In 1859 Alfred Russel Wallace drew a line through the Indonesian islands. West of it the animals are Asian; east of it they turn Australian. The reason is not on the land. It is in the depth of the water between.
Bali and Lombok sit about 20 km apart across the Lombok Strait. On Bali, Wallace found barbets and woodpeckers; on Lombok, cockatoos, honeyeaters and mound-building megapodes. Wallace, The Malay Archipelago, 1869
During the last Ice Age the sea fell about 120 m. The shallow shelves became dry land. The Lombok Strait is about twice as deep at its sill, so it stayed open water.
Depth below today’s sea level, linear scale 0 → 1,000 m. South of the strait the floor plunges to 3–4 km. All depths approximate.
At the Last Glacial Maximum, Sumatra, Java, Borneo and Bali were one landmass. Animals could walk to Bali. They could not walk to Lombok.
−116 m was the sea level on the Sunda Shelf 21,000 years ago, according to Field Museum reconstructions. Sea level peaked at +5 m around 4,200 years ago and then settled at today’s level.
The gold dashed line is the Lombok sill. The shaded gap between it and the lowest sea level is about 130 m of water that was always there.
Endpoints from Sathiamurthy & Voris 2006; curve between them simplified.
The islands between Wallace’s Line and the Sahul shelf were never joined to either continent. Species that arrived evolved in isolation.
Endemic vertebrate species in the Wallacea hotspot: species found nowhere else on Earth. The same profile counts about 1,500 endemic vascular plants (not shown, off this scale).
Source: CEPF Wallacea Ecosystem Profile.
Leave out the bats, which can fly across straits, and nearly nine in ten of Wallacea’s mammals live nowhere else.
of non-bat mammal species are endemic to Wallacea
km² of land in the hotspot
islands, from Sulawesi to Timor
people live there today
Source: CEPF, Wallacea Ecosystem Profile and portfolio overviews.
Wallace worked his way through the archipelago from 1854 to 1862, collecting as he went. The pattern was in his specimen boxes before anyone knew how deep the straits were.
miles travelled within the archipelago
natural-history specimens, mostly insects
years in the field, 1854–1862
The Malay Archipelago published
Source: Wallace, The Malay Archipelago (1869), Preface. Huxley redrew the line in 1868 to put the Philippines on the Asian side, which is why there are two lines in the north of the map.
The same straits still carry the Pacific into the Indian Ocean, and a 2023 study found that animals crossed the line more easily in one direction than the other.
Skeels et al. (Science, 2023) modelled more than 20,000 vertebrate species. Asian lineages had evolved in humid conditions like Wallacea’s islands, so they colonised eastward more readily. Australian lineages had adapted to drier land and rarely took hold in the west.
Underneath, the Indonesian Throughflow moves about 15 million m³ of water per second (≈15 Sv) from the Pacific to the Indian Ocean through these passages, including the Lombok and Makassar straits.
INSTANT program 3-year mean, 2004–2006 (Gordon et al. 2010).
Like this? Get a new map in your inbox every morning.
One hand-drawn map a day — real places, real sources. No spam, unsubscribe any time.