The distribution of ants on the Wessel and English Company Islands, in the seasonal tropics of Australia’s Northern Territory

Abstract

A total of 74 ant species (from 23 genera) was recorded from 195 quadrats (50 × 50 m) from 39 continental islands off Arnhem Land, Northern Territory. In general, the ant fauna comprised generalist species widespread on the north Australian mainland. The functional group composition was also comparable to that of similar environments on the north Australian mainland. The ant fauna was not tightly structured. There were few habitat specialists, no species showed a clear preference for smaller islands, and only a few species showed unequivocal preferences for larger islands. There were no clear cases of congeneric, or otherwise ecologically similar, species replacing each other on different islands. In contrast to the north Australian mainland, there were no significant differences between habitats in ant species richness. However, the functional group composition varied significantly between the eight main habitats sampled across the islands, in a manner consistent with that reported for the mainland.

The number of ant species recorded per island was most closely related to island size (80% of the deviance explained), but there was only slight or no relationship between island size and the number of species at the quadrat scale. Functional group composition varied between islands, with small islands supporting a relatively high proportion of Generalised Myrmicinae species. Low-lying (and presumably intermittently inundated) islands supported a higher proportion of Dominant Dolichoderinae and few Specialist Predators and Tropical Climate Specialist species. Very small islands supported a relatively high proportion of Dominant Dolichoderinae species. These differences are largely attributable to inter-island differences in habitat availability and disturbance regimes, and to differences between functional groups in dispersability, competitive ability and ecological flexibility. Species richness was little influenced by the extent of island isolation.

Patterning in the ant fauna of these islands parallels that reported for islands elsewhere.