Livestock grazing, habitat protection and diversity of bees and wasps in the Central Monte desert.

Reserves aim mainly at preventing or mitigating human impacts on natural ecosystems. It is important to assess how well reserves meet this goal. We evaluated whether habitat protection offered by Ñacuñán Biosphere Reserve (Central Monte desert, Argentina) results in detectable changes in habitat structure and the species richness and composition of bees and wasps. We conducted pan trap sampling and flower visitor observations in six pairs of protected and unprotected sites. Our results suggest that thirty five years of cattle exclusion in Ñacuñán have had detectable effects on habitat structure. However, these changes in habitat structure translated only into partial and conflicting effects on hymenopteran richness, and did not have detectable effects on hymenopteran composition. Our study should be repeated with a greater sampling effort and throughout multiple years before our results can be applied to guide management decisions. [ABSTRACT FROM AUTHOR]