Mutualism denied? Nectar-robbing bumble bees do not reduce female or male success of bluebells

Although mutualisms are often viewed as fragile constructs, subject to invasion by "cheaters" that gain from the mutualism without providing compensating benefits, few studies have explored whether or not apparent cheating behavior by one player actually denies benefits to the other species. Panicled bluebells (Mertensia paniculata) experience high rates of nectar robbery by bumble bees (Bombus mixtus and B. frigidus) in the Wrangell Mountains of southcentral Alaska. Nevertheless, experimental prevention of nectar larceny in two seasons did not enhance components of female reproductive success (nutlet initiation, seed-set, and seed mass) or male success (pollen removal). Observational data show that the absence of a negative impact of nectar robbery is a consequence of bumble bee behaviors; the two bee species that rob mature flowers buzz-pollinate the same flowers at an earlier stage in floral development. The shift in bee behavior is driven by the presentation of different rewards at different times in the flowering period; young flowers contain pollen and older flowers produce nectar. Although flowers typically receive and donate pollen before most nectar is produced, the mature flowers producing a robbed nectar reward may nonetheless contribute to the reproductive success of bluebell plants. Flowers sequestered from pollinators in the early stage were still capable of initiating nutlets, demonstrating that older flowers can enhance female reproduction if pollinators are rare (when many early-stage flowers may go unvisited). Moreover, the removal of mature flowers reduced the visitation rate to early-stage flowers on the same plant. Because individual bumble bees switch frequently between nectar robbing and pollen collection, the nectar reward in mature flowers may act as a key enticement to pollinators, which then enhance plant reproduction by legitimately visiting early-stage flowers. Rather than representing a case of cheating behavior, nectar robbery may be an integral part of this plant-pollinator mutualism.

Duke Scholars

Author William F. Morris Biology