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. Kilimanjar o, Tanzania, is pollinator-limited in seed set. I investigate how qual ity and quantity of floral rewards affect behaviour of the nectar-fora ging birds that pollinate L. deckenii, to determine if enhanced floral rewards could make the lobelia sufficiently attractive to overcome po llinator limitation of female success. 2 Experimental sucrose solution s were introduced into flowers in a range of concentrations and volume s. Enhanced rewards increased the visitation rate of one of the two po llinator species (Nectarinia johnstoni, scarlet-tufted malachite sunbi rd). However, based on previous studies of the way in which pollinator behaviour affects seed set in this lobelia species, the increased vis itation is expected to be insufficient to produce full seed set. 3 Mor eover, enhanced rewards promoted territorial defence of the experiment al inflorescences by Nectarinia johnstani. Defence by these sunbirds r educed visitation rates by the second pollinator, a behaviourally subo rdinate species (Cercomela sordida, mountain chat), which contributes substantially to pollination of Lobelia deckenii. Permanent sunbird te rritoriality might also reduce gene flow in the lobelia population, an d reduce offspring quality in any mutant lobelias offering richer nect ar. As a result, the potential benefits of greater floral rewards, eve n to the point of very rich nectar, are probably small, and the potent ial costs due to sunbird territoriality may outweigh any benefits. 4 T his study suggests that the relation between attractive investment and fitness gains in Lobelia deckenii is not a monotonically increasing f unction, and that selection may favour low quality rewards even with c oncurrent pollinator limitation of female success.