The Wallace Line & Biogeography

When the age of exploration yielded to a period not simply of unlocking new areas of the globe, but of serious, timely examinations of those areas, the study of Earth’s biology began in earnest. In the middle of the 18th century, Carl Linnaeus noticed patterns in the similarities and differences between organisms worldwide, prompting him to develop a system for classifying plants and animals. This development led humans to take a broader view of the planet and its beings. Why were birds similar from place to place, but not exactly alike? Why did some places feature unique wildlife?

These ponderings led to the groundwork for what we today call biogeography. Broadly, this term means the study of the distribution of species and ecosystems across Earth and through time. Biogeography can be split into smaller segments: phytogeography relates to plants; zoogeography deals with animals; mycogeography deals with fungi. The study requires a wide lens, as it combines elements of evolution, ecology, geology, human history, climatology, and even physical things we love around here, such as seas, rivers, and mountains.

Though Linnaeus unwittingly pushed humanity toward this field of study, one discovery involving oceans, islands, and a giant, invisible line ushered in a new way to view the living beings on the planet.

Map included in Alfred Russell Wallace's The Geographical Distribution of Animals
Map of the world's biogeographic realms according to Miklos Udvardy - graphic by carol

Today, scientists carve the globe into eight biogeographical realms. These regions represent the largest groupings of terrestrial organisms.

The first image above comes from the system developed by Alfred Russell Wallace in 1872. Most of the macro-regions remain relatively unchanged since that time. Wallace was a brilliant naturalist and biologist from England, who independently developed the idea for natural selection around the same time as Charles Darwin (he claims the notion arrived in a fever dream). Today, history mostly remembers Darwin and his finches when it comes to evolution, but Wallace can take some solace in the fact that he went down as the “father of biogeography.”

This designation is the result of exploration Wallace undertook in the late 1850s throughout Asia and Australia. We can understand the oddity and specialness of a discovery he made by examining the geography of the Malay Archipelago.

Alfred Russel Wallace in 1986
Take another look at the large regions
The Malay Archipelago - graphic by Sbb1413

Looking at the map of the major biorealms again, the orange area of Australasia and the yellow Indomalayan region smash together in a curious location.

The Malay Archipelago is a string of over 25,000 islands between the mainlands of Asia and Australia. Though we know islands are major drivers of natural selection, adaptation, and evolutionary change, these tracts are close enough together that one might expect to see a lot of genetic overlap or a gradual spectrum of change from north to south or east to west. These islands do not sit in major isolation like the Galapagos or Chathams.

Yet, when Wallace sailed through the myriad islands in 1859, he noticed something spectacular. The islands in the Malay Archipelago contained a massive faunal boundary, as if a mountain chain a hundred miles high stretched across the surface of the ocean.

Wallace's original drawing of the boundary (line highlighted for clarity)

Hopping from island to island, Wallace documented the animals he found on each. Strikingly, though some of the islands were remarkably close – Lombok and Bali find themselves separated by just 22 miles of ocean – the critters were alien across an unseen line. To the west of the line, the animals seemed to come from Asia: apes, big cats, elephants, monkeys, and rhinoceroses. To the east of the line, the Australian influence is rather stark: marsupials, monotremes, and many rodents native to the big island. Across the entire archipelago, a hard stop existed. Even the birds seemed to obey the barrier. Some of the islands across the line are closer to each other than they are to other islands on their own side, so something strange was happening. In time, this stripe became known as the Wallace Line.

Two major questions arise immediately from such a phenomenon. First, why is there a different biogeographic distribution of fauna if these islands are so close? Second, if the animals seem to have such a hard time moving from one island to another, how did tigers, elephants, and rhinos end up in these places?

The Wallace Line and its animal separations - graphic by PBS

Wallace deduced the answer lay in geology.

He knew that sea levels were likely lower during the last glacial period, as seawater was locked up as ice. This reality could explain the second big biogeographic question the line posed. It wasn’t aliens who dropped tigers, elephants, and rhinos onto islands in the middle of the ocean, but rather their own ambulatory abilities. The same is true of the marsupials on the Australian side.

All the islands west of the line are connected to Asia by the Sunda Shelf. Today, the sea depth there is relatively shallow. When water levels were lower, this shelf would have been walkable by any mobile creature. When the oceans rose, big animals that can’t swim found themselves captive. Likewise, the islands east of the line are connected to Australia by the Sahul Shelf and the same situation likely occurred there.

The grey areas display likely areas above sea level during the last glacial period - graphic by Altaileopard

Still, why is there a line between islands at all?

Wallace surmised the answer again came from the ocean, this time because of depth. Instead of shallower regions connecting islands to the mainland, he believed the depth along the line must be rather substantial. Only certain birds and swimming creatures would be able to cross such a boundary. No walking possible.

Though Wallace and other scientists of the era did not have knowledge of plate tectonics, it turns out he was correct about the depth, he just didn’t know why. Despite the Malay Archipelago looking like a coherent group of next-door neighbors, the halves once existed much farther apart. Only in the relatively recent past (in geologic terms) did these areas smash into each other.

The Malay Archipelago is the site of a gnarly junction of four tectonic plates: the Australian, the Pacific, the Eurasian, and the Indian. So, the islands that were near each other before the most recent outcome of continental drift all developed similar animals, while a deep gulf lay between the separate groups. This distance helps explain why even birds, which could bridge the distances if they needed, seem to be so different across the Wallace Line.

Earth's 15 principal tectonic plates - graphic by USGS

Why hasn’t there been much crossover between birds and other mobile creatures?

This answer is a bit harder to nail down. Partially, there has been a bit of migration. Plants, which spread in a much different manner (wind), are fairly similar across the islands. Some avian species can cross the line, but the short answer is we are likely in a sweet spot of geological activity. As time moves forward, we will likely see some species begin to migrate.

For now, the Wallace Line maintains its invisible hold. Variations on the line have cropped up over the years, but Wallace’s notion remains largely intact scientifically. Since plate tectonics did not gain wide acknowledgment until the 1960s, Wallace’s observations in the 1850s seem rather incredible. Who needs to be recognized for natural selection?

Further Reading and Exploration

Biogeography: Species Distribution – ThoughtCo

Wallace Line – Encyclopedia Britannica

The Invisible Barrier Keeping Two Worlds Apart – PBS Eons

On the physical geography of the Malay archipelago by Alfred Russell Wallace

Wallace’s Line in the Light of Recent Zoogeographic Studies – Quarterly Review of Biology

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