Review Article

The ecology of ants (Hymenoptera: Formicidae) on islands

Morrison, L.W.

Abstract: Ants are nearly ubiquitous on islands. Species may reach oceanic islands by three mechanisms: (1) mating flights of reproductives, (2) rafting, and (3) human-assisted dispersal. Past land bridge connections may be important for continental shelf islands. Patterns of species accumulation may be similar even when primary mechanisms of dispersal differ. Colonizing propagules are limited primarily by the presence of suitable habitat and interspecific competition or predation from other ants. Although island area is usually a significant predictor of species richness, the diversity of habitat types is often a better predictor. Distance, or isolation, has often been found to be only a weak predictor of ant species richness, probably because many islands that have been studied are not very distant relative to most species' dispersal abilities. Interspecific competition appears to result in mutually exclusive distributions within islands for some of the more aggressive species, although the effects of habitat affinities have not been well studied. With the exception of very small islands, competition does not appear to result in exclusions of species from entire islands, and robust patterns of nestedness have been observed. Populations go extinct from and immigrate to small islands in ecological time, although such turnover appears to be relatively low compared with other arthropods. Only a few archipelagoes, however, have been the focus of turnover studies. Some species may reach extremely high population abundances on islands but subsequently decline. The exact mechanisms underlying these population fluctuations are not known with certainty, although the presence of honeydew-producing insects has often been associated with the most dramatic cases. Endemicity varies greatly and ranges from 0 to > 96%, peaking at intermediate distances in the Pacific and reaching the highest levels on islands in the Indian Ocean. Anthropogenic disturbances appear to present the greatest driver of change to insular ant communities and result in increasing numbers and abundances of exotic ant species, which may have deleterious effects on the rest of the community.