Pollinator effectiveness in a composite: a specialist bee pollinates more florets but does not move pollen farther than other visitors

Abstract

Premise: Variation in pollinator effectiveness may contribute to pollen limitation in fragmented plant populations. In plants with multiovulate ovaries, the number of conspecific pollen grains per stigma often predicts seed set and is used to quantify pollinator effectiveness. In the Asteraceae, however, florets are uniovulate, which suggests that the total amount of pollen deposited per floret may not measure pollinator effectiveness. We examined two aspects of pollinator effectiveness—effective pollen deposition and effective pollen movement—for insects visiting Echinacea angustifolia, a composite that is pollen limited in small, isolated populations. Methods: We filmed insect visits to Echinacea in two prairie restorations and used these videos to quantify behavior that might predict effectiveness. To quantify effective pollen deposition, we used the number of styles shriveled per visit. To quantify effective pollen movement, we conducted paternity analysis on a subset of offspring and measured the pollen movement distance between mates. Results: Effective pollen deposition varied among taxa. Andrena helianthiformis, a Heliantheae oligolege, was the most effective taxon, shriveling more than twice the proportion of styles as all other visitors. Differences in visitor behavior on a flowering head did not explain variation in effective pollen deposition, nor did flowering phenology. On average, visitors moved pollen 16 m between plants, and this distance did not vary among taxa. Conclusions: Andrena helianthiformis is an important pollinator of Echinacea. Variation in reproductive fitness of Echinacea in fragmented habitat may result, in part, from the abundance of this species.