Many bee species, including rare species, are important for function of entire plant-pollinator networks

It is important to understand how biodiversity, including that of rare species, affects ecosystem function. Here, we consider this question with regard to pollination. Studies of pollination function have typically focused on pollination of single plant species, or average pollination across plants, and typically find that pollination depends on a few common species. Here, we used data from 11 plant-bee visitation networks in New Jersey, USA, to ask whether the number of functionally important bee species changes as we consider function separately for each plant species in increasingly diverse plant communities. Using rarefaction analysis, we found the number of important bee species increased with the number of plant species. Overall, 2.5 to 7.6 times more bee species were important at the community scale, relative to the average plant species in the same community. This effect did not asymptote in any of our datasets, suggesting that even greater bee biodiversity is needed in real-world systems. Lastly, on average across plant communities, 25% of bee species that were important at the community scale were also numerically rare within their network, making this study one of the strongest empirical demonstrations to date of the functional importance of rare species.

Individual bee foragers are less efficient transporters of pollen for the plants from which they collect the most pollen into their scopae

PREMISE

Bees provision most of the pollen they remove from anthers to their larvae and transport only a small proportion to stigmas, which can negatively affect plant fitness. Though most bee species collect pollen from multiple plant species, we know little about how the efficiency of bees' pollen transport varies among host plant species, or how it relates to other aspects of generalist bee foraging behavior that benefit plant fitness, such as specialization on individual foraging bouts.

METHODS

We compared the pollen collected and transported by three bee species for 46 co-occurring plant species. Specifically, we compared the relative abundance of pollen taxa in individual bees' scopae, structures where bees store pollen to provision larvae, with the relative abundance of pollen taxa on the rest of bees' bodies, which is more likely to be transferred to stigmas.

RESULTS

Bees carried five times more pollen grains in their scopae than elsewhere on their bodies. Within foraging bouts, bees were relatively specialized in their pollen collection, but transported proportionally less pollen for the host plants on which they specialized. Across foraging bouts, two bee species transported proportionally less pollen for some of their host plants than for others, though differences didn't consistently follow the same trend as at the foraging bout scale.

CONCLUSIONS

Our results suggest that foraging bout specialization, which is known to reduce heterospecific pollen transfer, also results in less efficient pollen transport. Thus, bee foragers that visit predominantly one plant species may have contrasting effects on that plant's fitness. This article is protected by copyright. All rights reserved.