Environment and Competition as Determinants of Local Geographical-Distribution of 5 Meat Ants, Iridomyrmex-Purpureus and Allied Species (Hymenoptera, Formicidae)

The distributions of five species of meat ants [Iridomyrmex purpureus S.S. (P), 'yellow species' (Y), viridiaeneus (V), 'small purple species' (SP), 'blue species' (B)] were investigated in the Gawler Ranges and Eyre Peninsula of South Australia, in relation to climate, soils and vegetation. P occurred mainly in two areas of relatively high total and/or summer rainfall that were predicted a priori, and Y was limited to sandy alkaline yellow duplex soils. V, SP and B are all found in the arid Gawler Ranges and, in this order, form a sequence of increasing penetration into areas of higher rainfall on Eyre Peninsula. It is proposed that a north-to-south gradient of increasing rainfall is accompanied by a gradient of increasing diversity (species richness) of local ant faunas. By analogy with the distribution of meat ant species across habitats differing in the diversity of their ant faunas in the Gawler Ranges, it is suggested that southern limits to the ranges of V and SP are determined by intensification of diffuse competition from increasingly diverse ant communities. Within their ranges, however, the relative frequencies of SP and B depend on simple asymmetrical competition between meat ant species. Diffuse competition at the periphery of a species' range is discussed briefly as selective pressure towards evolutionary change or stasis.