Island Biogeography

The equilibrium theory

Since Darwin and Wallace, islands have served as natural laboratories for studying evolution and the distribution of life. In 1963 ecologist Robert MacArthur and biologist Edward O. Wilson proposed a mathematical model of island species numbers, developed in The Theory of Island Biogeography (1967). They argued that the number of species on an island is set by a balance of two processes. The immigration rate of species not yet present falls as the island fills, while the extinction rate of resident species rises as species accumulate. Where the two curves cross lies the equilibrium species number. Large islands support bigger populations and so lower extinction rates, and islands near the mainland receive higher immigration, so large, near islands hold the most species. Even at equilibrium the species composition keeps changing, a process called turnover.

Species–area relationship and experiments

The theory accounted for the long-known species–area relationship S = cA^z, where S is species number and A is area; for islands z typically falls between 0.2 and 0.35, meaning a tenfold increase in area roughly doubles the number of species. Preston (1962) derived a similar relationship from species-abundance distributions. In the late 1960s Daniel Simberloff and Wilson (1969) tented and fumigated small mangrove islands in the Florida Keys to remove all arthropods and then tracked recolonization. Within about a year species numbers returned to near their original levels, nearer islands regained more species, and species composition changed, a classic field experiment supporting the predictions of equilibrium and turnover. Brown and Kodric-Brown (1977) added the rescue effect, in which immigration reduces extinction.

Conservation uses and critiques

The theory suggested that nature reserves are islands in a "sea" of developed land. Jared Diamond (1975) proposed design principles: reserves should be large, close together, compact and linked by corridors. Simberloff and Abele (1976) countered that several small reserves might conserve more species than a single large one, sparking the SLOSS (single large or several small) debate. Later habitat-fragmentation research showed that edge effects, the quality of the surrounding landscape and species differences matter more, moving beyond the theory (Laurance, 2008); Haddad et al. (2015) concluded that fragmentation reduces biodiversity by 13 to 75 percent. The theory was also criticized for treating species as equivalent and ignoring evolutionary speciation; Whittaker et al. (2008) proposed a general dynamic theory incorporating island geological age and speciation, and Hubbell (2001) offered a neutral theory.

FAQ

What is island biogeography theory in simple terms?

The number of species on an island is set where species arriving balance species dying out, and larger islands closer to the mainland hold more species.

How is it used in conservation?

Treating reserves as islands surrounded by development, it supports designing them to be as large as possible, close together and linked by wildlife corridors.

Sources reviewed: 2026-10-26

References

1. MacArthur, R. H., & Wilson, E. O. (1963). An equilibrium theory of insular zoogeography. Evolution, 17(4), 373–387.
2. MacArthur, R. H., & Wilson, E. O. (1967). The Theory of Island Biogeography. Princeton: Princeton University Press.
3. Simberloff, D. S., & Wilson, E. O. (1969). Experimental zoogeography of islands: The colonization of empty islands. Ecology, 50(2), 278–296.
4. Preston, F. W. (1962). The canonical distribution of commonness and rarity. Ecology, 43(2), 185–215.
5. Diamond, J. M. (1975). The island dilemma: Lessons of modern biogeographic studies for the design of natural reserves. Biological Conservation, 7(2), 129–146.
6. Simberloff, D. S., & Abele, L. G. (1976). Island biogeography theory and conservation practice. Science, 191(4224), 285–286.
7. Brown, J. H., & Kodric-Brown, A. (1977). Turnover rates in insular biogeography: Effect of immigration on extinction. Ecology, 58(2), 445–449.
8. Whittaker, R. J., Triantis, K. A., & Ladle, R. J. (2008). A general dynamic theory of oceanic island biogeography. Journal of Biogeography, 35(6), 977–994.
9. Losos, J. B., & Ricklefs, R. E. (Eds.). (2010). The Theory of Island Biogeography Revisited. Princeton: Princeton University Press.
10. Hubbell, S. P. (2001). The Unified Neutral Theory of Biodiversity and Biogeography. Princeton: Princeton University Press.
11. Laurance, W. F. (2008). Theory meets reality: How habitat fragmentation research has transcended island biogeographic theory. Biological Conservation, 141(7), 1731–1744.
12. Haddad, N. M., et al. (2015). Habitat fragmentation and its lasting impact on Earth's ecosystems. Science Advances, 1(2), e1500052.
13. Quammen, D. (1996). The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York: Scribner.