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Pollinator foraging modifies nectar sugar composition in Helleborus foetidus (Ranunculaceae):An experimental test¹

² Estación Biológica de Doñana, CSIC, Avenida de María Luisa s/n 41013 Sevilla, Spain ³ Centro de Investigación Científica de Yucatán, A.C. Calle 43 No. 130 Chuburná de Hidalgo 97200 Mérida, Yucatán, México ⁴ Instituto de Investigaciones Químicas, Centro de Investigaciones Científicas Isla de La Cartuja, CSIC-US, Avenida Américo Vespucio 49 41092 Sevilla, Spain

ABSTRACT

We experimentally tested the hypothesis that the extensive within-plant variation of nectar sugar composition in Helleborus foetidus (Ranunculaceae) and other species results from differences between flowers and nectaries in pollinator visitation history. Experiments were conducted to mimic single-nectary visits by wild-caught individuals of the main bee pollinators of H. foetidus, which were assayed for their capacity to modify the sugar composition of natural and artificial nectar. Experimental nectar probing with bee mouthparts induced extensive changes in proportional sugar composition 48 h after treatment, and bee taxa differed widely in their effects. Nectar probing by Andrena, medium-sized Anthophoridae, Apis mellifera, and Lasioglossum had no subsequent effects on nectar sugar composition, while probing by Bombus terrestris and B. pratorum induced an extensive reduction in percentage sucrose, a marked increase in percentage fructose, and a slight increase in percentage glucose. Results support the hypothesis that stochastic variations among flowers or nectaries in the taxonomic identity of recent visitors and their relative visitation frequencies may eventually generate very small-scale mosaics in nectar sugar composition. Changes in nectar sugar composition following bumblebee probing may be the consequence of nectar contamination with pollinator-borne nectarivorous yeasts.

Key Words: bumblebees • Helleborus foetidus • nectar sugar composition • nectarivorous yeasts • pollinator visitation • Ranunculaceae • Spain

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