Changes in ant species composition and community organisation along grazing gradients in semi-arid rangelands of the Northern Territory

The impact of cattle grazing on ant diversity, species composition and comm unity organisation was investigated in the Victoria River District of the N orthern Territory, using distance from water as a surrogate of grazing inte nsity. Two gradients were sampled, one on red earth at Kidman Springs (5 pl ots), the other on black cracking clay at Mount Sanford (6 plots). Sites we re sampled in April (both sites) and October (Kidman Springs only)1998. The two sites supported distinctly different ant communities, with 70 species at Kidman Springs and 36 at Mount Sanford. Species richness and total abund ance did not change significantly with distance from water, although specie s richness was always greatest at the most distant plot. However, multivari ate analysis showed that ant species composition changed markedly. Despite large changes in species composition, the abundance of only six of the 24 m ost common species (two at Kidman Springs and four at Mount Sanford) were s ignificantly correlated with grazing intensity. The abundance of Bassian ta xa was promoted by grazing, whereas the species richness of Torresian taxa and the abundance and species richness of Widespread taxa were suppressed. Ant functional group analysis showed community organisation was resilient, with slight changes being primarily driven by Hot Climate Specialists and d ependent on the season. However, the relative richness of five functional g roups correlated significantly with grazing intensity, all negatively. Ant variables suppressed by grazing were positively associated with patch cover variables and litter, and negatively correlated with annual grass and bare ground, and may serve as indicators of rangeland condition.