Indigenous versus exotic understory plantings: Contrasting impacts on urban bee diversity

Urban environments pose a threat to biodiversity through processes such as habitat degradation and biotic homogenization. Despite this, cities are increasingly recognized for their potential to conserve bees and other pollinators. Planting understory vegetation is one way of providing more floral resources to support urban bee communities and the ecosystem services they provide. However, the influence of vegetation origin and landscape context on urban bee communities is unclear, particularly in the Southern Hemisphere. We sampled the bee communities at 32 understory plantings dominated by exotic or indigenous (native to the local bioregion) vegetation around inner Melbourne, Australia. For each site, we recorded the amount of impervious surface and irrigated turf in 200-m buffers. Indigenous plantings were found to promote significantly greater alpha and beta diversity in bee communities compared to exotic plantings. Particular plant taxa were highly effective at attracting a variety of bees, with a maximum of 19 bee species (including specialists) hosted by indigenous Wahlenbergia capillaris (Campanulaceae). Apis mellifera was highly dominant and strongly associated with exotic plantings, whereas many indigenous bee species were positively associated with indigenous plantings. This study shows indigenous understory plants have a positive influence on indigenous bee communities relative to exotic plantings which tend to attract only A. mellifera. Planting indigenous plants in cities is therefore recommended as a conservation action for local bee species.