Wallace Line and Biogeography

Overview

This lecture discusses the Wallace Line, an invisible biogeographic boundary in Indonesia, its discovery by Alfred Russel Wallace, and how geological history shapes species distribution.

Discovery of the Wallace Line

• The Wallace Line divides Asian and Australasian animal species across a narrow strait in Indonesia.
• Western islands (e.g., Bali) have Asian fauna like elephants and tigers; eastern islands have species like Komodo dragons and cockatoos.
• Alfred Russel Wallace identified the line in 1859 during his exploration of the Malay Archipelago.
• Wallace noticed sudden, drastic changes in animal species when moving between nearby islands, notably from Bali to Lombok.

Wallace's Observations and Theories

• Wallace observed that differences between neighboring islands' fauna could be greater than between two continents.
• The boundary was not explained by climate or geography, but by deep historical divides.
• Wallace theorized that today's animal distributions reflected an ancient history of the Earth’s land and seas.

Geological Context and Plate Tectonics

• Wallace hypothesized that islands west of the line were once part of Asia (Sunda), and those to the east related to Australia (Sahul).
• He lacked the concept of tectonic plates, which was confirmed only in the late 1960s.
• Plate tectonics explains the moving and splitting of continents, creating regions like the Malay Archipelago.

Wallacea and Species Distribution

• The transitional zone between Sunda and Sahul is called Wallacea; these islands were never connected to major landmasses.
• Species on Wallacea islands often arrived from Australia, such as the Komodo dragon.
• Deep ocean trenches, like the Lombok Strait, still prevent many species from crossing, preserving distinct faunas.

Wallace's Legacy in Biogeography

• The Wallace Line remains a key example of how geological events influence modern biodiversity.
• Wallace helped shape the idea that animal distribution reveals the history of the Earth.
• He is recognized as a pioneer of biogeography, complementing Darwin’s work on evolution.

Key Terms & Definitions

• Wallace Line — An invisible boundary dividing Asian and Australasian animal species in Indonesia.
• Biogeography — The study of species distribution over geographic areas.
• Sunda — The western landmass formerly connected to Asia.
• Sahul — The eastern landmass including Australia, Papua, and Aru Islands.
• Wallacea — Transitional islands between Sunda and Sahul.
• Plate Tectonics — The theory that Earth's surface is divided into moving plates.

Action Items / Next Steps

• Compare the differences between the Wallace Line and Weber’s Line.