Do bumble bees prefer flowers on a flat land? Causes and consequences of foraging rate altered by inclinations of the ground

Background/Question/Methods Determining factors affecting pollinator visitation is key to understand the reproductive success of animal-pollinated plants. Many factors such as floral display size, reward amount, and the presence of co-flowering species are known to affect pollinator visitation rates to plants. The inclination of the ground where plants grow, which has been little studied, could be one such factor. Pollinators are expected to prefer plants growing on flat land to those on sloped planes, because sloping habitats are likely unsuitable for efficient foraging. To test this possibility, a series of laboratory experiments with bumble bees and arrays of artificial flowers were conducted.

Results/Conclusions In the first experiment, bees were allowed to forage in flower arrays tilted at various angles ranging from 0° (horizontal) to 90° (vertical), and found that their foraging performance decreased with increasing angles. This reduction was caused by an increased travel time between flowers, especially when bees flew upward and downward along the slope. In the second experiment, bees were presented with a pair of arrays, one horizontal and one sloping. The bees preferred visiting flowers on the horizontal array, suggesting that they disliked the reduction in foraging performance on the slope. These results suggest that plants growing on steep slopes have difficulties in attracting pollinators. To counteract such negative effects, therefore, plants growing on slopes may be selected for investing more resources in pollinator attraction such as floral display.