The effect of removing numerically dominant, non-native honey bees on seed set of a native plant
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Pollination services are compromised by habitat destruction, land-use intensification, pesticides, and introduced species. How pollination services respond to such stressors depends on the capacity of pollinator assemblages to function in the face of environmental disruption. Here, we quantify how pollination services provided to a native plant change upon removal of the non-native, western honey bee (Apis mellifera)—a numerically dominant floral visitor in the native bee-rich ecosystems of southern California. We focus on services provided to clustered tarweed (Deinandra fasciculata), a native, annual forb that benefits from outcross pollination. Across five different study sites in coastal San Diego County, tarweed flowers attracted 70 insect taxa, approximately half of which were native bees, but non-native honey bees were always the most abundant floral visitor at each site. To test the ability of the native insect fauna to provide pollination services, we performed Apis removals within experimental 0.25 m2 plots containing approximately 20 tarweed plants and compared visitation and seed set between plants in removal and paired control plots (n = 16 pairs). Even though 92% of observed floral visits to control plots were from honey bees, Apis removal reduced seed production by only 14% relative to plants in control plots. These results indicate that native insect assemblages can contribute important pollination services even in ecosystems numerically dominated by introduced pollinators.
KeywordsIntroduced species Native bees Pollinator visitation Pollination services Seed set
We gratefully acknowledge field and laboratory assistance of S. Gaylor, K. Powell, L. Rowe, S. Sandoval, C. Shough, and R. Trivedi. J. Nieh and two anonymous reviewers provided helpful comments on the manuscript. This work was performed in scrub habitats managed by the University of California Natural Reserve System, City of San Diego Open Space Parks, and the Otay-Sweetwater Unit of the San Diego National Wildlife Refuge. Financial support for this research came from a Mildred E. Mathias Graduate Student Research Grant from the University of California Natural Reserve System (AJN), a Ledell Family Grant (AJN), Natalie Hopkins Grant and Doc Burr Grant from the California Native Plant Society (AJN and HJC), a Grant-in-Aid of Research from Sigma Xi Scientific Research Society (AJN), and NSF Doctoral Dissertation Improvement Grant DEB-1501566 (KLJH and DAH).
Author contribution statement
AJN, HJC, KLJH, and DAH conceived and designed the experiments. AJN and HJC performed the experiments. AJN, HJC, KLJH, and DAH analyzed the data. AJN, HJC, KLJH, JRK, DAH wrote the manuscript.
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