Scale-dependent shifts in the species composition of flower visitors with changing floral density
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Responses of flower-visiting animals to floral density can alter interactions between plants, influencing a variety of biological processes, including plant population dynamics and the evolution of flowering phenology. Many studies have found effects of floral or plant density on pollinator visitation rates at patch scales, but little is known about responses of flower visitors to floral densities at larger scales. Here, I present data from an observational field study in which I measured the effects of floral density on visitation to the annual composite Holocarpha virgata at both patch (4 m2) and site (12.6 ha) spatial scales. The species composition of flower visitors changed with floral density, and did so in different ways at the two scales. At the site scale, average floral density within patches of H. virgata or within patches of all summer-flowering species combined had a significant positive effect on per-flowerhead visitation by the long-horned bee Melissodes lupina and no significant effects on visitation by any other taxa. At the patch scale, per-flowerhead visitation by honeybees significantly increased whereas visitation by M. lupina often decreased with increasing floral density. For both species, responses to patch-scale floral density were strongest when site-scale floral density was high. The scale-dependence of flower visitor responses to floral density and the interactions between site- and patch-scale effects of floral density observed in this study underscore the importance of improving our understanding of pollinators’ responses to floral density at population scales.
KeywordsForaging behavior Flower visitation Pollination Seasonal variation Density dependence
I thank Matthew Poonamallee, Emily Bergmann, and Susan Bury for assistance in collecting field data. I am grateful to John Rotenberry, Paul Aigner, Doug Yanega, the University of California-Riverside Biology Department Lab Prep staff, Derek Roff, Dmitry Maslov, Barbara Walter, Morris and Gina Maduro, Ed Platzer, Rhett Woerly, and the University of Califorina-Riverside Entomology Research Museum for providing advice, equipment, and/or assistance with logistical challenges. I thank John Rotenberry, Kurt Anderson, Rick Redak, and two anonymous reviewers for comments on the manuscript. This work was performed at the University of California Natural Reserve System’s Donald and Sylvia McLaughlin Reserve. Funding was provided by the University of California-Riverside, a National Science Foundation Graduate Research Fellowship, and a Mildred E. Mathias Graduate Student Research Grant from the University of California Natural Reserve System. All methods carried out in this study comply with the current laws of the United States of America.
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