Intra-individual variation in pollen availability: an experimental analysis of its impact on plant-pollinator...

Intra-individual variation in the quantity and quality of floral rewards can affect pollinator foraging and plant fitness. Few studies have examined how this variation affects pollinator behaviour, particularly in pollen-rewarding plants.

We quantified intra-individual variation in pollen production in two nectarless species (Solanum sisymbriifolium and S. rostratum), in which pollen is concealed inside poricidal anthers. Then, for one of these species (S. rostratum), we manipulated pollen availability to create plants with increased intra-individual variation (three flowers with high, natural levels and no pollen) or plants with natural levels of variation (three flowers with natural pollen levels). We exposed both treatments simultaneously to bumblebees (Bombus terrestris), recording visitation and buzzing behaviour.

Natural intra-individual variation in pollen production was high, with 46-58 % of the variation attributable to differences among flowers of the same plant, and coefficients of variation of 14-23 %. Analysis of foraging patterns capturing >2500 visits and 9700 buzzing events showed that bumblebees performed fewer and shorter buzzing visits on pollen-less flowers, whereas they were more likely to remain on a plant after visiting higher-reward flowers, potentially reducing outcrossing opportunities. These flower-level responses did not scale up to the plant level. Contrary to our expectations and to previous work in nectar-rewarding systems, plants in both variation treatments received visits in similar quantity and quality.

These findings suggest that B. terrestris can detect and respond to differences among flowers in pollen quantity despite high intra-individual variation, but that there is no plant-level effect. Given that bees respond to individual flowers rather than integrating information at the plant level, the behavioural patterns documented here might help plants to buffer fitness consequences of intra-individual variation in pollen rewards, which we show is substantial even at the level of pollen production and is likely to be exacerbated following visitation.