Bumble bee preference for flowers arranged on a horizontal plane versus inclined planes

Summary1 Determining factors affecting pollinator visitation is the key to understanding the reproductive success of animal‐pollinated plants. The inclination of the ground where plants grow, which has been little studied, could be one such factor. There may be differences in foraging performance when visiting flowers on horizontal planes compared with flowers on inclines planes. And if this is the case, pollinators may have a preference for horizontal or inclined planes. To test these possibilities, a series of laboratory experiments with bumble bees and arrays of artificial flowers were conducted. 2 In the first experiment, bees were presented with a pair of floral arrays, one on a horizontal plane and one sloping. The bees preferred visiting flowers on the horizontal array. 3 In the second experiment, bees were allowed to forage on a flower array tilted at various angles ranging from 0° (horizontal) to 90° (vertical). It was found that their foraging performance decreased with increasing angles, showing a 9·1% reduction from 0° to 90° arrays. This reduction was caused by an increased travel time between flowers, when moving upslope or downslope. 4 These results suggest that plants growing on steep slopes may be less preferred by pollinators. Future studies are needed to clarify how slopes affect pollinator behaviour in field conditions and the ecological influences on plant reproduction.