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 despite environmental disruptions. 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. To test the ability of the native insect fauna to provide pollination services to this native plant species, 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). Targeted pollinator removals differ from traditional single-visit measures of pollinator efficiency and effectiveness, which yield quantitative estimates of the pollen transfer ability of individual pollinator species, but shed no light on the portion of pollination services that are non-redundant with services provided by other pollinators.
Results/Conclusions
Tarweed flowers attract over 50 taxa of native insects, many of which are bees, but non-native honey bees accounted for 81% of floral visits at our study sites. Rates of Apis visitation in control plots were fourfold higher, on average, compared to those of all other taxa combined in removal plots. Apis also contacted threefold more capitula per visit in control plots compared to the number contacted by non-Apis visitors in removal plots. Despite these high levels of honey bee visitation, 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. Experimental removal of native pollinators could be used to assess the degree to which full seed set can be achieved by honey bees alone, absent any contribution by native pollinators. Such reciprocal removals, coupled with estimates of outcrossing rates assessed from resulting seeds using genetic markers, would provide useful information on both the quantity and quality of pollination services provided by native and non-native pollinators.