The MacArthur-Wilson Theory
In 1967, Robert MacArthur and E.O. Wilson proposed one of ecology's most elegant theories: the number of species on an island is determined by a dynamic equilibrium between immigration from the mainland and local extinction. This theory transformed how ecologists think about biodiversity, applying not just to oceanic islands but to any isolated habitat — forest fragments, mountain tops, or lake patches.
Immigration and Extinction Curves
The immigration rate decreases as more species colonize the island (fewer mainland species remain to arrive), while the extinction rate increases with species count (more species means more competition and smaller populations). Where these curves cross defines the equilibrium species number. The key insight is that this equilibrium is dynamic — species constantly turn over even as the total number remains roughly stable.
Area and Distance Effects
Two primary factors control island diversity. Area affects extinction: larger islands provide more habitats, support larger populations, and buffer against stochastic extinction. Distance affects immigration: remote islands receive fewer colonists. This creates a predictive framework: large, near islands are species-rich; small, remote islands are species-poor. The theory correctly predicted species counts on islands worldwide.
From Islands to Conservation
The theory revolutionized conservation biology. Habitat fragments act as ecological islands, and their species richness follows the same area-distance rules. This has profound implications for reserve design: a single large reserve preserves more species than several small ones of equal total area (the SLOSS debate). Understanding these dynamics is essential for managing biodiversity in an increasingly fragmented world.