Floral infidelity among bees rises when competition falls, study says

Removing just one species of bee from fields can make the remaining species less faithful to flowers, significantly lowering the pollination chances of plants, a study in Proceedings of the National Academy of Sciences suggests.

The research, conducted in subalpine meadows in the Rocky Mountains, appears to upend conventional wisdom that one pollinator is as good as another and fields won’t suffer from the loss of a species.

Ecologists were examining the notion of floral fidelity – the propensity of bumble bee species to be true to one source of nectar, in this case, the larkspur. Such fidelity is important to the plant, since bees are the dust mops of nature, carrying off pollen to fertilize another blossom, which ensures propagation of the plant.

Competition among species drives species into niches, and for bees, that may mean greater fidelity to one species over another. Wild bees tend to be generalists, and models assume that if one species is lost, another provides the same pollination service.

Removing a species of bumble bee, however, caused a significant drop in productive pollination - deposition of pollen from the same species. Remaining bees proved to be more promiscuous: foraging movements between plants of different species increased 156%, according to the study. From the perspective of the bee population, fidelity to one species dropped by nearly 78%.

“We found that the remaining bee species in the system did become more generalized,” said ecologist Berry J. Brosi of Emory University, in Atlanta, lead author of the study. “They were actually showing lower floral fidelity. Even over a single foraging trip, they were visiting multiple species of plants more often.”

Brosi and fellow researcher Heather Briggs, an ecologist at UC Santa Cruz, divided the meadow into plots a little bigger than tennis courts and got to work with nets. Some areas were left alone, while bees were netted from others and held in temporary captivity.

“We literally just run around and remove every individual of that species,” said Briggs. “We feel like we get around 95%-99% of them out. That’s possible because there are nests around the area, and once you deplete those numbers, maybe a few will come in, but that’s it for that area. So it’s actually feasible, even though it sounds crazy.”

Just in case, team members, who also hailed from the Rocky Mountain Biological Laboratory, in Crested Butte, Colo., patrolled the plots to keep the bumble bee species out during observation periods that stretched through the day. All told, they removed six of the 11 species in various plots.

The ecological service provided by wild bees has received heightened attention amid the collapse of colonies of domesticated honey bees, which are vital to pollination of crops nationwide. The cause of the sudden drop in population of those bees has been attributed to biological pathogens, compromised immune systems, pesticides and handling practices.

A study earlier this year showed that native pollinators are far more efficient, even when farmers rent colonies of domesticated honey bees. Another study in 2006 showed that wild bees can increase the effectiveness of honey bee pollination. A study this year suggested that production in a plant community could be increased with the introduction of new species, mainly by driving bees to greater floral fidelity – they covered their niches better.

Taken together, the studies suggest that current models of ecosystem function may be overstating how resilient an ecosystem may be to the loss of a species. Simulation models that test the notion tend to treat all pollinators as generalists, each making a fixed contribution, Brosi said. “By contrast, what our work shows is that interactions between species can actually shape the particular functional role that any one species can have.”

Rachel Winfree, a pollination ecologist at Rutgers University who was not involved in the study, said the work represented an important advance in vetting models with field work. In addition, she said, “it tests a very interesting and novel idea about how the balance between competition within, versus among, species could lead to counter-intuitive results.”