COS 135-5 - Not all pollinators are created equal: Differences in pollination efficiency and their effects on plant reproductive success and fitness

Abstract: Throughout the flowering time of many plant species, flowers may be visited by a diverse suite of pollinators that vary in their efficiency. The efficiency of a pollinator will be the result of functional traits which will determine the quantity and quality of both the pollen delivered and seeds produced. Nevertheless, it is often assumed that the most efficient pollinator is the one that visits most frequently and deposits the greatest quantity of pollen. Critically, however, this implicitly overlooks the quality of the pollen that a pollinator delivers. For example, pollinators with larger foraging ranges may deliver greater quality pollen that is genetically novel, which can result in more genetically diverse offspring that are more fit and may have a greater ability to withstand environmental stochasticity. Addressing this deficit will allow for more accurate measurements of pollination efficiency, essential for predicting how pollinator declines may affect the genetic diversity and persistence of plant populations.

To this end, we sought to demonstrate the importance of measuring pollination quality by examining how interspecific differences in pollination efficiency affect the reproductive output and fitness of a montane perennial (Delphinium nuttallianum; Ranunculaceae). Single-visit pollen deposition experiments were used to isolate the effects of individual pollinators. Stigmas were collected, with the number of pollen grains deposited and the ratio of conspecific to heterospecific grains counted to determine the quantity and quality of pollen respectively. Seed set per fruit was counted and all seeds were X-rayed to determine viability rates. Seeds were then grown in a greenhouse to obtain fitness measures (germination rate, survival, biomass) and will be genotyped to determine genetic diversity, the extent of inbreeding, and relatedness. Preliminary results found a trend where the larger, more specialized bumblebee species (Bombus appositus) resulted in a greater seed set but significantly less viable seeds than those produced by a smaller, more generalized bumblebee species (Bombus flavifrons). These results suggest that the pollinator that deposits the greater quantity of pollen may not be the highest quality pollinator. By examining differences in pollination services between these species, this study demonstrates how measuring both the quality of pollination in addition to quantity will lead to more accurate measures of pollination efficiency. Accurately understanding how different pollinators contribute to the reproduction and genetic diversity of plant populations is essential for the implementation of effective conservation strategies that focus on the long-term maintenance of plant populations and genetic diversity.