Functional ecology of wild bees in cities: towards a better understanding of trait-urbanization relationships

A functional ecological understanding of urban wild bee communities is of growing importance especially in regard to biodiversity conservation, ecosystem service maintenance and effective conservation programmes. In this paper, we review and summarize the published literature aiming to inform future research investigations in the growing field of wild bee functional ecology. Specifically, we: (1) review which functional trait-based analyses have been carried out on wild bees in cities thus far; (2) summarize which wild bee species traits have been considered; (3) evaluate any consistent wild bee trait-environment relationships (i.e. urbanization) across studies; and (4) synthesize findings and limitations to inform future research recommendations. We reviewed 48 studies based on a systematic Web of Knowledge search. We found consistent trait characteristics for 'nesting type', 'diet', 'body size', 'sociality' and 'phenology' across studies. More than one third of the studies were descriptive and the majority of studies were located in urban gardens in temperate Europe and North America, calling for more research from underrepresented geographic regions and from the entire spectrum of urban habitat types. Of these studies, only five analyzed functional diversity indices and three studies applied statistics to relate urban wild bee traits to urbanization factors. Future studies should consider trait-based statistics, and could incorporate functional trait-based ecological networks to examine network shifts across urbanization gradients. Our review suggests that we lack generalizable information about wild bee trait and urbanization relationships yet, making conservation recommendations challenging. Therefore, we propose more research that considers methodological recommendations to develop a comparable and comprehensive understanding of how urbanization affects the functional ecology of urban wild bees to link with specific urban conservation measures.