Betrayal By Bees Could Have Huge Impact On Plant Reproduction
July 23, 2013

Global Pollinator Decline Could Have An Impact On Flowering Plants And Food Crops

April Flowers for - Your Universe Online

A new study led by Emory University finds removing even one species of bumblebee from an ecosystem has a swift and clear impact: the floral "sweethearts" of that species produce significantly fewer seeds.

The research team focused on the interactions between bumblebees and larkspur wildflowers in Colorado's Rocky Mountains. The findings, to be published by the Proceedings of the National Academy of Sciences, reveal how reduced competition among pollinators disrupts floral fidelity, or specialization, among the remaining bees in the system. This leads to less successful plant reproduction.

"We found that these wildflowers produce one-third fewer seeds in the absence of just one bumblebee species," says Emory University ecologist Berry Brosi. "That's alarming, and suggests that global declines in pollinators could have a bigger impact on flowering plants and food crops than was previously realized."

Approximately 90 percent of the plants on Earth need animals, mostly insects, to transfer pollen from one plant to another so they can fertilize and reproduce. Globally, bees are by far the most important pollinators. They have co-evolved with the floral resources they need for nutrition.

Scientists have reported dramatic declines in the populations of some bee species during the past decade. This decline has sparked research into the potential impact of such declines.

Previous research has indicated plants can tolerate losing most pollinator species in any ecosystem as long as the remaining pollinators pick up the slack. The limit to those studies, however, is that they are based on theoretical computer modeling.

Brosi, along with ecologist Heather Briggs of the University of California-Santa Cruz, wanted to see if this theoretical resilience would hold up in real-life scenarios. To find out, their team conducted field experiments to learn how the removal of a single pollinator species would affect the plant-pollinator relationship.

"Most pollinators visit several plant species over their lifetime, but often they will display what we call floral fidelity over shorter time periods," Brosi explains. "They'll tend to focus on one plant while it's in bloom, then a few weeks later move on to the next species in bloom. You might think of them as serial monogamists."

Plants clearly benefit from floral fidelity because a pollinator visit will only lead to plant reproduction when the pollinator is carrying pollen from the same plant species. "When bees are promiscuous, visiting plants of more than one species during a single foraging session, they are much less effective as pollinators," Briggs says.

The field experiments were conducted at the Rocky Mountain Biological Laboratory near Crested Butte, Colorado. Although the sub-alpine meadows of the facility, at 9,500 feet, are too high for honeybees, during the summer months they are awash with bumblebees. The researchers focused on the interactions of the bumblebees with the dark-purple wildflowers known as larkspurs. Larkspurs are visited by 10 out of the 11 bumblebee species found at the facility.

Studying a series of 20-square-meter wildflower plots, the researchers evaluated each one in both a control state - left in its natural condition - and in a manipulated state - using nets to remove the bumblebees of just one species.

The behavior of the bumblebees was observed in both the controlled plots and the manipulated ones. "We'd literally follow around the bumblebees as they foraged," Briggs says. "It's challenging because the bees can fly pretty fast." In some cases, the researchers were able to follow the bees to 100 or more flowers, and sometimes they were only able to record between five and ten movements.

"Running around after bumblebees in these beautiful wildflower meadows was one of the most fun parts of the research," Brosi says. Brosi's team was mostly made up of Emory undergraduate students, and the project was funded by the college's Scholarly Inquiry and Research at Emory (SIRE) grants and NSF support via the Research Experience for Undergraduates (REU) program.

Exacting non-destructive methodologies are required at the Rocky Mountain Biological Laboratory so researchers don't have a negative impact on the bumblebee populations. "When we caught bees to remove target species from the system, or to swab their bodies for pollen, we released them unharmed when our experiments were over," Brosi says. "They're very robust little creatures."

Brosi adds no researchers were harmed in the making of this study, either. Stings were very uncommon during the experiments. Bumblebees are quite gentle on the whole."

The research team saw a cascading effect of removing one species across the steps of the pollination process, from patterns of bumblebee visits to plants, to picking up pollen, to seed production. Around 78 percent of the bumblebees in the control groups were faithful to one species of flower, while only 66 percent of the bees in the manipulated groups showed such floral fidelity. The reduced floral fidelity in the manipulated plot groups meant the bees in those groups carried more different types of pollen on their bodies than those in the control groups.

Brosi said the study findings have direct implications for plant reproduction. When one of the bumblebee species was removed, the larkspurs produced about one-third fewer seeds compared to the flowers in the control groups.

"The small change in the level of competition made the remaining bees more likely to 'cheat' on the larkspur," Briggs says.

Previous studies revealed how competition drives specialization within a species. The new bumblebee study, however, is one of the first to show a link between this mechanism and the broader functioning of an ecosystem.

"Our work shows why biodiversity may be key to conservation of an entire ecosystem," Brosi says. "It has the potential to open a whole new set of studies into the functional implications of interspecies interactions."