Urbanization has proven to have negative consequences for several species, due to drivers such as habitat fragmentation, noise and light pollution, increased temperatures and more. However, there are groups of species that perform better in urban areas, including racoons, raptors, and recently added to this list are solitary cavity-nesting bees. Several recent studies have found that these bees have higher abundances and richness values in urban areas, but little work has been carried out to understand the mechanism driving their increase. Our study set out to investigate how landscape factors, such as land usage, and local factors, such as the floral community, impact solitary cavity nesting bees. We hypothesized that floristic values, such as floral density, richness and community structure would be an important determinate in reproductive output, as has been shown in multiple previous studies. Additionally, we hypothesized that rates of nest establishment and survivability of offspring would increase with urbanization, possibly explaining the increased population sizes found in previous studies. To study this, we set out trap nests from March to December in 19 grassland restorations around Louisville, KY ranging from rural to urban in both 2018 and 2019. Additionally, bi-weekly floral surveys were conducted, and finally land use (e.g., % impervious surface, % lawn, % forest) was characterized using GIS tools.
Results/Conclusions
We found a significant increase in larval survivability along an urbanization gradient. While there was no change in fecundity along this gradient, the eggs that were laid had a greater chance of surviving to wintertime in urban areas (p<0.001). Landscape level variables such as restoration size and surrounding habitat complexity did not have a significant influence on any measures of solitary bee health. Floral richness, diversity and density had no impact on any measures of bee health, instead the most important floristic value was community identity. Solitary bee fecundity significantly increased with the percentage of native flowers in the community (p<0.05). The total number of individuals surviving to winter, was therefor a result of both landscape and local factors, with urban restorations with predominantly native plants having the highest values (p<0.05). These results highlight the importance of small-scale grassland restorations in urban areas and begin to get at the mechanism for why solitary cavity-nesting bees perform better in urban areas.