AN EQUILIBRIUM THEORY
OF INSULAR ZOOGEOGRAPHY
ROBERT H. MACARTHUR1 AND EDWARD O. WILSON2
1 Division of Biology, University of Pennsylvania, Philadelphia, Pennsylvania.
2 Biological Laboratories, Harvard University, Cambridge, Massachusetts
Evolution, Vol. 17, No. 4. (Dec., 1963), pp. 373-387
SUMMARY
A graphical equilibrium model, balancing immigration and extinction rates of species, has been developed which appears fully consistent with the fauna-area curves and the distance effect seen in land and freshwater bird faunas of the Indo-Australian islands. The establishment of the equilibrium condition allows the development of a more precise zoogeographic theory than hitherto possible theory than hitherto possible. One new and non-obvious prediction can be made from the model which is immediately verifiable from existing data, that the number of species increases with area more rapidly on far islands than on near ones. Similarly, the number of species on large islands decreases with distance faster than does the number of species on small islands.
As groups of islands pass from the unsaturated to saturated conditions, the variance-to-mean ratio should change from unity to about one-half. When the faunal buildup reaches 90% of the equilibrium number, the extinction rate in species/year should equal 2.303 times the variance divided by the time (in years) required to reach the 90% level. The implications of this relation are discussed with reference to the Krakatau faunas, where the buildup rate is known
A "radiation zone," in which the rate of intra-archipelagic exchange of autochthonous species approaches or exceeds extraarchipelagic immigration toward the outer limits of the taxon's range, is predicted as still another consequence of the equilibrium condition. This condition seems to be fulfilled by conventional information but cannot be rigorously tested with the existing data.
Where faunas are at or near equilibrium, it should be possible to devise indirect estimates of the actual immigration and extinction rates, as well as of the times required to reach equilibrium. It should also be possible to estimate the mean dispersal distance of propagules overseas from the zoogeographic data. Mathematical models have been constructed to these ends and certain applications suggested. The main purpose of the paper is to express the criteria and implications of the equilibrium condition, without extending them for the present beyond the IndoAustralian bird faunas.
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