The effects of tallgrass prairie restoration on native bee communities
Bees are arguably the most important pollinators on a global scale. Though bees are represented by high taxonomic diversity, with 20,000 species documented worldwide, many species are in trouble. Declines of honeybees and North American native bees, such as bumblebees, have been reported over the past decade. The current approach to native bee conservation is habitat enhancement, providing nesting and foraging resources in hopes that pockets of suitable habitat will sustain diverse bee communities. On a larger scale, habitat restoration—as opposed to small-scale habitat enhancement—could potentially support larger and more diverse bee communities in natural areas. In Illinois, native habitat loss has been severe. Over 80% of land cover is agricultural and only 0.01% of tallgrass prairie, the dominant pre-settlement ecosystem, remains. We investigated the effects of restoration on native bee communities using a chronosequence of prairie restorations in northeastern Illinois, ranging from abandoned old fields as a control to remnant prairies as reference sites. Specifically we asked: Are bees responding to restoration? Is there a signal of restoration on the taxonomic or trait composition of resulting bee communities?
We collected 6,685 bees representing 73 species, 29 genera and 7 families thus far. Bee abundances in restored prairies were weakly negatively correlated with restoration age (p=0.03 R2= -0.03). Bee diversity was not significantly different by site type and, within restored sites, did not change with age. Although typically correlated in other bee studies of natural and human-dominated ecosystems, bee diversity was not significantly correlated with floral diversity, floral abundance, or the proportion of natural area surrounding sites at any scale. Although these results are surprising, bee community composition did differ significantly by restoration stage (PERMANOVA, p=0.001), which was partly explained by differences in forb diversity (PERMANOVA, p=0.02). There was weak evidence that bee community composition based on traits was different by restoration stage (PERMANOVA, p=0.07). Taxonomic and trait composition of bee communities were significantly correlated (Mantel, p=0.05). Forb communities were significantly different by restoration stage (PERMANOVA, p=0.001), not surprising considering many restoration techniques directly influence plant community structure (e.g., seeding, mowing and burning). Even though floral attributes were not significant predictors of bee diversity, the community composition of bees and forbs were correlated (Mantel, p=0.001).