Interactions for Pollination among Coflowering Shrubs

Abstract

To examine competition for pollination in a natural community, I measured pollinator sharing, pollinator dependence, and limitation of fruit set for four colflowering shrub species in The Great Swamp, Rhode Island. These species included all potential competitors for pollination and have had the long, extensive associations deemed necessary for competitive coevolution. The four shrubs shared the major pollinators (bees), and three of the four species depended upon pollinators for fruit set; but pollination limitation due to competition was detected in only one species. Despite the lack of competition, the shrubs interacted negatively and stronly for bumble bees, which were major pollinators. The species differentially attracted bumble bees, and their relative attractive (competitive) abilities were predicted by an index based on average daily sucrose production per flower x local floral density as: Ilex opaca > Kalmia latifolia > Gaylussacia frondosa > Kalmia angustifolia. Bumble bees rarely visited the less attractive species during flowering overlap in 3 of 4 yr. Competitive avoidance was achieved, not by specialization and divergence as is commonly hypothesized, but by having traits that confer tolerance of rare pollinator visits (e.g., long—lived flowers or self—pollination) or by having diverse pollinator assemblages (small bees and bumble bees). K. latifolia was the only shrub specialized for pollination (by bumble bees), and this specialization increased, rather than decreased, the risk of pollination limitation because K. latifolia was a poorer competitor for bumble bees than Ilex. Pollination limitation was prevented or reduced by the extended longevity of unpolinated flowers. Flowers could remain viable > 2 wk until Ilex ceased flowering and bumble bees switched to K. latifolia K. angustifolia, the poorest competitor, was visited by many species of small bees and could also self—pollinate, so fruit set was maintained even in the absence of bumble bees, Gaylussacia lacked these traits, and it was pollination limited: flowers are short—lived and cannot self—pollinate. For this species small bees were important in maintaining fruit set when bumble bees switched to more attractive species. These traits for competitive avoidance are advantageous when pollination is uncertain for any reason, such as unfavorable weather for pollinator foraging. As a result, their presence cannot be unambiguously attributed to past competition, although they currently are important in reducing competition among these shrubs. Such traits as generalization for resource use of tolerance of sparse resources may be as important as specialization or greater competitive ability for the coexistence of species in competitive communities.