Single Large or Several Small (Sloss) Debate

Equilibrium Theory of Island Biogeography

In the 1960s, Robert MacArthur of Princeton University and Edward O. Wilson of Harvard University proposed a new explanation for the distribution of animal and plant species on islands: the equilibrium theory of island biogeography. They envisioned the biota of an island to be a dynamic rather than static entity, with species new to the island arriving occasionally (“immigration”), while ever so often the population of some species on the island would dwindle to the point that the species disappeared (“extinction”). They asserted further that the immigration rate would be a declining function of the number of species present on the island (because the more species that are already on an island, the fewer there would remain in the vicinity that could possibly immigrate), while the extinction rate would be an increasing function of the number of species already present (because the more the species that are present on the island, the smaller the population sizes of each would be, on average, and therefore the more at risk of extinction each would be). A rising extinction curve would cross a falling immigration curve at a particular point, which would constitute a dynamic equilibrium number of species (Figure 1). A further consequence predicted by this model was that, other things being equal, a large island would have a larger equilibrium number of species than a small island because the extinction curve would be lower. This would follow from the fact that if two islands had the same number of species, the larger one would have larger population sizes, on average, of each species, so extinctions would happen less frequently. Another prediction of the model is that, other things being equal, the more isolated an island is, the fewer the species that would exist at equilibrium (because immigration rates would be lower).

The equilibrium theory struck a highly responsive chord among biogeographers and ecologists, who quickly began seeking data on island biological communities, or parts of communities (such as all the bird species, or all the mammals, or all the plants), to attempt to match them to the predictions of the theory. Especially common were studies showing that, within single archipelagoes, larger islands tended to have [Page 2545]more species than smaller ones, just as the theory predicted.

Figure 1 The dynamic equilibrium model of biogeography

Source: Adapted from MacArthur, R. H., & Wilson, E. O. (1967). The theory of island biogeography. Princeton, NJ: Princeton University Press

Note: P is the number of species in the entire regional species pool, and S is the equilibrium number of species attained when the extinction rate E equals the immigration rate I.

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