Independent Discovery of the Equilibrium Theory of Island Biogeography

Abstract

A correlation of this kind [between number of species and logarithm of area of island] is as interesting as it is unexpected, for it suggests the existence of an equilibrium valuefor the number of species in a given island, a value which acts as a limit to the size of the fauna. The processes which determine the equilibrium value for an island of given size must be, on the one hand, the extinction of species, and, on the other hand, the formation of new species within the island, and the immigration of new species from outside it. The above quotation does not come from one of MacArthur and Wilson's (1963, 1967) two seminal publications on the equilibrium theory of island biogeography. It was written 15 yr earlier by another author. It appears on page 117 of Eugene G. Munroe's (1948) doctoral thesis on the distribution of butterflies in the West Indies. The earlier and independent discovery of the equilibrium theory by Munroe is more than one of those amusing little incidents in the history of science. It warrants further examination for two reasons. First, unlike some purported cases of prior discovery of important ideas, Munroe did not have just some vague, poorly articulated notion of the species equilibrium. He clearly presented the empirical species-area relationship that stimulated his inductive discovery, investigated the generality of this pattern, and developed detailed verbal and mathematical models to explain it (Munroe 1953). Munroe's concept of the species equilibrium was identical in all important respects to MacArthur and Wilson's. Second, given the striking similarity of the two models, it is worthwhile to ask why Munroe's discovery went unrecognized (but see Gilbert 1984) while MacArthur and Wilson's has been hailed as one of the successes of the evolutionary ecology of the 1960s.