A trade-off between the frequency and duration of bumblebee visits to flowers

Pollinator behavior influences plant reproduction in many ways. A tradition al measure of pollination, the number of visits received, may be a poor pre dictor of plant reproductive success, particularly when there are trade-off s between visit quantity and components of visit quality. For example, the duration of pollinator visits may be negatively correlated with the number of visits received by a flower. We tested for a trade-off between the numbe r of bumblebee visits and the duration of those visits in an experimental p opulation of snapdragons (Antirrhinum majus: Scrophulariaceae). The duratio n of a bumblebee visit to a flower increased significantly with the time in terval since the flower had last been visited. Over the lifetime of a flowe r the correlation between the total number and average duration of visits r eceived by a flower was weakly negative. However, at the whole-plant level the correlation was positive: plants whose flowers received more visits als o received visits of longer duration. Factors affecting the relationship be tween quantity and duration of pollinator visits to flowers also were inves tigated. Two factors weakened the negative dependence of average visit dura tion on number of visits received by individual snapdragon flowers: (1) the correlation between the total number of visits to a flower and the average interval between visits was only -0.53, as visits to individual flowers we re not very evenly spaced over time, and (2) newly opened flowers received fewer and shorter visits than older flowers. Comparing whole plants, nectar production per flower varied dramatically across individuals, a probable e xplanation for the positive correlation between visit number and average du ration per flower observed at the plant level. The potential for a trade-of f between these two components of pollinator service exists when visit dura tion depends on reward quantity, whether the trade-off is realized will dep end on variation in nectar production and on whether pollinators forage sys tematically.