Experimental small‐scale flower patches increase species density but not abundance of small urban bees.

Abstract: Large flower plantings are often used to combat negative effects of habitat loss on pollinators, but whether these floral additions are effective at smaller scales remains unclear, particularly in urban settings. To test the effectiveness of small‐scale floral additions on enhancing urban bee populations, as well as their impact from 1 year to the next, different quantities of potted sweet alyssum (Lobularia maritima) flowers were placed across sites in Ann Arbor, Michigan for two consecutive years and the resulting Halictid bee visitors were monitored. Overall, we found the number of flowers added at the local level was significantly and positively correlated with small Halictid bee abundance and species density in an urban landscape. At smaller flower quantities, dynamics were clearly linear, where incremental increases in number of flowers showed significant increases in bee abundance and species density. At larger quantities of floral additions, however, dynamics were nonlinear in that incremental increases in flower quantity had no effect on bee abundance and highly variable effects on bee species density. When comparing the change in small Halictid bee abundance and species density from 1 year to the next, we found a significant increase in bee species density in the second year of small‐scale floral additions, but no significant difference in bee abundance. Synthesis and applications. Our results show that small flower plantings can have positive effects on small bee communities in urban systems even over a short period of time and therefore confirm that encouraging citizens to plant flowers can be an effective conservation strategy for certain urban pollinator populations. In addition, our finding that smaller flower plantings may have higher impacts on small pollinators than larger plantings suggests resource managers interested in pollinator conservation should consider spreading multiple, smaller floral plantings across the urban landscape, rather than pooling all resources into one large flower patch. [ABSTRACT FROM AUTHOR]