s by a Victorian biologist to explain how and why land animals were distributed across the planet has been redrawn following a study of Christmas Island in the eastern Indian Ocean.

Alfred Russel Wallace (1823–1913) was a respected biologist, who divided the earth into six continental bio-realms. The divide between the animal suites on Australasia and Asia, now known as Wallace’s Line, is especially notable with kangaroos, koalas and echidnas to the east, and tigers, orangutans and elephants to the west. It has largely been shaped by tectonic-plate movements – previously, these two land areas were separated by a vast ocean.

Christmas overlooked 

Dr Jason Ali, Associate Professor of the Department of Earth Sciences, said: “When Wallace proposed the boundary, the little dot of land that is Christmas Island (it’s slightly smaller than Lantau) was quite simply overlooked. But we discovered that most of Christmas Island’s animal species have their origins on the eastern side of Wallace’s Line and are in fact Australian. Redrawing Wallace’s Line is the logical solution to the finding. 

“Also, only with the modern analytical techniques where biologists look at an organism’s DNA can they confidently work out its nearest blood relatives. The pioneering biogeographers of Wallace’s time never had this luxury.” 

Dr Ali, who worked on the research alongside colleagues Professor Jonathan Aitchison and Professor Shai Meiri, from the University of Queensland and Tel Aviv University respectively, did not set out to dispute Wallace’s theory. 

“Our study initially had a geological focus,” he said. “We were trying to work out when this rather unusual island, Christmas Island, that is just to the south of Indonesia, remerged. Its top was sub-aerial prior to about 16 million years ago. It then ‘drowned’ before popping back up about 5 million years ago due to the warping of the underlying tectonic plate it rests upon.

“There is just a handful of islands on Earth that have done this. Obviously, only when the body is exposed can it accommodate land animals. As I looked at the island’s biological inventory, I realised that many of the native species had ancestors that originated to the east of Wallace’s Line. This is really weird because the landmass sits well over 1,000 kilometres to the west of this key biogeographical divide. They shouldn’t be there!” 

Dr Ali joined HKU in 1997, and for the next 10 years spent much of his research time working on geological projects in the Philippines, Tibet and south-west China in Guizhou, Sichuan and Yunan provinces. About 12–13 years ago he became interested in biogeography – how life is distributed around the planet. “The key thing is that my geological background gives me insights into biogeographical systems that practically all of the biologists who work in the field do not have,” he said. “Also, I am pretty much the only Earth scientist who works in the field, hence there are lots of key topics I can get stuck into with minimal competition.” 

He has worked for more than 20 years with Professor Aitchison, who was at HKU until 2011–2012 before he moved to Brisbane, where he heads a large school. “I enjoy working with Jonathan because we look at problems from slightly different angles, so we can often uncover some neat things – it’s like having stereoscopic vision of a problem. My connection with Professor Shai Meiri is somewhat shorter – two to three years. He’s a biologist working out of Tel Aviv who studies mainly reptiles and amphibians. He makes sure that I don’t do anything silly with the animal data!”

Extended Wallace Line

Wallace’s Line has been redrawn based on the discovery of Australasian-derived species on Christmas Island.

Southern end amended 

Dr Ali is careful to point out that the findings don’t actually change the rest of the path of Wallace’s Line – just its southern end, so the change is more like an amendment. “However, since the work appeared, I have had numerous requests for copies, which suggests that people are interested and/or intrigued. Hopefully, people will incorporate the findings into their studies of Christmas Island and the broader area.

“The finding forces us to think again about the barrier between two of Earth’s most distinctive faunal realms,” said Dr Ali. 

As a follow-up, over the last few months he has been working on a major review of the development of ideas related to the biogeography of the Indo-Australian Archipelago. “Following Wallace’s publication, many people proposed their own divides and transition zones for the region,” said Dr Ali. “It’s really complicated and messy, so I thought why not deconstruct it? The manuscript’s working title says it all: Wallace’s Line, Wallacea, and associated divides and areas: a tortuous tangle of ideas and labels.”

Cover of Biological Journal of the Linnean Society

Dr Ali’s research paper titled ‘Redrawing Wallace’s Line based on the fauna of Christmas Island, eastern Indian Ocean’ was published in the Biological Journal of the Linnean Society.

The finding forces us to think again about the barrier between two of Earth’s most distinctive faunal realms.