The Effects of Land Use on the Structure of Ground‐Foraging Ant Communities in the Argentine Chaco

We examined the responses of a ground‐foraging ant community to a gradient of land‐use intensity in a grazing agroecosystem in the Chaco of northern Argentina. The gradient extended from a highly degraded condition characteristic of traditional grazing practices, through an area of less severe disturbance where grazing was less concentrated, to two areas in which grazing had been managed for 3 and 18 yr, respectively. Ground cover changed along this gradient from bare to litter‐covered, ground‐layer vegetation changed from sparse to a structurally complex mixture of grasses and forbs, and canopy cover increased in areas of intermediate grazing intensity and then decreased. Community diversity varied among the sites depending on both season and scale of analysis. Site‐scale ant species richness was slightly higher in sites of intermediate disturbance in the summer‐wet season but was much greater in the least disturbed site in the winter‐dry season. The same dry‐season pattern was evident in both species richness and diversity at the scale of transects within sites, whereas species richness at the scale of individual traps within transects was significantly lower at sites of intermediate disturbance than at either highly restored or highly degraded sites. Abundances of individual ant species and functional groups also changed along the land‐use gradient. Litter‐inhabiting cryptic species and specialized predators responded positively to grazing management, whereas opportunists and the hot‐climate specialist Forelius nigriventris were prevalent in highly disturbed areas. Other functional groups exhibited redundancy and species turnover along the gradient. Detrended correspondence analysis (DCA) revealed that the ant faunas at the extremes of the land‐use gradient were more similar than expected. We hypothesize that the interaction of local‐scale habitat features with historical and biogeographic influences may determine the responses of this ant community to land use, and that highly degraded areas may have conservation value because they are regional sources of arid‐adapted ants.