Roles of scale, matrix, and native habitat in supporting a diverse suburban pollinator assemblage.

Wild pollinators provide important services to both wild and human-dominated ecosystems, yet this group may be threatened by widespread anthropogenic landscape change. Suburban sprawl is one of the fastest growing types of land use change in North America, and it has certain characteristics, such as abundant floral resources, that may be beneficial for many pollinators. We examined the effects of sprawl on the wild bee assemblage of the shortgrass steppe on the Front Range of Colorado, USA. Diversity, abundance, and community composition of bees in remnant grassland fragments surrounded by suburban residential land use were compared with those in extensive, continuous grassland. No overall effect of suburbanization on bee abundance was observed, and abundance was extremely variable even within study sites. Bee species richness was positively but nonlinearly related to grassland habitat area. Bee species density was higher and more variable in suburban sites. Suburban sites and smaller habitat area were both related to relative increases in the proportions of small bee species, social bees, and solitary cavity-nesting bees in the assemblage; small suburban habitat areas also favored species of the family Halictidae over Apidae, and individuals of the genus Halictus over those of the genus Lasioglossum. In this landscape, large native habitat areas surrounded by suburban sprawl may actually increase species richness and species density over that of continuous grassland, probably by means of habitat complementation or supplementation between grassland remnants and the surrounding suburban matrix. However, at habitat areas < 20 ha, species richness became quite variable; sites < 8 ha contained less than half the species of controls.