Pollinators aid in reproduction of 35% of crops, an ecosystem service valued at $29 billion. Honeybees alone provided crop pollination services estimated at $14.6 billion in 2000. Recent declines in honeybees have increased interest in services provided by native bees. The status of many native bee populations has not been assessed. Of those that have, more than 50% are declining, due in part to anthropogenic habitat alteration. Urban areas can provide refuges for bees, but many do not have adequate green spaces. Rooftop gardens might provide additional habitat to help stabilize bee populations. Yet, use of rooftop gardens by bees is understudied in the southwestern United States, a region of high bee biodiversity. We compared native bee and honeybee visitation to a rooftop garden and nearby ground-level garden. We recorded floral visitors during weekly visual encounter transects from April-October 2017. Individuals on flowers were recorded as native bees or honeybees, plant species visited was also noted. Because the transect varied in size among sites (roof was 37.7 m; ground was 286.8 m), we standardized the observations to bees/m for statistical analyses using paired t-tests with Bonferroni corrections.
Results/Conclusions
We recorded 1,503 observations: 846 native bees and 657 honeybees. Preliminary data analysis showed no significant differences in native bee visits to the rooftop (0.055±0.065 bees/m) and the ground-level gardens (0.064±0.054 bees/m; t(193)=1.110, p=0.13). Similarly, there were no significant differences in honeybee visits to rooftop (0.073±0.081 bees/m) and ground-level gardens (0.068±0.056 bees/m; t(177)= -0.433, p=0.33). There were no significant differences between the native bee and honeybee observations at either site. This indicates that rooftop gardens could serve a role in increasing available habitat for both native bees and honeybees. Rooftop gardens are still relatively rare in the arid southwestern U.S., yet could be a mechanism to protect the high biodiversity of native bees in this region. The visual encounter survey methods of this study did not allow us to address the potential of differential use of the rooftop garden among native bee species. For example, bee food specialists might require careful design of rooftop gardens, while generalists might readily use any garden with flowering plants. This year we are collecting bees at each location for species identification. The increased taxonomic resolution will aid our understanding of how the diverse bee community uses urban gardens and help to inform conservation actions.