The Nature of the Taxon Cycle in the Melanesian Ant Fauna

Abstract

Undisturbed ant faunas of islands in the Moluccas-Melanesian arc are for the most part "saturated," that is, approach a size that is correlated closely with the landmass of the island but only weakly with its geographic location (figure 1). In the Ponerinae and Cerapachyinae combined the saturation level can be expressed approximately as F=3A0.6, where F is the number of species in the fauna and A the area of the island in square miles. Interspecific competition, involving some degree of colonial warfare, plays a major role in the determination of the saturation curve. It deploys the distribution of some ant species into mosaic patterns and increases the diversification of local faunas. Perhaps because of the complex nature of the Melanesian fauna, differences between local faunas appear that give the subjective impression of randomness. Despite the action of species exclusion, the size of local faunas occurring within a set sample area increases with the total size of the island (figure 2). Water gaps break populations and initiate speciation in Melanesia. Endemic insular faunas build up primarily by the process of multiple invasion. Expanding species now on Melanesia originated almost exclusively from tropical Asia, New Guinea, and Australia. Faunal dominance, measured by the degree of faunal interpenetration, is a direct function of land area and is less directly related to insular faunal size (figure 4). Taxa originating in Melanesia exclusive of New Guinea are almost all confined to the archipelagoes of their birth. 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 (for example, from New Guinea to Fiji) but not in the reverse direction (figure 9). Taxa originating in Melanesia exclusive of New Guinea are almost all confined to the archipelagoes of their birth. On New Guinea, expanding species occur primarily in marginal habitats. In the inner rain forest habitats they are replaced by the large endemic faunas. On archipelagoes with small endemic faunas, the expanding species are ecologically "released," becoming abundant in the inner forest habitats and otherwise increasing their ecological amplitude. As a group they are characterized by great diversification among themselves. No genus among those studied has produced more than three Asia-based or four New Guineabased Stage-I species (figure 8). No genus studied, including the largest and most successful, has a total of more than seven Stage-I species on New Guinea. Sympatric Stage-I species in the same genus tend to be ecologically and morphologically very dissimilar. The following rule is predicted: the ecological amplitude of individual species, both expanding and endemic, should be negatively correlated with the size of the local fauna to which they belong and hence the size of the island on which they occur. The prediction is based on the phenomenon of ecological release of Stage-I species and appears to be supported by some fragmentary evidence relating to Fijian endemic species. Expanding species evidently play a major role in the fragmentation and speciation of older taxa. By dominating the faunas of smaller islands they maintain hiatuses in the ranges of the disjunct taxa. By saturating the marginal habitats they restrict the older taxa to the inner rain forest. Autochthonous and retreating taxa show certain common biological characteristics coadaptive with restriction to the inner forest habitats. These involve nest site preference, colony size, and foraging behavior (table 5). Three general attributes of success are recognized in the expanding Melanesian ant taxa: the acquisition of a significant ecological difference, which presumably reduces interspecific competition, the ability to penetrate the marginal habitats, and the ability to disperse across water gaps. it is suggested that the attributes are causally related in the sequence given. Success in the marginal habitats gives expanding species the advantage needed to encompass and progressively replace older resident taxa.