When skies grow gray and winds turn cold, Pittsburghers get romantic notions about tropical locales. But in some of the lushest, most biodiverse spots on the planet, the natives -- the plants, that is -- are starving for sex.

Plants in the tropics simply aren't getting pollinated as much as they would like. The supply of birds, bees and other critters that serve as pollinators can't keep up with the demand.

That's what biologists at five universities, including the University of Pittsburgh, conclude in a global analysis of pollination published this week in the Proceedings of the National Academy of Sciences.

Ecologists have feared that many pollinators may be on the decline because of pesticide use, habitat loss and invasive species. In that case, a shortage of pollinators may mean that many plant species in the tropics may be pushed toward extinction.

But Tia-Lynn Ashman, associate professor of biological sciences at Pitt, emphasized that's only one possibility. It may be the fierce competition between plants for pollination is the evolutionary force that drives the creation of so much diversity.

"Is what we found demonstrating that pollinators are on the decline globally . . . or is this simply a consequence of so many competing species?" Dr. Ashman said. "We really don't know."

The study, led by biologist Jana Vamosi of the University of Calgary, was the first to try to assess the competition for pollinators worldwide. To do so, they performed what is called a meta-analysis -- combining the data from more than 1,000 smaller scale studies of pollen supply-and-demand from every continent, with the exception of Antarctica, into a single database.

The findings come at a time of growing concern that pollinators may be on the decline. European scientists have documented declines in bumblebees and other pollinators. The U.S. Fish and Wildlife Service lists more than 50 pollinator species as threatened or endangered. According to the Ecological Society of America, wild populations of honeybees -- the most important agricultural pollinator in the United States -- have declined 25 percent since 1990.

But as environmental crises go, "this one seems to have a fairly low profile," said May Berenbaum, chair of the entomology department at the University of Illinois at Urbana/Champaign. She chairs a National Research Council panel now assessing the status of pollinators in North America.

The panel, which is due to issue its report this summer, was initiated by the perception that pollinators are on the decline, Dr. Berenbaum said, but subsequently expanded its charge to determine whether that is, in fact, the case.

A shortage of pollinators could have broad consequences, however. Most flowering plants require some sort of aid in pollination and more than 150 U.S. food crops require pollinators.

Unlike Europe, monitoring data on pollinators has not been widely available for North America, she explained.

"We're not just talking insects," Dr. Berenbaum emphasized. Insects account for the vast majority of pollination, but creatures such as bats and hummingbirds also contribute significantly. Worldwide, more than 100,000 invertebrate species and more than 1,000 vertebrates, including reptiles, play a role.

"It's a pretty complex problem," Dr. Ashman said.

In the tropics, for instance, the new study was not able to determine whether pollen limitation was the result of declines or is a long-term condition that both causes and is exacerbated by the great diversity of plants. A shortage of pollinators, Dr. Ashman explained, can cause plants to evolve in ways that make themselves more attractive to a particular pollinator, or to change the timing of their flowering so that there is less competition for pollinators.

"What we don't know is whether the high biodiversity causes fierce competition or whether the fierce competition leads to high biodiversity," she said. If the shortage of pollinators is a recent phenomenon, however, it may cause the demise of species."

"We know that they are suffering from inadequate pollination services," Dr. Ashman said. "What we don't know is the ultimate consequence."