Pollen on stigmas as proxies pollinator effectiveness

How do you measure the effectiveness of a pollinator? One common method is to sit patiently by a flower and watch for visits. While this can tell you the quantity of visits, it doesn’t always tell you about the quality of visits. 

Tia-Lynn Ashman and colleagues argue that there is another way of measuring pollinator interactions through examining the pollen on flower stigmas, in a new article in Annals of Botany. However, like pollinator visits, this method has its problems. “Our aim with this viewpoint is to stimulate awareness by discussing caveats, limitations and assumptions of using patterns of pollen receipt and pollen tubes in styles to interpret pollinator-mediated facilitative and competitive processes. We do so by explicating a few examples of how multiple processes can produce similar CP [conspecific pollen] –HP [heterospecific pollen] patterns and vice versa and how multiple CP–HP patterns can result from the same pre-pollination processes (e.g. competition). We hope to shed light on the complexity, but also provide some guidelines on how to better evaluate and interpret data on stigmatic pollen loads in a way that helps move this growing field forward.”

Problems that Ashman and colleagues consider include, pollinator visitation rates, the pollinator pool diversity and floral morphology that helps avoid pollination by pollen from outside the species. They also look at competition after deposition in developing pollen tubes, to fertilize flowers, and in difficulties in sampling.

While pollen on stigmas doesn’t provide all the answers, the authors say it is still a valuable tool. “[S]tigmatic pollen loads can add important information to our understanding of pollination processes at the community scale and in the wild. They directly address post-pollination interactions that cannot be captured by visitation metrics alone.”