BARTHELL, J. F.**; THORP, R. W.; MASON, C.; GARVIN, E.; JOHNSON, E.; WELLS, H.; WENNER, A. M.; University of Central Oklahoma; University of California, Davis; University of Tulsa; University of Tulsa; University of Tulsa; University of Tulsa; University of California, Santa Barbara: An Island Ecosystem after Honey Bees: Is a Noxious Weed now Competing for Native Pollinators?

Feral honey bees, Apis mellifera, have steadily declined on Santa Cruz Island, California (USA), since the release of the varroa mite, Varroa destructor, in 1993. Indeed, no honey bees have been detected at flowers since 2004 and swarm traps designed to monitor for the presence of colonies on the island have remained empty since 1996. Before their decline, honey bees showed strong preference for visiting the invasive weed yellow star-thistle, Centaurea solstitialis. Conversely, native bees preferred a native gumplant, Grindelia camporum, that flowers at the same time and in the same location as yellow star-thistle. Since an obligate out-crossing species like yellow star-thistle must maintain pollen flow among plants to reproduce, we hypothesized that yellow star-thistle would competitively draw native pollinators from its native counterpart (gumplant) in the absence of honey bees. We simulated invasion in a patch of gumplant by removing all existing yellow star-thistle in the area and then re-introducing thistles next to designated gumplants. Visitation rates at the gumplants in gumplant-thistle pairs were compared with visitation of those in gumplant-gumplant (control) pairs. The results show that gumplants paired with yellow star-thistle had significant reductions in the number of native pollinators that visited them in comparison with the controls. Our observations are consistent with predictions that successfully invasive plant species, in the absence of co-evolved pollinators, compete for native pollinators that may, in turn, reduce fitness in native plant species.