Diversity and composition of seed-carrying ant assemblages in the Monte desert: a regional assessment of the effects of grazing

Abstract

The responses of ant communities to human-induced changes depend on the type of disturbance and the characteristics of the environment which they inhabit. In the Monte desert, cattle grazing reduces seed abundance, which may affect seed-carrying ant species. We evaluated the effects of cattle grazing on the composition and diversity of seed-carrying ant assemblages at sites with contrasting grazing pressure in four localities between 26–39°S; at each site we also measured grazing intensity and seed abundance. We identified a total of 44 ant species. In each locality there was a marked difference in grazing intensity between grazed sites and sites with low or no grazing, and seed abundance was higher in the latter, but we did not find differences in the diversity of the ant assemblages between grazing conditions; only the species richness in one of the four localities showed differences, although with higher values in the grazed sites. At each locality, species turnover among sites (beta diversity) was less important than the local (alpha) diversity. At a regional scale, species composition varied among localities but not between sites with contrasting grazing pressure, and species turnover was explained by differences among localities and not between grazing conditions, indicating that these assemblages were structured geographically and not by livestock activity at the sites. Therefore, the structure of these assemblages was determined to a greater extent by environmental conditions at each locality than by the effects of cattle grazing, although a species-specific effect cannot be discarded. Implications for insect conservation. Both the high diversity of ant species involved in granivory, at a local and a regional scale, and the differences in the composition of ant assemblages among localities reinforce the importance of considering the entire extension of the Monte desert when implementing biodiversity inventories and monitoring schemes.