Can pollinator habitat plantings restore both biodiversity and ecosystem services?
Federally funded pollinator habitat plantings have increased dramatically in the United States. One objective of restorations is to simply increase pollinator abundance and overall species richness. Another more specific goal is to benefit native bees that provide pollination services to crops. A third objective is to conserve native pollinator biodiversity with a focus on rare species. Whether pollinator plantings as currently implemented meet these objectives has rarely been tested. We conducted a 3-year field study in New Jersey and eastern Pennsylvania in which we compared pollinator communities (aggregate bee abundance and richness) between 15 federally funded restoration plantings and paired old-field controls. We then used our database of 11,800 specimens of 152 species collected from 5 crops to determine which species are the most important ecosystem service-providing bees (herein ES bees). We used a second, independent database of 16,404 specimens of 250 species of bees not collected from crops to determine which species are regionally rare. We tested whether pollinator restorations had greater (1) abundance, (2) species richness, (3) abundance of ES species, and (4) abundance of rare species, as compared to old-field controls.
In the restoration plots and old fields combined, we collected 5064 specimens of 136 species. Compared to old fields, restoration plots had greater aggregate abundance (P <0.001) and richness (P <0.001). Using the regional crop pollination database, we found that twelve bee species (8% of bee species collected visiting crop flowers over an 8 year period) accounted for 62% of all visits to five crop plants; we defined these 12 as ES bees. In the restoration plots, ES bees were three times more abundant compared to old fields (P <0.001). Using the regional non-crop bee database, we found 113 species (approximately a third of all bee species collected in the study region from 1970-2013, accounting for only 2% of the total specimens) were collected in only one or two sites regionally; we defined these as rare species. There was a trend towards higher abundance of rare species in restoration plots than in old fields (P = 0.09). These results suggest that pollinator plantings benefit ES bees as well as native bee richness and abundance of rare bees. Overall our study indicates that pollinator restorations as they are currently implemented have value for biodiversity conservation broadly defined, as well as provisioning of ecosystem services.