Scale-dependent shifts in the species composition of flower visitors with changing floral density

Abstract

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.

Floral biology of Senecio macrophyllus M. BIEB. (Asteraceae), a rare Central European steppe plant

“…On the other hand, the neighborhood of shrubs contributes to the great number and diversity of visiting insects that probably have their nests among bushes. The observation underlines the significance of habitat conditions for the insect behavior and this is in accordance with other authors who stress their importance [21,22,27,36,39].…”

“…Taking into account the results of other authors we may conclude that the diversity of Apoidea visitors, including Bombus species, was connected with the presence of components of grassland and bush outskirts accompanying S. macrophyllus or plant species occurring abundantly in adjacent abandoned fields [16,17,55]. Abundantly rewarding and/or highly energetic plant species may act as magnet plants attracting visitors to the study ragwort [27,39]. These might be, for example, Medicago falcata, Trifolium pratense, Centaurea scabiosa L., Stachys recta L., or Salvia pratensis for Bombus terrestris, and B. lapidarius.…”

“…Receptive stigmas had bubbling on their surface when placed in this solution. Vigorous bubbling was counted as receptive, whereas weak bubbling was assumed as non-receptive [39]. These observations were made under a stereoscopic microscope.…”

“…Prolonged pollen presentation should be favored when pollinators are infrequent or inefficient at delivering the pollen they remove, whereas gradual dosing should optimize delivery by frequent pollinators [24,[33][34][35]. In recent years, investigations underline the impact of spatiotemporal changes in plant communities, population size, and density of individuals on the interaction between plant species and flower visitors [11,[36][37][38][39].…”

Flowering, pollen characteristics and insect foraging on Campanula bononiensis (Campanulaceae), a protected species in Poland

“…Their maintenance is one of the most important challenges and can be obtainable by conservation of plant species biodiversity. It is believed that the understanding of the floral biology of wild species threatened with extinction may contribute to their protection and through this our understanding of different interactions linking organisms within biocenosis may be improved [3]. In entomophilous plants, besides the nectar, pollen serves as an attractant [4].…”