Quick look
A recent study by Iowa State University ecologists shows that pollinators help maintain plant biodiversity, suggesting that significant decreases in pollinators could cause a "plant-pollinator extinction vortex."
AMES, Iowa - Ecologists have long seen a strong connection between biodiversity and pollinators - the butterflies, birds, bats, bees and other insects that help the flowers they snack on fertilize by transferring pollen from male anthers to female stigma.
Previous research has shown diverse landscapes draw more pollinators, as a wider variety of pollen and nectar attracts attention from a wider variety of animals - some which only feed on certain plants. Essentially, pollinators go where the food is, said Brian Wilsey, a professor of ecology, evolution and organismal biology at Iowa State University.
A recent study by Wilsey and doctoral graduate Nathan Soley showed the converse is also true: Pollinators support diversity in plant communities. In an article published this month in Ecology, Wilsey and Soley described a four-year experiment they conducted in plots of restored prairie that examined how plant diversity was affected by purposely protecting wildflowers from pollinators. Among animal-pollinated plants, viable seed production fell by 50% and the diversity of species fell by 27%, they found.
"Our study is the first we are aware of to show that plant biodiversity at the community level can be limited by a lack of pollinators," Wilsey said.
Handling 68,000 flowers
The study was inspired by a familiar dynamic in prairie restoration areas, said Wilsey, a grassland ecologist. When first planted, restorations usually feature a varied assortment of species that includes wildflowers. However, wind-pollinated grasses will often dominate within a few years, crowding out other types of plants. A lack of pollinators could help explain those rapid shifts, if they play a role in maintaining biodiversity.
Ecologists have assumed pollinators support biodiversity, but the relatively scant amount of research quantifying the effect has focused on individual species, Soley said.
"Things get a lot more complicated at a community level," he said. "I think it's important to test our assumptions if we're going to be all in on pollinators."
Wilsey and Soley's experiment was conducted on about 50 acres of university land south of Ames. In 54 circular plots 2 meters wide, the researchers covered flowers with sheer fabric bags that let light in but kept pollinators out. They also augmented pollination in the flowers of another set of plots by transferring pollen by hand, using Q-tips and small paint brushes. Over four growing seasons, they either bagged or hand-pollinated about 68,000 flowers during their weekly trips to the study site, Soley said.
Wilsey and Soley measured seed production and biodiversity within each of the plots, comparing data from areas where pollination was enhanced or prohibited to undisturbed control plots. While the effects became more dramatic over time, it was clear in the first year that the exclusion treatments would have an impact because pollen visibly piled up inside the flower bags, Soley said.
The study's results suggest significant declines in pollinators could cause biodiversity losses that further reduce pollinator populations, causing a self-reinforcing downward trend in both that the researchers call a "plant-pollinator extinction vortex."
"Before this study, I would have never thought that pollinators were this important to maintaining biodiversity. It really opened my eyes," Wilsey said.
Implications for restorations
Pollinators are essential because of their role in food production. According to the U.S. Department of Agriculture, about 35% of global food crops depend on animal pollination to reproduce, making the seeds and fruits that humans harvest.
In addition to providing critical support for pollinators and other wildlife, diverse landscapes improve water and soil quality. In prairies, which used to cover most of Iowa, a variety of life makes ecosystems more resilient to droughts, floods and invasive species. Beyond pollinators, the known pro-biodiversity factors include low nutrient availability, proximity to other quality habitat and a lack of human degradation, Wilsey said.
One major implication of knowing pollinators help maintain plant biodiversity is the need to consider the presence of pollinator habitat when establishing prairie restoration areas. That's especially true for urban projects, Wilsey said. The human-enhanced pollination plots in the study showed no change in biodiversity when compared to the control plots, an indication that there were sufficient bees and other pollinators in the area. But that's less likely to be the case in more human-impacted environments.
Future experiments could use the same methods in different settings to see how much of a difference pollinators make for biodiversity when there are other complicating factors, Wilsey said.
The experiment, now entering its seventh year, is continuing at the study plots near Ames. It's time-intensive work, but it's worth it to better understand how pollinators and plant communities affect each other, Soley said.
"These mutualisms are important to preserve," he said.
