New project on pollinator competition and foraging decisions!

Last summer, we started a project comparing the visitation rates of honeybees and other, mostly solitary pollinators to flower patches of different sizes. We simultaneously collected data about the potential rewards of those patches from their peak flowering season well into floral senescence. We got some interesting results: first, we found that honeybees visited the larger patches per flower more frequently than the smaller patches, while other pollinators visited each at an equal rate. This was consistent with research suggesting that honeybees can converge on the best resources via information-sharing within the colony. But do honeybees respond to long-term seasonal declines across an entire habitat? We found that while solitary pollinators ceased visiting all resources as blooms senesced – perhaps as a result of synchronized foraging seasons – honeybees persisted in foraging. Though they responded to declining value relative to each patch, they did not respond to the seasonal decline in the larger habitat. This raised some interesting questions for us: Are honeybee colonies, as information-sharing systems, able to respond to long-term changes, and how? How are honeybees competing with other pollinators? How do honeybees, bumblebees and solitary pollinators track long-term changes in the floral community?

One exciting aspect of springtime field ecology here is the abundance and diversity of flowering plants – so we can consider these questions in the context of a complex habitat composed of resources with highly variable and changing rewards. We planned a project that broadly takes stock of the reward values of abundant floral resources in a ~50m² region, and the visitation rates of various pollinators.

[1] Penstemon spectabilis in full bloom

Every week, we assess the region for abundant and blooming plants, and select 4-6 species for surveying. We want to track each species from its earliest flowering date past when it’s no longer a potentially valuable resource, so we rotate a species off of the list when it ceases flowering altogether. For each species, we mark four individual patches to survey that week. We’re using distinctive markers that can be seen aerially by Cassie’s drone so that we can use our ground data as a training set for detailed flower maps. Such ground data includes manual flower counts, estimates of senescence, and nectar samples and sugar content analysis.

[2] Sampling nectar from a penstemon flower

As far as pollinator visitation goes, our protocol is the same as it was last summer: we record visitation for ten minutes at a selected ten inflorescences at a given patch (inflorescences loosely defined not in botanical terms, but in terms of how a pollinator would approach the patch, as this varies quite a bit among plant species). Since our broader research questions revolve around the differences between solitary and social, information-sharing pollinators, we sort the pollinators we see into three categories: honeybees, bumblebees, and others. Honeybees live in large colonies of 10,000+ individuals, while bumblebees live in small groups (50-400 individuals in a nest, depending on the species) and other pollinators are mostly solitary.

We’re coupling the pollinator observations with honeybee collection – something that we didn’t get the chance to do last summer. The honeybee collection essentially entails running around with a net and trying not to get stung when transferring the agitated bee into a vial. By identifying the pollen (using a microscope) carried by honeybees that we collect in the field, we can get a sense of whether individual foragers are visiting multiple species of flowers, or just one – and we can relate this to the flowering data as another way of seeing how honeybees track changes in the composition of resources.

[3] a honeybee with pollen baskets

It might be interesting to collect pollen from bumblebees and solitary bees as well, but unfortunately there are far fewer of these than honeybees, and the destructive sampling isn’t worth it. While there’s been plenty of hype in the past decade about Colony Collapse Disorder, less attention has been paid to wide declines in bumblebee and solitary bee populations. Interest in these declines is growing, but there’s still a lot that we don’t know. Many studies find that honeybees, great generalists, are outcompeting and displacing highly specialized native bees in their rapidly shrinking habitats. Check back soon  to see what we learn about the nature of competition at variable resources throughout high flowering season at the Bernard Field Station!

Further reading

Kerstin Walther-Hellwig, Gerriet Fokul. “Increased Density of Honeybee Colonies Affects Foraging Bumblebees. Apidologie 37, 517-532.” Apidologie 37 (2006): 517–32. doi:10.1051/apido:2006035.

Media credits

[1] photo by Matina Donaldson-Matasci
[2] photo by Matina Donaldson-Matasci
[3] public domain image