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Syrphid fly feeds on nectar of California sunflower, helping to pollinate it. (Courtesy: University of California, Santa Barbara)

Plants growing in rainforests and other biological hot spots may face a greater risk of extinction because of a decline in birds, bees and other pollinators, according to a new study.

As the number of insects, birds and bees decreases worldwide, flowering plants are facing greater competition for the sexual services of those pollinators, which help the plants reproduce.

Researchers who looked at 482 field trials from all continents – except Antarctica – concluded many plants lacked enough pollen to reproduce properly. Most of the trials compared naturally pollinated plants to those receiving pollen by hand.

Jana Vamosi, a researcher at the University of Calgary led the study, which was published in this week's online issue of the Proceedings of the National Academy of Sciences.

"If plants can't survive, neither can animals," Vamosi said.

The global analysis found ecosystems with more species, such as the jungles of South America and Southeast Asia, had more pollen problems compared to less diverse habitats on northern continents.

Scientists usually think of diversity as helping to stabilize ecosystems, but that protective effect doesn't appear to be happening when it comes to pollination.

The problem may arise because competition among plant species for pollinators is greater in biodiversity hot spots, and tropical plant species aren't able to evolve as rapidly as their environment changes, the researchers suggest.

"The pattern raises the alarm, however, that species in species-rich regions face two challenges that increase the risk of extinction: habitat destruction, which is occurring at alarming rates in the tropics, and reduced pollinator activity," said study co-author Susan Mazer, a biology professor at University of California in Santa Barbara.

Biologists can't yet tell if the problem is new or long-term, or what's causing it. More research is planned to see if plants are becoming separated from others of the same species, forcing pollinators to fly longer distances to deliver pollen.

The pollinators also face threats from habitat loss, as well as pesticide use, invasive species and the extinction of vertebrates.