Inter-annual bee community response to urbanization

Cities are complex landscapes capable of perpetuating multi-taxa declines through a variety of mechanisms operating synergistically at local and landscape scales. While bee species response to urbanization varies, urban communities may become functionally homogenous as disadvantaged traits are displaced from the landscape. Due to the ecological and economic necessity of bees, understanding what bees exist within city spaces and how they respond to increasing urban and climatic changes over time has conservation implications. Examining plant-bee interactions can further elucidate bee resource use through time and space and has utility in informing local-scale floral enhancements in urban green spaces. To investigate bee community and plant interaction response to urban change and climate over time, we surveyed green spaces across an urbanization gradient from May-September during 2021-2023 in Toronto, Ontario and compiled climate data. Over the three-year period, we sampled 19,000 bees representing 194 species and documented 13,134 plant-bee interactions. Overall bee abundance and richness did not differ by urban intensity or year; however, ordination analysis revealed clustering indicating potential urban influence in altering community compositions. Ground-nesting, generalist, and native species were most represented across each urban intensity. Plant-bee interaction network metrics likewise did not differ interannually or by urban intensity. Instead, metrics such as weighted nestedness varied significantly throughout the season likely due to differences in resource availability. As a result, plant genera such as Cornus and Solidago can be important early and late season flora recommendations for bees. These results demonstrate that some cities can support diverse bee communities.