Book: The Theory of Island Biogeography

Overview

MacArthur and Wilson's The Theory of Island Biogeography (1967) introduced a concise, influential framework linking species richness on islands to simple demographic and geographic processes. The book proposed that the number of species found on an island represents a dynamic balance between colonization from a source pool and local extinction, shaped primarily by island size and isolation. This equilibrium perspective reframed how ecologists think about spatial patterns of biodiversity and moved the field toward making testable, quantitative predictions.

Core theory

The central idea is that immigration and extinction rates determine species richness. Immigration declines as an island accumulates species because fewer new species remain to arrive; extinction rises with species number because more populations are represented, and small populations face higher risks. Two geographic factors shift these rates: proximity to the mainland or source pool increases immigration, while larger island area reduces extinction by supporting bigger, more stable populations. The point where the immigration and extinction curves intersect defines an equilibrium species number, although the identity of species present can change through ongoing turnover.

Model mechanics

MacArthur and Wilson used simple, graphical models to show how immigration and extinction curves intersect under different scenarios of island size and distance, producing clear qualitative predictions. They articulated mechanisms such as the "target area" effect, where larger islands receive more colonists, and the "rescue effect, " where frequent immigration reduces local extinctions. The model deliberately abstracts away from species-specific differences and habitat heterogeneity, treating species as functionally equivalent to reveal general patterns driven by area and isolation.

Empirical tests and examples

The theory spurred a wave of empirical work designed to test its predictions. Classic studies include observational and experimental work on mangrove islands, island recolonization after volcanic disturbances such as Krakatau, and the manipulative defaunation experiments by Simberloff and Wilson on small mangrove islands. These studies largely supported the qualitative predictions, larger and less isolated islands tend to have more species and higher colonization rates, but also revealed complexities, including habitat specificity, differential dispersal abilities, and the importance of local ecological interactions that the simple model did not encompass.

Legacy and implications

The Theory of Island Biogeography had profound impacts on ecology, evolution, and conservation. It provided a conceptual and practical basis for reserve design and landscape conservation, influencing debates about whether a single large reserve or several small reserves (SLOSS) better preserves biodiversity. The equilibrium framework also laid groundwork for later theoretical developments, including metapopulation models and neutral theory, and stimulated research that integrated spatial structure, stochasticity, and species differences. While subsequent work has refined and extended the original model, MacArthur and Wilson's insight, that spatial geometry and demographic rates jointly shape biodiversity patterns, remains a cornerstone of spatial ecology.