Bumble Bee Selection Of Mimulus Guttatus Flowers: The Effects Of Pollen Quality And Reward Depletion

Ecology, Dec, 1999 by Alastair W. Robertson, Claire Mountjoy, Brian E. Faulkner, Matthew V. Roberts, Mark R. Macnair

ALASTAIR W. ROBERTSON, [1]

Hatherly Laboratories, Department of Biological Sciences, Prince of Wales Road, University of Exeter, Exeter EX4 4PS, UK

Abstract. The ability of flower visitors to monitor returns when collecting pollen from flowers has been seldom studied despite the importance of pollen as a food resource, particularly for bees. Californian populations of Mimulus guttatus are polymorphic for pollen quality: many plants produce a high proportion of cytoplasmless pollen grains that render the grains incapable of fertilizing ovules or of supporting bees nutritionally. We found that different genotypes maintained a consistent proportion of inviable pollen within a genotype and over time. The number of pollen grains per flower was also consistent within a plant at each date but declined over time. We studied the ability of British bumble bees (Bombus spp.) to discriminate among plants of Mimulus guttatus on the basis of pollen quality and quantity at three scales: indoors with choices of two genotypes, in outdoor plots of several genotypes that varied in pollen quality, and outdoors at a whole-patch scale where two patches of plants differed in quality.

We found that bees could discriminate among plants on the basis of pollen quality provided that flowers still retained most of the pollen. In the two-genotype trials, bees chose genotypes primarily on the quantity of viable pollen, and nectar was much less important. Similarly, where patches of low- and high-pollen quality plants were established, bees responded by visiting the high-quality patch more often and by visiting more flowers within the patch. However, the results from the outdoor plots that contained genotypes of varying phenotypes were inconsistent. A meta-analysis of a large number of separate plots showed that the overall correlation between visitation rate and pollen quality was significant, but variation among plots was also significant. A possible explanation for this inconsistency was suggested in a greenhouse trial in which we showed that, when foraging density was high, depletion of the standing crop of pollen happened quickly, and this reduced the ability of the foragers to choose the hi gher-quality genotypes. The results have implications for the evolution of pollen production in Mimulus guttatus and reward production in other plants.

Key words: Bombus; bumble bees; Mimulus guttatus; pollen number; pollen quality; pollinator behavior; pollinator-mediated selection, reward depletion.

INTRODUCTION

Nectar and pollen are the most common rewards offered by flowering plants to flower visitors in return for providing a pollination service (Simpson and Neff 1983). For many flower visitors, nectar is the primary source of easily assimilated energy, while pollen is a source of protein. The availability of either may limit the fitness of colonies of social bees and generate selection for behavior that maximizes the uptake of these resources (Charnov 1976). Plants, on the other hand, are under selection to secure regular visitation and an efficient pollen delivery service at minimal cost. The amount of reward that is offered by a plant may determine both the rate of visitation and the subsequent behavior of flower visitors (Zimmerman 1988, Rathcke 1992) and therefore has consequences for pollen transfer. Several studies have shown that selection can act on plants through the effect of foraging preferences of nectar-gathering visitors (e.g., Pyke et al. 1988, Real and Rathcke 1991, Mitchell 1993, Hodges 1995).

Despite the wealth of studies demonstrating a relationship between nectar level and pollinator behavior (reviewed in Zimmerman 1988, Rathcke 1992), very little attention has been given to establishing whether the quantity and quality of the other major reward, pollen, has similar effects on pollinator behavior. Some workers have suggested that the ability of bees to detect differences in amounts of available pollen in flowers 15 limited (Hodges and Miller 1981, Haynes and Mesler 1984). Indeed, some evidence that pollen-collecting bees discriminate between flowers on the basis of pollen availability is equivocal. For example, a preference for the polleniferous flowers of some dioecious orgynodioecious species could be due to the greater levels of nectar these flowers also offer (e.g., Kay et al. 1984, Elmqvist et al. 1988, Ashman and Stanton 1991, Delph and Lively 1992 but see Eckhart 1991). Nevertheless, several papers report a preference for flowers based on visual signals of pollen availability; either phe nological changes that correspond with the pollen donation phase (Wainwright 1978, Galen and Plowright 1985, Pellmyr 1988, Gori 1989, Cresswell and Robertson 1994) or differences between species in pollen production that can be visually assessed (Armbruster and Webster 1982, Armbruster and Herzig 1984). However, we know of only three papers that show that bees discriminate amongst flowers on the basis of pollen when the relative amounts of available pollen is disguised (Buchmann and Cane 1989, Gori 1989, Harder 1990).