MUTUALISM DENIED - NECTAR-ROBBING BUMBLE BEES DO NOT REDUCE FEMALE ORMALE SUCCESS OF BLUEBELLS

Although mutualisms are often viewed as fragile constructs, subject to invasion by ''cheaters'' that gain from the mutualism without providi ng compensating benefits, few studies have explored whether or not app arent cheating behavior by one player actually denies benefits to the other species. Panicled bluebells (Mertensia paniculata) experience hi gh rates of nectar robbery by bumble bees (Bombus mixtus and B. frigid us) in the Wrangell Mountains of southcentral Alaska. Nevertheless, ex perimental prevention of nectar larceny in two seasons did not enhance components of female reproductive success (nutlet initiation, seed-se t, and seed mass) or male success (pollen removal). Observational data show that the absence of a negative impact of nectar robbery is a con sequence 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 o f different rewards at different times in the flowering period; young flowers contain pollen and older flowers produce nectar. Although flow ers 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 seq uestered from pollinators in the early stage were still capable of ini tiating nutlets, demonstrating that older flowers can enhance female r eproduction if pollinators are rare (when many early-stage flowers may go unvisited). Moreover, the removal of mature flowers reduced the vi sitation rate to early-stage flowers on the same plant. Because indivi dual bumble bees switch frequently between nectar robbing and pollen c ollection, the nectar reward in mature flowers may act as a key entice ment to pollinators, which then enhance plant reproduction by legitima tely visiting early-stage flowers. Rather than representing a case of cheating behavior, nectar robbery may be an integral part of this plan t-pollinator mutualism.