The structure of diverse floral visitor assemblages and the nature of spatial variation in plant-pollinator interactions have important consequences for floral evolution and reproductive interactions among pollinator-sharing plant species. In this study, I use surveys of floral visitor communities across the geographic range of Clarkia xantiana ssp. xantiana (hereafter C. x. xantiana) (Onagraceae) to examine the structure of visitor communities, the specificity of the pollination system, and the role of variation in the abiotic vs. biotic environment in contributing to spatial variation in pollinator abundance and community composition. Although the assemblage of bee visitors to C. x. xantiana is very diverse (49 species), few were regular visitors and likely to act as pollinators. Seventy-four percent of visitor species accounted for only 11% of total visitor abundance and 69% were collected in three or fewer plant populations (of ten). Of the few reliable visitors, Clarkia pollen specialist bees were the most frequent visitors, carried more Clarkia pollen compared to generalist foragers, and were less likely to harbor foreign pollen. Overall, the core group of pollinators was obscured by high numbers of incidental visitors that are unlikely to contribute to pollination. In a geographic context, the composition of specialist pollinator assemblages varied considerably along the abiotic gradient spanning the subspecies' range. However, the overall abundance of specialist pollinators in plant populations was not influenced by the broad-scale abiotic gradient but strongly affected by local plant community associations. C. x. xantiana populations sympatric with pollinator-sharing congeners were visited twice as often by specialists compared to populations occurring alone. These positive indirect interactions among plant species may promote population persistence and species coexistence by enhancing individual reproductive success.
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Acknowledgements I am grateful to M. A. Geber for invaluable advice and encouragement. B. N. Danforth, J. Ehleringer, P. L. Marks, M. V. Price, and two anonymous reviewers provided many helpful comments on the manuscript. This research could not have been completed without assistance from J. S. Ascher with bee identification and from J. E. Carlson with logistics. I also thank B. N. Danforth, T. L. Griswold, and R. W. Brooks for help with bee identifications. The research was supported by a NSF Doctoral Dissertation Improvement Grant (DEB-0104582) to D. M. and M. A. Geber and NSF (DEB-969086) to M. A. Geber.
- Geographic variation
- Positive interactions