POLLINATOR BEHAVIORAL-RESPONSES TO REWARD SIZE IN LOBELIA-DECKENII - NO ESCAPE FROM POLLEN LIMITATION OF SEED SET

1 Lobelia deckenii, an endemic giant rosette species of Mt. Kilimanjaro, Tanzania, is pollinator-limited in seed set. I investigate how quality and quantity of floral rewards affect behaviour of the nectar-foraging birds that pollinate L. deckenii, to determine if enhanced floral rewards could make the lobelia sufficiently attractive to overcome pollinator limitation of female success. 2 Experimental sucrose solutions were introduced into flowers in a range of concentrations and volumes. Enhanced rewards increased the visitation rate of one of the two pollinator species (Nectarinia johnstoni, scarlet-tufted malachite sunbird). However, based on previous studies of the way in which pollinator behaviour affects seed set in this lobelia species, the increased visitation is expected to be insufficient to produce full seed set. 3 Moreover, enhanced rewards promoted territorial defence of the experimental inflorescences by Nectarinia johnstani. Defence by these sunbirds reduced visitation rates by the second pollinator, a behaviourally subordinate species (Cercomela sordida, mountain chat), which contributes substantially to pollination of Lobelia deckenii. Permanent sunbird territoriality might also reduce gene flow in the lobelia population, and reduce offspring quality in any mutant lobelias offering richer nectar. As a result, the potential benefits of greater floral rewards, even to the point of very rich nectar, are probably small, and the potential costs due to sunbird territoriality may outweigh any benefits. 4 This study suggests that the relation between attractive investment and fitness gains in Lobelia deckenii is not a monotonically increasing function, and that selection may favour low quality rewards even with concurrent pollinator limitation of female success.