1. Grassland ecosystems are imperiled by agricultural activity worldwide. Restoring grassland habitat is important to conserving grassland fauna and preserving ecosystem services, but more knowledge is needed on the impact that local and landscape factors have on patterns of diversity. We focused on whether prairie grassland restorations along a gradient of increasing agricultural cover in the surrounding landscape would be inhabited by less diverse and/or more homogenous native bee communities. Native bees are a specific target for many grassland restoration efforts, and supporting their local and β-diversity in reconstructed habitats is of mounting interest. We also investigated if higher floral resource richness within restorations could help ameliorate negative effects of agricultural landscapes. 2. We sampled 16 prairie restorations in Minnesota (USA) that varied along a gradient of increasing agricultural land cover around the site. We characterized floral resource richness at all sites beginning in mid-May and ending in mid-September. We used GLMMs and multivariate analyses to disentangle how floral resource richness and percentage of surrounding land cover in agricultural production are associated with the local and β-diversity of bee communities. 3. Local bee diversity increased with increasing local floral resource richness, independent of the surrounding landscape. Bee β-diversity was not impacted by local floral resource richness or percentage of agricultural cover in the surrounding landscape, indicating local and landscape factors are not substantially impacting the homogeneity of bee communities across restorations. 4. Synthesis and applications. We found that, regardless of agricultural cover in the surrounding landscape, more florally rich plantings attract more diverse bee communities. We recommend that habitat plantings prioritize local scale diversity, and that potential sites where the landscape is dominated by agricultural production should not be overlooked for restoration.

Abstract. Pollinators and pollination services are under threat globally, and invasive plants have been implicated in their decline. Results of previous studies suggest that consequences of invasion for pollinators and plant-pollinator interactions are context specific. Investigating factors such as the density of an invasive plant and its phenology may provide a nuanced understanding of invasive species impacts. We conducted a 2-yr study in Montana to investigate how local pollinator abundance, richness, community composition, and visitation patterns varied with invasive Centaura stoebe density and phenology, and whether C. stoebe altered the reproduction of a co-flowering native plant, Heterotheca villosa, through changes in pollinator visitation. In an observational study, we found that during its peak bloom in August, Centaurea stoebe provided abundant floral resources to late-season pollinators. However, prior to C. stoebe bloom, native floral density and pollinator abundance and richness of these plots were lower compared to plots where C. stoebe was low or absent. Pollinator community composition in plots without C. stoebe was different compared to plots with C. stoebe (both high and low C. stoebe density), and these differences in pollinator composition strongly depended on the time of season. In an experimental study, we found that there was little evidence of competition between C. stoebe and H. villosa for pollinators at low relative densities of C. stoebe. Using experimental pollen supplementation, we observed no evidence of pollen limitation of seed set in H. villosa with increasing density of experimentally added C. stoebe. Our results suggest that the impact of an invasive plant on pollinators and plant-pollinator interactions depends on the relative density of the invasive plant and the timing of its bloom. Differences in pollinator visitation patterns over the growing season suggest that although C. stoebe provides abundant resources to lateseason pollinators, displacement of native plants at high C. stoebe density may indirectly harm pollinators that are active before C. stoebe blooms or that prefer native plants. Based on our results, restricting C. stoebe to low densities may help mitigate negative repercussions to native plant reproduction and may even be beneficial to some pollinators.

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