Across the Wallace Line
- Jan 31
- 7 min read
Dear Reader,
Last week, work was to take me to Bali, Indonesia. After years of reading articles and books on biogeography, featuring the islands of Bali and Lombok, I was eager to ferry across the narrow strait that divides them in the footsteps of Alfred Russell Wallace. Wallace’s Line — an invisible barrier between two biogeographic realms — is a holy grail for biogeographers. By observing the differences in biodiversity on either side, the young naturalist, Alfred Russell Wallace, arrived at a broad explanation for the distribution of species across the world, namely, the theory of natural selection.
A series of unfortunate events — though nothing like what Wallace suffered — meant I couldn’t travel to Bali. Instead, I had to retrace Wallace’s journey through his words alone, immortalised in his book ‘The Malay Archipelago’. The book was written six years after he returned from his travels, well after the groundbreaking 1858 Darwin-Wallace paper on evolution by natural selection. It offers a sweeping overview of his eight-year journey and the observations that led to his (r)evolutionary ideas.
So much has been written about the Darwin-Wallace paper, with many theories and whispered conspiracies about their academic relationship, and as many rebuttals. If you’re interested, here’s a brilliant, balanced overview of the entire affair.
In this essay, I would like to explore the geology and natural history of the Malay Archipelago, as presented in Wallace’s book (without delving into his rather problematic, racial notes on its peoples). I hope this serves as inspiration for travellers (like me!) who dream of crossing Wallace’s Line someday, to better understand some key tenets of biogeography, and the deep-time stories of these islands.
Devayani
The Malay Archipelago
The world’s largest group of islands, the Malay Archipelago, spans over 6,100 km (3,800 miles) along the equator from east to west. At its widest, it stretches about 3,500 km (2,200 miles), and includes Indonesia, Malaysia and the Philippines, covering parts of the Indian and Pacific Oceans. For comparison, the islands could spread across all of Europe from the extreme west and far into Central Asia — not the actual area of the islands but their spread alone.
One of the world’s most active tectonic belts runs through the archipelago, passing beneath well-known destinations such as Sumatra, Java, Bali, Lombok, Flores, Ternate, and Gilolo. [I’ve written about Indonesia’s Volcano Trails before]. There’s a marked break between Morotai and North Sulawesi islands, before the fiery belt continues past the Siau and Sanguir islands of Indonesia, to the 7000+ islands of the Philippines. As tectonic upheavals occur along plate boundaries and faults, this region experiences intense volcanic and earthquake activity. Ironically, the intense volcanic activity that has shaped these islands is also responsible for their fertile, tropical vegetation.
Geology suggests that mainland Malaysia (Malacca) and Thailand (Siam) may once have been connected to the islands of Java, Sumatra, Borneo, Bali, and the Philippines. Volcanic activity can cause landmasses to subside, eventually creating a sea. Yet the fauna across these islands offers overwhelming proof that they were once connected. Elephants and tapirs are found in Sumatra and Borneo, rhinoceros wander across Sumatra and Java, wild cattle rampage across Borneo and other parts of Southern Asia — these mammals wouldn’t have swum across the sea. The birds and insects also hint at the connection between the continent and the island. We can infer that there were land bridges between these islands in the recent past.
If an island’s biodiversity closely resembles that of the mainland, it indicates a more recent separation between the two landmasses. The lesser the resemblance between island and mainland biodiversity, the older the separation.
For example, Sumatra's biodiversity is very similar to that of Malaysia. Borneo bears some similarity, whereas Java has more distinct biodiversity. This indicates that Java split up from Malaysia a long time ago, followed by Borneo, and more recently, Sumatra.
While tectonic activity explains some differences in biodiversity distribution, we need to dive deeper to understand why certain species are found where they are, rather than elsewhere.
Wallace’s Line
Most tourists visit Bali for its Hindu temples, water sports, silver jewellery workshops, and lace crafts, and then may hop over to Komodo Island to see the dragons — not the mythical beasts, but the endemic monitor lizards that once devoured (and wiped out) the island’s pygmy elephants!
Yet a veritable mystery lies just across the Lombok Strait — a narrow stretch of water between Bali and Lombok islands, and birdwatchers may be able to piece together the puzzle better than most folk! In Wallace’s own words:
In Bali we have barbets, fruit-thrushes, and woodpeckers; on passing over to Lombock these are seen no more, but we have an abundance of cockatoos, honeysuckers, and brush-turkeys, which are equally unknown in Bali, or any island further west.
Imagine that! By crossing a mere 40-60 kilometre (25-37 mile)-wide channel, we can travel between two biogeographic realms: one with distinctly Asian species, and the other with Australian species. Wallace goes on to describe the stark difference between Java and Borneo, where monkeys, wild cats, civets, otters and squirrels abound, whereas in Sulawesi (Celebes) and the Maluku (Moluccas) islands, the prehensile-tailed Cuscus, a marsupial, is the only native mammal. There are similar differences in the distributions of insects, reptiles, and even flora.
