The bees' knees: A review and meta-analysis of urban green-space habitats for insect pollinators

As insect diversity declines worldwide, conservationists and land managers are increasingly looking to take advantage of novel green spaces for insect conservation in urban areas. However, to establish effective habitat for conservation, we must first have a robust understanding of the features of urban green spaces that provide resources to support a diverse community of insects. Here, we present a systematic review and meta-analysis of the literature to evaluate the relationships between bee, beetle and butterfly diversity (species richness and abundance) and habitat features (such as the abundance and species richness of vegetation and flowers) in urban green spaces. We identify clear relationships between total insect diversity and green area; vegetation height, abundance and species richness; and floral abundance and species richness. Bees are the most studied taxon of the three investigated here and show a clear positive relationship with vegetation species richness and the species richness and abundance of flowers. We highlight how uncertainty in the relationship between these variables and individual insect orders is driven by differing resource requirements across life stages and functional groups. This is particularly apparent with respect to beetles and butterflies, which were also the more data-deficient taxa investigated. Overall, we suggest that land managers and conservationists could build large areas of habitat in cities through the revegetation of many small spaces that are sufficiently close to be ecologically connected, with a focus on providing abundant and varied floral resources.