Equilibrium Theory of Island Biogeography and Ecology

Oceanic islands and archipelagoes are intrinsically important to biologists; 5% of the land surface of the earth is insular, and if South America, which has been an island throughout most of its existence, is included the figure rises to 19%. Significant portions of the evolutionary histories of many economically and biologically important species occurred on oceanic islands, and if the earth were not liberally sprinkled with isolated bits of land in addition to the "world continent," its biota would be much poorer. But the intrinsic importance of islands, scientific or economic, has not inspired the intense research in island biogeography which justifies this review of recent advances. Rather it is the realization that oceanic islands are paradigms for geographic entities ranging in size from tiny habitat patches (52, 53) to continents (86, 92, 112) or even the entire earth (74). It is almost a platitude that Darwin’s observations in the Galapagos Archipelago and Wallace’s in the Malay Archipelago crystallized the then nascent concept of organic evolution by natural selection (13, 110), and many other classical evolutionary advances rest originally on insular observations. Because islands are so clearly isolated from other land masses, island population data contributed heavily to the realization that most speciation is allopatric (54). Wallace’s Malaysian observations allowed strong inferences about changing sea levels, past land connections, and the position of a line separating two great biogeographic provinces (110). Insular isolation is important ecologically because it allows us to be virtually certain that an organism encountered on an island is a true nesiote. Consequently, problems in Community structure and function, such as the distribution of individuals into species or the trophic relationships among populations, are more readily attacked in an island setting; any organism found there is assuredly a member of the biotic community.