Pollinators are essential for life on Earth

Ground-breaking research on pollinators and pollination benefits the scientific community.

By Na Wei, PhD, Scientist

Pollinators promote flowering plant diversity

A new study1 published in the world’s most prestigious science journal Nature by Na Wei, Holden scientist, discovered how pollinators contribute to the maintenance of flowering plant diversity. “For years, scientists have been puzzled by how numerous rare plant species coexist with abundant species in diverse communities,” says Wei. “We believe that pollinators can be one critical piece of this puzzle.”

Pollinator service is often limited in nature, and so plants compete with one another for those pollinators. One way to overcome pollinator limitation is for plants to form specialized relationships with some pollinators, thereby ensuring that pollinators are available for plant fertilization. Wei and colleagues monitored 416 pollinator species that visited 79 different flowering plant species in serpentine grasslands in California. These pollinators are diverse, including bees, wasps, flies, butterflies, moths and hummingbirds.

This research team found that specialization between flowering plants and pollinators was greater than expected, and rare plants seemed more likely to form specialized relationships with pollinators to ensure their reproduction and persistence. What’s more, flower characteristics were important predictors of specialization. For instance, plants having pea-like flowers were more specialized than plants with aster-like flowers. Also, plants having longer flower tubes were more specialized than plants with shorter flower tubes.

They also found that not all rare plant species had specialized pollinators but had to use the shared pollinator services. For these rare plants, pollinators that were primarily attracted by abundant species can stop by and pick up rare species pollen as well. This benefits rare species at the cost of abundant species. “This type of asymmetric facilitation has also been reported in a study conducted in a diverse grassland community in Brazil. However, Wei and colleagues’ study goes deeper,” wrote pollination biologist Marcelo A. Aizen of National University of Comahue, Argentina, in a Nature News and Views article2. “It reaches a mechanistic understanding

that addresses not only the pollination benefits but also the costs of pollinator sharing in terms of the loss of a species’ own pollen and receipt of foreign pollen.”

The results of this study have clear implications for conservation of native plants and natural systems. It suggests that to conserve rare plant species and diverse plant communities, we also need to conserve the diversity of pollinators that plants depend on for reproductive success. Wei and colleagues emphasized in their paper that “In light of pollinator losses worldwide and climatically induced shifts in plant abundances, understanding how pollinators contribute to the persistence of rare plant species is arguably one of the most critical tasks for biodiversity conservation in the Anthropocene.” This study that accelerates our understanding of biodiversity conservation has been reported by many mainstream news in multiple languages including English, Chinese, Korean and Indian.

Not all pollinators are equal

There are nearly 300,000 flowering plant species, and around 80% of them depend on insects and other animal pollinators for fertilization and reproduction. But pollinators are not equally effective in helping plants. In a recent study3 published in journal Oecologia this year, Wei and colleagues uncovered how ‘'good’ pollinators are by examining the amount and diversity of pollen grains carried by pollinators.

Pollen grains that are collected and stored in the ‘pollen baskets’ on 'pollinators’ legs are for their own use and are not available for plant fertilization. This research team thereby focused on the pollen grains — outside pollen baskets — that cling to the hairs and other parts of pollinators’ bodies. They brushed 251 individual bees and 95 flies to collected pollen grains. They then counted the number and identity of these pollen grains under a microscope.

This team found that pollen grains varied widely among pollinators. Some carried approximately 80,000,000 pollen grains, whereas others carried only 10 pollen grains. So what makes such a big difference? “Pollinator sex and identity play a big role,” says Wei. “Female bees carried a lot more pollen grains than male bees, and male bees are more or less like flies.” Moreover, pollinator body size was also a good predictor of pollen load and diversity, with larger pollinators carrying more and diverse pollen grains and thus more generalized in visiting plants.

“However, this does not mean that flies are not important pollinators compared to bees,” says Wei. “Instead, our findings indicate that pollinators that carry fewer pollen grains may need to visit plants more frequently to be effective pollinators.”

Pollinators are the vehicles of microbes

When pollinators visit plants, they not only deposit pollen to fertilize flowers but also can deposit microbes that can be beneficial or harmful to plants.

In a recent study4 published in journal Molecular Ecology this year, Wei and colleagues discovered that many microbes arrived at flowers through pollinators. Some of these microbes were yeasts living in nectar and some were known bacterial and fungal pathogens to plants. Wei and colleagues conducted this field experiment on strawberries, one of the most popular fruits in the United States. As strawberry cultivation relies heavily on pesticide use, this research team also tested whether bactericide and fungicide use influences pollinator visits and flower microbes. They found that agrochemical use did not influence pollinator visits, but influenced flower microbes, especially from fungicide use.

In another recent study5 published this year in Malus: International Ornamental Crabapple Society Bulletin, Wei and her summer interns observed pollinators that visited 93 crabapple trees in the National Crabapple Evaluation Project (NCEP) plot at Holden Arboretum and that visited 214 of the 308 crabapple trees in the NCEP plot at Secrest Arboretum. Wei and her team discovered that crabapple trees were visited by diverse pollinators, including many bee species, wasps, flies and butterflies. The pollinators that visited crabapples differed between Holden and Secrest Arboretum, with more honeybees and flies at Secrest and more mining bees (Andrenidae) that are solitary and ground-nesting at Holden. They also found that crabapple trees that had more flowers attracted more and diverse pollinators, and white flowers were visited more than red and pink flowers. The team is currently working on linking pollinator visits to microbes on crabapples to decipher the outcomes of pollinator visits.

Concluding Remarks

“It is difficult to overstate the importance of pollinators for plants and humans,” says Wei. “We hope that our findings benefit not only the scientific community, but also our society and environment.”

References Cited

1. Wei N, Kaczorowski RL, Arceo-Gómez G, O’Neill EM, Hayes RA, Ashman T-L. 2021. Pollinators contribute to the maintenance of flowering plant diversity. Nature 597:688-692. https://rdcu.be/cxoEL

2. Aizen MA. 2021. Pollination advantage of rare plants unveiled. Nature 597:638-639. https://www.nature.com/articles/ d41586-021-02375-z

3. Cullen N, Xia J, Wei N, Kaczorowski RL, Arceo-Gómez G, O’Neill EM, Hayes RA and Ashman T-L. 2021. Diversity and composition of pollen loads carried by pollinators are primarily driven by insect traits, not floral community characteristics. Oecologia 196:131-143. https://doi.org/10.1007/s00442-021-04911-0

4. Wei N, Russell AL, Jarrett AR and Ashman T-L. 2021. Pollinators mediate floral microbial diversity and network under agrochemical disturbance. Molecular Ecology 30:2235-2247. https://onlinelibrary.wiley.com/doi/10.1111/mec.15890

5. Wei N, Incarnato M and Kaufman E. 2021. Plant–pollinator interactions in crabapples: A case study at Holden Arboretum and Secrest Arboretum, Ohio. Malus: International Ornamental Crabapple Society Bulletin.

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Na Wei, PhD, is a Plant Biologist and Research Scientist at Holden. She leads the evolutionary ecology lab. Her research program seeks to elucidate the ecological and evolutionary mechanisms that influence plant adaptation to environmental change. Her research tackles these grand questions using interdisciplinary approaches, integrating plant biology, microbial ecology, pollination biology, evolutionary biology, genetics, and genomics. Na received her PhD in ecology and evolutionary biology from the University of Michigan, Ann Arbor.