Island Biogeography Calculator

The Theory of Island Biogeography, proposed by Robert MacArthur and Edward O. Wilson in 1967, predicts that the number of species on an island is determined by a balance between colonization from the mainland and local extinction. Larger islands and islands closer to the mainland tend to support more species. The theory applies not only to oceanic islands but also to any isolated habitat patches such as forest fragments or mountaintops.

Equilibrium species richness (Ŝ) is the stable number of species predicted to exist on an island when the rate of new species colonizing equals the rate of existing species going locally extinct. It does not mean the exact same species are always present — individual species still turn over — but the total count stabilizes around this equilibrium value.

Larger islands support more species because they provide more habitat diversity, more resources, and can sustain larger populations that are less vulnerable to extinction. The species-area relationship follows a power law: S = c·A^z, where A is area and z typically ranges from 0.20 to 0.35 for oceanic islands. Doubling island area does not double species count — it produces a more modest, predictable increase.

Islands farther from the mainland receive fewer colonizing species because dispersal across open water is more difficult over longer distances. This reduces the colonization rate, which in turn lowers the equilibrium species richness. Near islands will therefore generally be more species-rich than distant islands of the same size, all else being equal.

Species turnover is the rate at which species are replaced on an island — some go locally extinct while new ones colonize. Even at equilibrium, where total species richness is stable, individual species continuously come and go. The turnover rate at equilibrium equals both the colonization rate and the extinction rate when they are balanced, and can be expressed as species replaced per unit time.

Colonization rate (c) represents how quickly species from the mainland establish on the island, and it decreases as the island fills with species (fewer new species remain on the mainland to colonize). Extinction rate (e) represents how quickly established species go locally extinct, and it increases as the island becomes more crowded. The MacArthur-Wilson model predicts equilibrium where these two rates intersect.

The species-area exponent z describes how steeply species richness increases with area. For true oceanic islands, z typically falls between 0.25 and 0.35. For habitat islands (e.g., forest fragments), z is often lower, around 0.15–0.25. A commonly used default value is z = 0.25. The constant c in the relationship S = c·A^z varies by taxonomic group and geographic region.

Yes — the Theory of Island Biogeography has been widely extended to any isolated habitat patch, including forest fragments, parks, lakes, mountain peaks, and urban green spaces. In conservation biology, this framework underpins the design of nature reserves, informing decisions about patch size and connectivity. The same mathematical principles apply, though calibration of rates may differ from oceanic contexts.