Urban areas represent a spatially small impact in relation to other land uses such as livestock and agriculture, but they undergo rapid changes. Such changes involve the size, shape, interconnectivity, and composition of natural patches. Habitat loss generated by urbanization affects the diversity and abundance of bees and other flower visitors in many sites. In general, the presence of urban areas represents a strict boundary to flower visitors and restricts their movement between natural and suburban habitat patches. The aim of this work is to evaluate how the flower visitor assemblage change along an urban-natural gradient in northwest Argentina. We established five areas in the Yungas ecoregion and sampled three sites with different degrees of urbanization (urban, suburban, and natural), at each area, reaching 15 sites. At each site, we sampled flower visitors during 5-min observation periods done over flowering plants. We found 197 morphospecies of flower-visiting insects along the gradient and an invariant richness, abundance, and Shannon diversity. The assemblage presented the same taxonomic group distributions in the three categories established. However, in urban sites, solitary bees and bees with soil borrowing nesting type predominate, while eusocial and cavity nesting bees were the main flower visitors in suburban sites. Our results suggest that the cities of northwestern Argentina are not a strict boundary for flower visitors; however, urbanization seems to be selecting and favoring certain flower-visitor species traits.
