FLORAL BIOLOGY, MICROCLIMATE, AND POLLINATION BY ECTOTHERMIC BEES IN AN EARLY-BLOOMING HERB

Abiotic factors may constrain the functioning of species interactions such as plant-pollinator mutualisms. I investigated how thermal enviro nment affects the interaction between the early-blooming daffodil, Nar cissus longispathus (Amaryllidaceae) and its major bee pollinator (And rena bicolor; Andrenidae), focusing simultaneously on plant and pollin ator sides of the interaction. I studied fruit and seed set, flower du ration, and the intrafloral thermal environment of N. longispathus, an d the thermal biology, foraging behavior, and thermoregulatory ability of A. bicolor over a 6-yr period in southeastern Spain. N. longispath us flowers from February to April, when unsuitable weather often limit s pollinator activity, yet most flowers are successfully pollinated in all years and sites. Fruit set was weakly pollen limited, but among f lowers setting fruit the proportion of ovules developing into seeds wa s not. Individual flowers lasted for 17 d on average, remaining functi onal during this period. On sunny days, the air inside N. longispathus flowers was significantly warmer than outside. Mean temperature exces s inside flowers was as high as 8 degrees C, and was positively relate d to solar irradiance. Within flowers, air temperature was highest aro und the anthers; this intrafloral gradient was consistent with variati on among perianth parts in radiation transmittance. Andrena bicolor fo raged in N. longispathus flowering patches only on sunny days with air temperature >12 degrees-13 degrees C, and foraging behavior and flowe r visitation rate were temperature dependent. Bees were able to fly at relatively low thoracic temperatures (T-th; range 22 degrees-31 degre es C) and this was essential for successfully foraging at N. longispat hus. Under the range of irradiance and air temperature found at foragi ng sites, A. bicolor individuals inside flowers were able to reach T-t h suitable for flight by passive means alone. Under laboratory conditi ons, A. bicolor was unable to raise or otherwise regulate T-th by phys iological means, but free-flying individuals thermoregulated behaviora lly. Basking was used to raise T-th and intrafloral microclimate, by i nfluencing the proportion of foraging time devoted to basking, played an important role in thermoregulation. Flower visitation rate was posi tively related to the average temperature inside visited flowers, and the probability of basking immediately after one floral visit declined with increasing flower temperature. I conclude that the favorable mic roclimate within N. longispathus flowers, their long duration, and the thermal biology of A. bicolor, were critical elements in this early-s eason pollination system.