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 m(2)) 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.
Visitation
Per-flowerhead visitation rates to Holocarpha virgata ssp. virgata (Asteraceae) and densities of flower-visiting insects in 2 x 2 m plots spanning the available range of floral densities at a variety of sites and dates within the flowering season
SiteData
Latitude and longitude of the center point of each site used in Visitation.csv and area occupied by summer-flowering plants within 200m of the center of each site. See ReadMe file associated with Visitation.csv for further information.
ObsData
Dates of each observation recorded in Visitation.csv along with average H. virgata floral densities within H. virgata patches and average densities of floral resources within patches of all summer-flowering species observed within 200m of the center of each site during each observation. See ReadMe file associated with Visitation.csv for further information.
Nectar
Total nectar sugar produced by H. virgata flowerheads in 1 x 1 m plots of varying floral densities at the sites and dates used in Visitation.csv. Flowerheads were bagged in the morning before insects became active and nectar measured in the late morning or early afternoon, after the conclusion of flower visitation observations. See ReadMe file associated with Visitation.csv for more information.
NectarTiming
Total nectar sugar measured in 11 bagged flowerheads of H. virgata at multiple points throughout a single day, August 22, 2010. Flowerheads were bagged before pollinators became active in the morning. At each measurement point, the flowerhead was emptied of nectar. See ReadMe file associated with Visitation.csv for further information.
Pollen
Pollen produced by H. virgata flowerheads in the 1 x 1 m plots also used for nectar measurements. Flowerheads were collected in the morning before insects became active. See ReadMe file associated with Visitation.csv for further information.
HandlingTimes
Flower handling times recorded during the flower visitation observations described above (and reported in Visitation.csv). See ReadMe file associated with Visitation.csv for further information.
SearchSpeeds
Search speeds calculated from flight times and estimated distances between flowerheads, in 2009 (the summer before the rest of the data in this dataset were collected), for the main visitors to H. virgata. See ReadMe file associated with Visitation.csv for further information.
BodySize
Intertegular spans of each of the four most common flower visitors to H. virgata. See ReadMe file associated with Visitation.csv for further information.