Floral-rich habitat restorations are increasingly used to restore and enhance wild pollinator communities. A number of studies have found that these habitat plantings increase overall bee abundance; however, it may be that this increase only occurs in the more common species. Relatively few studies have specifically addressed the benefit to rare species. The objective of this study was to examine whether pollinator habitat restorations benefit rare and/or common species. A key challenge to studying rarity is categorizing which species are rare, especially for understudied and hyper-diverse taxa such as insect pollinators. Sampling designs bias which species are collected and thus categorized as rare; therefore, species rarity should be determined using large independent surveys. We used a large-scale dataset of 30,135 specimens and 443 bee species specimens collected throughout the Mid-Atlantic region to grouped species into quartiles based on regional species frequencies. We then collected wild, native bees from 22 pollinator restorations, each paired with a nearby old-field control. Sites were distributed across New Jersey and eastern Pennsylvania, USA and sampled over five years. We tested for differences in mean abundance of either rare (lower quartile) or common (upper quartile) species between habitat plantings and old fields using a paired t-test.
From the restoration field study, we collected 59 species (892 specimens) belonging to the rarest frequency quartile in the independent data set, and 80 species (11,400 specimens) from the most common frequency quartile. Paired t-tests indicated that rare species were not more abundant in habitat plantings then old fields (mean difference: 0.88 ± 6.5; 95% CI). In contrast, common bees were more abundant in habitat plantings (mean difference: 110 ± 54; 95% CI). Further, effect sizes were much larger for common species compared to rare species (Cohen’s d: common species = 0.87, rare species = 0.06). These results suggest that habitat restorations as currently implemented may not provide adequate benefit to rare species. If the mechanisms underlying rarity are idiosyncratic across species, restoration practices might not be able to target all rare taxa. For example, some rare bees are specialists on particular plants for pollen and restorations that lack those plants will not benefit these bee species. A better understanding of what drives rarity of particular species, followed by targeted management practices could provide greater benefit for rare species.