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

Abstract

The impact of cattle grazing on ant diversity, species composition and community organisation was investigated in the Victoria River District of the Northern Territory, using distance from water as a surrogate of grazing intensity. Two gradients were sampled, one on red earth at Kidman Springs (5 plots), the other on black cracking clay at Mount Sanford (6 plots). Sites were 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 abundance did not change significantly with distance from water, although species richness was always greatest at the most distant plot. However, multivariate analysis showed that ant species composition changed markedly. Despite large changes in species composition, the abundance of only six of the 24 most common species (two at Kidman Springs and four at Mount Sanford) were significantly correlated with grazing intensity. The abundance of Bassian taxa 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 dependent on the season. However, the relative richness of five functional groups 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. Key words: biodiversity, cattle grazing, indicators, piosphere, rangeland management, semi-arid, tropical savannas