Annual variation across functional traits: The effects of precipitation and land use on four wild bee species

Abstract

Understanding the impacts of urbanization and climate change on organisms has become increasingly critical in ecology and conservation as these anthropogenic stressors negatively impact wildlife biodiversity, especially pollinators such as bees. We analysed the demographic (abundance and sex ratio) and morphological (body size and wing wear) responses to urbanization and inter-annual variation of four common wild bee species across an urban gradient in Toronto, Canada. We observed more significant shifts in bee demography with inter-annual precipitation variation than with urbanization, with diverse patterns depending on species. The drier active season saw a decrease in abundance for Agapostemon virescens and Ceratina calcarata, whereas Bombus impatiens and Xenoglossa pruinosa increased when compared with the previous year. Wetter active seasons resulted in smaller body sizes and greater wing wear for all bee species examined. For larger bees (A. virescens, B. impatiens and X. pruinosa), increasing urbanization resulted in significantly larger females only for A. virescens, whereas foraging effort reduced as urban intensity increased. The small, cavity-nesting bee, C. calcarata exhibited reduced body sizes and increased foraging effort with increasing urbanization. Moderate urbanization better supported most wild bee assemblages and morphology, suggesting that moderate land use intensity provides green spaces and adequate resources for these bee species.