Variability in the effectiveness of <i>Penstemon strictus</i> pollinators and the role that water availability plays

Abstract

Plant-pollinator interactions play a vital role in the functioning and stability of our ecosystems. Without pollination, angiosperms would not be able to set seed and reproduce, and most organisms would lose their primary source of nutrients and resources. Owing to the fact that pollination services are becoming increasingly threatened by anthropogenic impacts, a thorough understanding of the factors that influence the success of pollen transfer is important, relevant, and timely. In this study, we evaluate how water availability influences pollinator effectiveness. Pollinator effectiveness is defined as the amount of conspecific pollen deposited on the stigma in a single pollinator visit. Many researchers have compared and assessed pollinator effectiveness, while others have investigated how water availability influences floral and vegetative traits, but few have studied the relationship between environmental factors and conspecific pollen deposition. Using a water manipulation experiment on 80 paired Penstemon strictus plants growing in Crested Butte, Colorado, we were able to show that different types of pollinators vary in their behavior, handling time, and effectiveness. Our water treatment did not exert a significant effect on any of the floral or vegetative traits that we measured, so the pathway by which we predicted water availability to operate on pollinator performance was irrelevant. In our analysis of pollinator behavior, handling time and effectiveness, we therefore have not factored in water treatment or soil moisture (yet). With respect to pollinator effectiveness, we did not find a significant correlation between handling time and conspecific pollen deposition for any of the pollinators. We did, however, find a significant relationship between behavior and conspecific pollen deposition. Buzz pollination or sonication, a particular pollen foraging behavior employed by Bombus bifarius, yielded the greatest amount of pollen received by the stigma. Based on these findings, we suggest that behavior in conjunction with physical traits of pollinator species and genera, such as size, hairiness, and pollen-bearing structures, exert the largest influence on pollinator effectiveness.

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