Bee, wasp, and flower fly diversity of green roofs and urban meadows in New York City

Background/Question/Methods

Urbanization is a leading cause of native species loss. However, properly designed urban landscape can provide habitat for native species. Urban meadows and green roofs provide many ecosystem services, but their habitat value has not been well studied. Here, we compare the bee (Hymenoptera: Apoidea), flower fly (Diptera: Syrphidae), and wasp (Hymenoptera) assemblages of green roofs and urban meadows in New York City to determine whether the habitat value of these sites are comparable. Furthermore, we investigate whether insect diversity varies among replicate roofs and whether certain plant assemblages and media depth affect diversity on these roofs.  Ten replicate green roofs nine urban meadows spanning the five boroughs of the city were sampled.

Results/Conclusions

Bees and wasps were significantly more abundant in meadows than on roofs, but flower flies were significantly more abundant on roofs. Meanwhile, wasps were proportionately less abundant on roofs than in meadows relative to the other insect groups. Additionally, bee and flower fly assemblages were not compositionally distinct between roofs and meadows; however, wasp assemblages were clearly distinct between the two habitat types.

The abundance of flower flies on green roofs may be attributed to a preference for edge habitat, abundant larval food sources (e.g., aphids), or decreased predation and parasitism. The differences in wasp assemblages between the habitats may arise due to the disproportionate impact of habitat degradation on higher trophic levels. Overall, these results suggest that the habitat value of green roofs and urban meadows varies for each taxon, but, with appropriate measures, these areas can be designed to provide important habitat for beneficial insects within the urban matrix.