The Nature of the Taxon Cycle in the Melanesian Ant Fauna

Abstract

Ant faunas in the Moluccas -Melanesian arc are for the most part "saturated," conforming to the rule F=3aO.6, where F is the number of species in the fauna and A the area of the island in square miles. Interspecific competition plays a major role in determination of saturation curves. Other animal groups possess varying saturation curves, the significance of which is not understood. Expanding ant species now in Melanesia originated almost exclusively from tropical Asia, New Guinea and Australia. Faunal dominance, measured by degree of faunal interpenetration, is a direct function of land area and is less directly related to insular faunal size. The following "taxon cycle" is postulated. A taxon maintains its headquarters in a given land mass indefinitely, expanding and contracting . cyclically, or else it declines to extinction. The headquarters can be shifted from a larger to a smaller land mass (e.g., from New Guinea to Fiji) but not in the reverse direction. Three general attributes of success are recognized in expanding ant taxa Acquisition of a significant ecological difference, which presumably reduces interspicific competition, ability to penetrate marginal habitats, and ability to disperse across water gaps. It is suggested that attributes are causally related in the sequence given. Success in marginal habitats gives expanding species the advantage needed to encompass and progressively replace older resident taxa.