Island biogeography

Island biogeography predicts that the number of species on an island tends toward a dynamic equilibrium between immigration and extinction.¹ Immigration is usually highest on islands close to a source area and declines as the island’s species list becomes fuller. Extinction is generally higher on small islands, where populations are smaller and resources and habitats are scarcer. The equilibrium is therefore represented by the intersection of declining immigration and rising extinction curves.

The model does not imply a fixed roster of species. Instead, species turnover can occur while total richness remains roughly stable: one species arrives as another disappears. MacArthur and Wilson’s 1967 formulation made geographical isolation and island area central variables in explaining broad patterns of diversity, while recognizing that competition, predation, disturbance, and habitat quality also shape the outcome.²

Island area and isolation influence richness through partly different mechanisms. Larger islands usually contain more habitat types and larger populations, reducing extinction risk; they may also be easier targets for dispersing organisms. Nearer islands receive more colonists because propagules, seeds, and animals have shorter distances to cross. The resulting species–area relationship is often expressed as S = cA^z, where S is species richness, A is area, and c and z are fitted constants.

Distance alone is not destiny. Wind direction, ocean currents, dispersal behavior, landscape structure, and the identity of the source community can make a remote island more accessible than its map position suggests. “Islands” can also be habitat patches in a terrestrial matrix, such as forest remnants in farmland, although edge effects and human disturbance may make these systems depart from the classical oceanic model.

Island biogeography became influential in conservation because protected areas and habitat fragments resemble islands of suitable habitat in a hostile matrix. Its logic encouraged attention to reserve size, connectivity, and the risks of isolating populations.³ Larger reserves often support more species, while corridors or stepping-stone habitats can increase movement and recolonization after local extinctions.

Reserve design is not a simple rule that larger or closer always wins. A single large reserve may retain more interior habitat and viable populations, whereas several smaller reserves can represent different habitats and reduce the chance that one catastrophe affects every population. Modern conservation planning therefore combines island-biogeographic reasoning with metapopulation ecology, genetics, fire regimes, invasive-species management, and the particular requirements of threatened species.

The equilibrium model describes averages and tendencies rather than a universal law. Islands can gain species without reaching equilibrium, lose species through delayed extinction after habitat reduction, or accumulate non-native species introduced by people. “Extinction debt” is especially relevant when populations remain temporarily present after isolation but are too small to persist over the long term.

Island studies have also revealed that evolution can alter the very rates the model treats as inputs. Adaptive radiation, body-size change, flightlessness, and unusual defense traits are common subjects in isolated environments; the Hawaiian Islands provide prominent examples of diversification and endemism. Conversely, island populations can be exceptionally vulnerable to introduced predators, pathogens, and competitors because they often evolved without those pressures.⁴ The theory is thus a starting framework, not a complete account of island diversity.

Immigration rate

The rate at which species arrive on an island from a regional source pool.

Extinction rate

The rate at which established island species disappear locally.

Species–area relationship

The recurring pattern in which larger areas generally contain more species.

Turnover

Replacement of some island species by others over time without necessarily changing total richness.

Extinction debt

Future species loss caused by past habitat reduction or isolation, despite temporary persistence.