Pollinator foraging behavior and plant reproductive strategies: Theory and application to Lobelia deckenii on Mt. Kilimanjaro, Tanzania

Pollination in Lobelia deckenii (Lobelia sect. Rynchopetalum: Lobeliaceae), a giant rosette species occurring on Mt. Kilimanjaro, depends on the foraging activities of two nectar-feeding birds, the scarlet-tufted malachite sunbird (Nectarinia johnstoni: Nectariniidae) and the mountain chat (Cercomela sordida: Turdidae). L. deckenii flowers produce large volumes of nectar (at a rate of approximately 5 {dollar}mu{dollar}l{dollar}cdot{dollar}hr{dollar}sp{lcub}-1{rcub}{dollar}), but it is very dilute (mean concentration of 0.23 M sucrose equivalent). Measurements of bird visitation rates to individual inflorescences, the number of flowers probed during each visit, and the effective number of pollen grains deposited by single probes of a flower indicate that the two species are approximately equally efficacious in the delivery of pollen, although the sunbird is a specialized nectarivore and the chat an adventitious nectar feeder. The patterns of variation in these measurements of foraging behavior are used to derive a probability model of pollen acquisition by single flowers of L. deckenii. The model predicts large variation among flowers in effective pollen receipt, and further predicts that most flowers will be pollen limited in seed set. These predictions are supported by empirical measurements.; When natural nectar in the lobelia flowers is replaced with sugar solutions in a range of concentrations and volumes, the foraging behaviors of the pollinating birds are altered. Sunbirds probe fewer flowers in response to increased volumes of floral reward, and aggressively defend lobelia inflorescences when concentration is high, which reduces the visitation rate by the behaviorally subordinate mountain chats. When these altered foraging behaviors are incorporated in the probability model used to analyse pollen delivery under natural circumstances, the predicted result is, for some experimental reward levels, a decrease in female reproductive success of the lobelia.; Although female reproductive success in L. deckenii is limited by pollen acquisition, producing a large number of ovules in each flower may represent the optimal use of limited reproductive resources when variance in pollen receipt is large, and may maximize the total number of seeds produced on a plant, even though many flowers are pollen limited in seed set.