Timing is critical for the mutualistic relationship between pollinators and angiosperms, and temperature can be an important indicator of season for both. Early-emerging native pollinators, such as solitary cavity-nesting bees, depend on early spring floral resources for both sustenance and reproduction. We used the Urban Heat Island Effect (UHI) to examine how temperatures influence both groups’ phenologies. The UHI can model the rise in temperature we expect in the next few decades. We conducted this study around Louisville, Kentucky, USA, which has one of the strongest UHIs in the United States, with average temperatures 3-5°C higher than surrounding natural areas, encompassing the 1.5-2 degrees Celsius rise in temperature predicted between 2030 and 2052. We set out clear-sided trap nests for solitary bees and monitored them for reproductive activity in 15 sites across an urban-rural gradient. Each nest box contained 10 cavities and was visited weekly in Spring, 2022. For floral surveys, we established four transects in the cardinal directions radiating from each trap nest. We used 1 meter quadrats along these transects to assess the spring ephemeral floral resources available to pollinators. Solitary bees in urban habitats tended to start nesting earlier than bees in suburban or rural habitats and showed less variation in start date; suburban bees showed the greatest variation in start date. Rates of brood cell construction were relatively steady and similar in rural sites, but varied within urban and suburban sites. Similarly, the first date of flowering was highly variable in suburban sites; however, it showed little variation in either rural or urban sites. This study suggests that the urban heat island effect has major implications for these species’ phenology. Not only is this result important in understanding how climate change will affect the persistence of species in the future, but urban ecosystems are an important system to study in their own right; they are the most rapidly expanding ecosystem on the planet. Understanding which species are more vulnerable to extinction or extirpation in urban settings is important to forming future conservation plans.
