Background/Question/Methods
Decades of overgrazing have left many rangelands in northwestern Mexico in poor condition. To improve productivity rangeland is cleared and seeded with exotic buffelgrass (Pennisetum ciliare L.). Nearly 10% (18,000 km²) of the state of Sonora, Mexico’s second largest state, has been cleared and seeded with buffelgrass, yet there have been few studies of the effects that this practice has on Sonoran ecosystems. Our objective was to determine how the conversion of native rangeland to buffelgrass pastures affects native ant assemblages. Ants are one of the most abundant and diverse taxa in central Sonora and play important roles in many ecosystem processes in this subtropical, semi-arid environment. The abundance, diversity, and composition of ant assemblages on rangeland cleared and sowed with buffelgrass were compared to those of uncleared native rangeland along a precipitation gradient in central Sonora. The ant assemblages of paired study sites (native rangeland vs. buffelgrass pastures) were sampled with pitfall traps during the monsoon season. Data were analyzed based on species occurrences in pitfall traps.

Results/Conclusions
Abundance and species richness were greater in native rangeland than in buffelgrass pastures across the precipitation gradient. The difference in species richness was greater in sites with higher precipitation. Composition also differed significantly both between native rangeland and buffelgrass pastures, and high and low precipitation sites. Certain xerophilic species were favored by buffelgrass pastures, while other specialist species were completely absent from this habitat. Rank-abundance curves revealed that ant assemblages of native rangeland exhibited significantly greater evenness than those of buffelgrass pastures. The most abundant species in buffelgrass pastures achieve a significantly greater relative abundance than the most abundant species in native rangeland. These results show that Sonoran ant assemblages are significantly affected by the conversion of native rangeland to buffelgrass pastures. In particular, attine ants, which play important roles in ecosystem functioning, are highly sensitive to land conversion, while other hot-climate specialists are favored by conditions in buffelgrass pastures. Continued land conversion will likely further reduce the diversity and evenness of Sonoran ant assemblages and threatens several native ant species with local extinction.