Ecology 1967

The Theory of Island Biogeography

Robert H. MacArthur & Edward O. Wilson

How many species an island holds is a running balance of arrivals and losses.

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In depth · the introduction

Why does a big island near the coast teem with life, while a tiny, far-flung one feels half-empty? In the 1960s two biologists answered with a single moving picture — and ended up writing the rulebook for saving nature.

The big idea

The number of species living on an island, MacArthur and Wilson said, is a tug-of-war between two forces. New kinds keep arriving — blown, swum or drifted in from the mainland (immigration). And kinds already there keep dying out (extinction). As the island fills up, fewer brand-new kinds are left to arrive, while crowding pushes more of the residents to vanish. The species count settles at the point where arrivals exactly balance losses.

The twist is that this balance is restless. Even after the number stops changing, the cast keeps turning over: one kind winks out, another arrives to take its place. So an island holds a roughly steady number of species, but not the same species forever — a “dynamic equilibrium.”

How it came about

Robert MacArthur was a brilliant young mathematical ecologist at Princeton; Edward O. Wilson was a Harvard expert on ants who had tramped across the islands of the South Pacific. Wilson had noticed something curious — island ant faunas seemed to hold about a fixed number of species, even as which species were present kept shifting. MacArthur supplied the mathematics, and together they published the idea in 1963 and the full book in 1967.

Then came the test. Wilson and a young student, Daniel Simberloff, picked tiny mangrove islands in the Florida Keys, tented them, and fumigated away every last insect and spider. They watched, week by week, as new creatures recolonized — and the islands climbed back to roughly their original numbers of species, but with a noticeably different mix. The bold prediction had come true.

Why it mattered

For the first time, ecology had a crisp, testable answer to a basic question: how many species can a place hold, and what sets the number? But the deeper payoff came later. As humans began carving forests and grasslands into ever-smaller fragments — islands of habitat in a sea of farms and cities — those fragments turned out to obey the very same rules. The theory became the foundation of conservation biology: it shapes how big we make nature reserves, how we connect them, and why a chopped-up landscape loses species. Wilson and MacArthur had, almost by accident, handed us a tool for the age of extinction.

A way to picture it

Think of a parking lot with a single entrance. When it's empty, cars pour in fast and almost none leave. As it fills, two things happen: arrivals slow down (few spaces left) and departures speed up (it's packed, people are leaving). Pretty soon the lot settles at a steady level of fullness — yet individual cars are still constantly coming and going. That settled fullness is the island's equilibrium number of species; the endless coming-and-going is the turnover. A bigger lot (a bigger island) settles fuller; one that's hard to reach (a far island) fills less.

Where it sits

A century earlier, Darwin's finches had made the Galápagos the proving ground of evolution. MacArthur and Wilson turned islands into the proving ground of ecology, replacing Darwin's qualitative wonder with equations for how many species a place can hold. Their idea reached far past real islands — mountaintops, lakes, even patches of city park behave like islands of habitat — and it underpins today's struggle against extinction as wild land shatters into smaller and smaller pieces. In the Library it sits downstream of Darwin (1859) and alongside the population thinking of Hamilton (1964).

The original document

Original source text

R. H. MacArthur & E. O. Wilson · The Theory of Island Biogeography (Princeton Univ. Press, 1967); first stated in Evolution 17 (1963): 373–387

Islands are everywhere

Insularity is moreover a universal feature of biogeography.

Before the model, the authors widen the meaning of “island”: the principles seen on remote archipelagos, they argue, govern any patch of habitat cut off from its surroundings.

…the insular nature of streams, caves, gallery forest, tide pools, taiga as it breaks up in tundra, and tundra as it breaks up in taiga.

A balance of immigration and extinction

The core argument: as species accumulate on an island, the rate at which new species arrive falls — the readiest colonists come first and the pool of newcomers shrinks — while the rate at which resident species die out rises, as populations crowd and shrink. Where the falling immigration curve meets the rising extinction curve, the number of species reaches a dynamic equilibrium. It is dynamic because, at that balance, species keep arriving and going extinct: the count holds steady while the cast keeps changing — the species turnover that was the theory's boldest, most testable claim.

Area and distance

Two levers move the curves. A larger island lowers the extinction curve (bigger populations resist dying out), so it holds more species; an island nearer a source raises the immigration curve, so it too holds more. From this the long-known species–area relationship, and the decline of richness with isolation, follow as consequences of one mechanism rather than separate facts.

A warning that became a science

…the same principles apply, and will apply to an accelerating extent in the future, to formerly continuous natural habitats now being broken up by the encroachment of civilization.

[ … ]

The book then formalizes the rates, the colonization curves, the species–area exponent, and stepping-stone dispersal between islands. The full development is available at the source below.

Princeton University Press · 1967