Yet do all species toe this invisible, yet distinct, Wallace’s Line?
Mammals like tigers, rhinos, and primates moved from Asia across the western islands; rodents and bats spread out across the entire archipelago, and marsupials migrated from Australia across the eastern islands. Goannas and monitors originated in Asia and conquered all the islands. With birds, the pittas, hornbills, woodpeckers and kingfishers from Asia spread out across the western islands; the shorebirds and migrants from Asia can be found across the archipelago, whereas the megapodes (stocky, chicken-like birds), the honeyeaters and parrots from Australia made their way to the eastern islands.
![Tracing the distribution and spread of fauna across Wallace’s Line. Image Source: imps.cdu.edu.au. [access the interactive version]](https://static.wixstatic.com/media/7a7272_d77b099b73c4482abd2905826cab64e7~mv2.gif/v1/fill/w_500,h_300,al_c,pstr/7a7272_d77b099b73c4482abd2905826cab64e7~mv2.gif)
In a sense, Wallace’s Line is not as ‘uncrossable’ as some sources indicate; several families have managed to establish a presence across the islands. In recent years, human interventions have also altered species distributions and obscured biogeographic evidence.
Of Straits & Plates
Before we dive into why the Lombok Strait presents a barrier to the movement of species, here’s a thought-provoking excerpt from ‘The Malay Archipelago’:
Wherever the geologist can explore the earth’s surface, he can read much of its past history, and can determine approximately its latest movements above and below sea-level; but wherever oceans and seas now extend, he can do nothing but speculate on the very limited data afforded by the depth of the waters. Here the naturalist steps in, and enables him to fill up this great gap in the past history of the earth.
This line seems almost prophetic, as Wallace’s theory was proposed at a time we knew nothing about plate tectonics, and yet the faunal boundaries indicate a deep-time separation of geologic origin. Today, we know that an ancient ocean trench, created by the movement of plate tectonics, lies beneath Wallace’s Line or the Lombok Strait. This trench —unbridged by land even during Ice Age sea-level drops — has separated the Sunda (beneath Southeast Asia) and Sahul (submerged Australia) continental shelves for over 50 million years.
After breaking away from Gondwanaland, Australia’s northward drift caused it to collide with the southeastern fringe of the Eurasian plate. This collision created the volcanic arc of the Indonesian archipelago. The newly-formed islands, bathed in shallow seas, served as stepping stones for species from mainland Asia and Australia. Yet the deep-sea trench persisted and posed a veritable barrier for many vertebrates — as reflected in the erratic distribution of species across geologically similar, and not too distant islands.
Wallace had read of the shallow sea connecting the islands of Sumatra, Java, and Borneo with the Asian continent and a similar shallow sea connecting New Guinea and some adjacent islands to Australia. Yet he was the first to realise that while the connection between these landmasses was critical, it was their separation that set the stage for evolution.
Since Wallace’s time, several researchers have built on his work to delineate biogeographic realms. Other attempts to characterize faunal and botanical boundaries include the Sclater line (1894), Lydekker’s line (1896), Weber line (1904), and Mayr’s faunal balance (1944).
An interesting new development is the demarcation of ‘Wallacea’, a group of islands including Lombok, Sumbawa, Flores, Sumba, Timor and Sulawesi, which are separated from both Asia and Australia by deep-water straits. The Weber’s Line runs right through the heart of Wallacea, marking where Asian and Australian biodiversity is approximately equally represented. The rest of Wallacea has a unique assemblage of species which were able to cross the open ocean, as well as some endemics.
A Final Note
In the past 175 years, Wallace’s legacy across the Malay Archipelago has continued in a dark, ironic way: hunting and poaching of rare species, mostly birds, have driven several species to the brink of extinction. The ‘Indonesian Songbird Crisis’, as the media terms it, is a dark chapter of illegal wildlife trade for caged birds, religious traditions, and songbird contests (kicau mania), with wild, trapped birds posing additional risk of avian flu when brought into contact with domesticated birds.
The songbird crisis would undoubtedly have repercussions across the food web, and I can’t imagine present-day naturalists would be able to empirically arrive at or validate Wallace’s theory. To think of an entire island chain’s biodiversity, with its all-important clues to biogeography, near-obliterated in the evolutionary short timescale of two centuries, is a bleak reflection of Man’s rapacious nature.
In March 2024, I visited the Lee Kong Chian Natural History Museum in Singapore. During Wallace’s travels, he employed a field assistant, Ali, from Singapore; the museum documented their journey with key highlights and notes from the different islands. I’m not sure whether the museum still hosts this exhibition (if someone can confirm, I’d appreciate it), but perhaps the statue of Ali and Wallace and some exhibit literature can still be found there.
