Jana Vamosi, Ph.D, postdoctoral associate at the University of Calgary and Tiffany Knight, Ph.D., assistant professor of biology in Arts & Sciences at Washington University in St. Louis, and their collaborators have performed an exhaustive global analysis of more than 1,000 pollination studies which included 166 different plant species and found that, in areas where there is a great deal of plant diversity, plants suffer lower pollination and reproductive success. For some plant species, this reduction in fruit and seed production could push them towards extinction.
One reason that pollen becomes limiting to plants in regions of high diversity may be increased competition between the plants -- there are more plant species vying for the services of pollinators. Also, when there are a lot of species around, plants become more separated from other individuals of the same species, causing pollinators to have to fly long distances to deliver pollen. When pollinators do arrive, they may deliver lots of unusable pollen from other plant species.
Knight and her colleagues found this pattern to be especially true for species that rely heavily on pollinators for reproduction -- those that require outcrossing -- and for trees, in relation to herbs or shrubs, because individuals of the same species tend to be separated large distances when species diversity is high.
To test for pollen limitation of each plant species, scientists added pollen to a number of plants and compared them with control plants that were pollinated naturally. Vamosi, Knight and their colleagues created a database of more than 1,000 pollination experiments conducted worldwide.
"If pollinators are doing a good job, you wouldn't expect a treatment effect," Knight said. "But for some of our plants we saw a huge treatment effect. We saw that a lot of the plants are incredibly pollen-limited.
"Biodiversity hotspots, such as tropical rainforests, are a global resource -- they are home to many of the known plants used for medicine and may be a source for future cures ,and they absorb huge amounts of carbon dioxide and generate volumes of clean oxygen. Our research suggests that plants in these areas area also very fragile. They already suffer from low pollen receipt, and future perturbations of the habitat may exacerbate the situation."
According to Knight, there is no doubt that a reduced number of pollinating species - bees, flies, birds, even bats -- is one contributor to pollen limitation. But it's not the only one. Habitat fragmentation is a proven cause of pollen limitation, as well as development.
"These findings have global implications given the importance of biodiversity hotspots for medicine, food, nutrient cycling, and alternative resources for pollinators of domesticated crops world wide," said collaborator Tia-Lynn Ashman, Ph.D., of the University of Pittsburgh.
"The concern is that we are losing habitat really rapidly globally, especially in tropical areas, and losing pollinators there as well," Knight said. "We show that these areas are sensitive to pollen limitation just because they are diverse. Any perturbation in the tropical areas -- and there are lots right now - is going to hurt the situation even more than we think and perhaps drive certain species to extinction."
Vamosi, Knight and collaborators Janette A. Steets of the University of Alaska Fairbanks, Susan J. Mazer of the University of California, Santa Barbara, Martin Burd of Monash University, Melbourne, Australia and Tia-Lynn Ashman of the University of Pittsburgh, published their results in the Jan.16, 2006 online issue of PNAS. The study was supported by the National Science Foundation's National Center for Ecological Analysis and Synthesis