Some tropical plants pick the best hummingbirds to pollinate flowers

Being picky may increase access to genetic diversity and thus give the plants a competitive advantage over their neighbors, but there is a risk, the researchers say. If the preferred pollinators decline for any reason, the plants may not reproduce as easily and could decline as well.

These findings stem from the discovery that the showy red and yellow blooms of Heliconia tortuosa, an exotic tropical plant, recognize certain hummingbirds by the way the birds sip the flowers' nectar. The plants respond by allowing pollen to germinate, ultimately increasing the chances for successful seed formation.

Researchers from Oregon State University and the Smithsonian Institution announced their results in this week's issue of the Proceedings of the National Academy of Sciences, a professional journal. "To our knowledge, these findings provide the first evidence of pollinator recognition in plants," they wrote.

Matt Betts, an associate professor in the Oregon State University College of Forestry is the lead author. Adam S. Hadley, also at Oregon State, and W. John Kress of the Smithsonian Institution are co-authors. The National Science Foundation provided support for the research.

In experiments at the Las Cruces Biological Station in Costa Rica, Betts and Hadley began by trying to pollinate Heliconia plants by hand. Although such methods are commonly used in plant propagation, the researchers were puzzled by their lack of success. So in an enclosure known as an aviary, they exposed Heliconia to six species of hummingbirds and a butterfly. The team discovered that two types of hummers -- violet sabrewings and green hermits -- achieved more than 80 percent success in fertilizing the plants.

By controlling the sources of pollen, the researchers excluded the possibility that fertilization could be explained by specific birds carrying higher quality pollen.

"The ones that turned it on tended to have long curved bills that could reach the nectar," said Betts, who works in OSU's Department of Forest Ecosystems and Society. "The ones that couldn't turn it on had shorter bills and couldn't get as much nectar."

By modifying their hand pollinating methods to mimic birds extracting nectar, the researchers were able to achieve similar success in fertilizing the plants. "That closed the loop on the mechanism," said Betts.

The two most effective hummingbird species also shared another characteristic: Compared to five other species, they tended to travel more widely across the landscape. The researchers hypothesized that since far-ranging species tend to collect pollen from more distant plants, the pollen would exhibit more genetic diversity and enhance the plant's competitive fitness.

Pollen from nearby plants could come from close relatives and thus have reduced genetic diversity, the authors wrote.

"The mechanism may have evolved to enable the plant to sort out pollinators that are likely to be carrying high-quality pollen from those carrying poor-quality pollen," added Betts. "It's a big energy savings. If you bother to make a seed and fruit every time you get pollen, that's a lot of energy expenditure; you could be making a seed from your siblings' genes. If you make a seed or fruit only from distant high-quality pollen, it could be an adaptive advantage."

Examples of co-evolution of plants and pollinators have been known since Charles Darwin's day, but the mechanisms that underlie these networks are poorly understood. It's possible, Betts said, that other examples of pollinator recognition could occur in tropical forests.

"It is now well-known that the high cognitive capacity of many vertebrate pollinators allows them to recognize and specialize on particular flower species," he and his co-authors wrote. "A growing body of research indicates that plants may also exhibit complex decision-making behavior."

Betts has conducted research for six years at Las Cruces. The results of this and other studies there, he said, suggest that the integrity of these ecosystems could depend on maintaining corridors to enhance pollinator movement and survival. In some areas, tropical forests have been broken up into smaller fragments as development and agriculture have expanded.

"We need to be more careful in how we manage landscapes in order to maintain the movements and occurrence of these key species," he said. "We know that if we make corridors to connect patches, if we have bigger patches of tropical forests, those species will be maintained, and this plant and its pollinators will do a lot better."