Berry J. Brosi, an assistant professor at Emory University in Atlanta, and Heather M. Briggs, a graduate student at the University of California, Santa Cruz, both in environmental science, studied 20 plots of meadow at the Rocky Mountain Biological Laboratory in Crested Butte, Colo., each about 22 yards on a side.

With help from a number of students they tested what happened if they removed the most populous bumblebee species by catching them with butterfly nets, and patrolled the plots to keep them out.

The prevailing view, Dr. Brosi said, based on mathematical models, was that all the other bumblebee species would take up the slack and the plants would do fine. But that was not so for the tall larkspur, a lovely purple wildflower.

Researchers found that the remaining bumblebees became less faithful to one flower species than they had been before the removal of the most numerous bees. They took advantage of less competition to play the field, or the plot.

All well and good for the bees, at least in the short term, but the larkspur, which the researchers targeted for this study, did not do as well because bees that once would have stuck with the larkspur now carried pollen from a variety of other flowers when they visited their once exclusive floral partner.

But the larkspur needs pollen from its own species to reproduce and suffered for the bumblebees’ wayward ways.

Because of the unfaithful bees, the researchers reported Monday in Proceedings of the National Academy of Sciences, plants produced about 30 percent less seed. The finding, they report, shows a surprising effect from a loss of biodiversity that could have implications for a variety of ecosystems.

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