Plants face extinction threat due to lack of sex competition among pollinators
Rhett A. Butler, mongabay.com
January 16, 2006
The decline of birds, bees and other pollinators may be putting plants at risk of extinction according to a new study.
The National Science Foundation-sponsored study analyzed hundreds of field studies investigating fruit production in hundreds of wild plant species found in the world’s “hotspots”—the planet’s most diverse ecosystems.
The research found that ecosystems with the greatest number of species, including the tropical rainforests of South America and southeast Asia, have larger deficits in pollination compared to the less-diverse ecosystems. Scientists who took part in the study say that plant species in biodiverse hotspots may be continually faced with new competitors and cannot evolve as rapidly as their environment changes. If this is the case, then pollen limitation may be a chronic problem for species in biodiversity hotspots. Otherwise the high level of pollen limitation may result from habitat changes. Nevertheless, the finding is troubling for global biodiversity conservation.
“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 Susan Mazer, co-author of the article and a professor of biology at UC Santa Barbara.
Two news releases from sponsoring institutions appear below.
Increased sexual competition among flowering plants in biodiversity hotspots may lead to extinctions
University of California – Santa Barbara
16-Jan-2006
The decline of birds, bees and other pollinators may be putting plants of the world’s most diverse ecosystems at risk of extinction, according to a new study that analyzed hundreds of field studies investigating fruit production in hundreds of wild plant species.
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The finding raises concern that more may have to be done to protect the Earth’s most biologically rich areas.
The meta-analysis was sponsored by the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara and was funded by the National Science Foundation. The article, “Pollination Decays in Biodiversity Hotspots,” reporting the results, is published in the January 17 issue of the Proceedings of the National Academy of Sciences and is available online at the academy’s website at www.pnas.org.
The analysis shows that ecosystems with the greatest number of species, including the jungles of South America and Southeast Asia and the rich shrubland of South Africa, have bigger deficits in pollination compared to the less-diverse ecosystems of North America, Europe and Australia.
“This is truly a synthetic work,” said Susan Mazer, co-author of the article and a professor of biology at UC Santa Barbara. “Our detection of global patterns required the simultaneous analysis of many studies conducted independently by plant ecologists all over the world.” The meta-study analyzes 482 field experiments on 241 flowering plant species conducted since 1981. The study took several years to complete and all continents except Antarctica are represented.
“This analysis can tell us things about ecological processes at the global scale that individual studies are not designed to tell us,” she said, noting that the synthesis could not have been done 25 years ago because few careful field studies of this type had yet been conducted. Each individual study represented in the meta-study is highly labor-intensive and species-specific.
“The global pattern we observed suggests that plant species in species-rich regions exhibit a greater reduction in fruit production due to insufficient pollination than plant species in regions of lower biodiversity,” said Mazer. The investigators suggest that high biodiversity “hotspots” are characterized by stronger competition among plant species for pollinators, such that many plant species simply do not receive enough pollen to achieve maximum fruit and seed production.
“Many plants rely on insects and other pollen vectors to reproduce,” said first author Jana Vamosi, an evolutionary biologist and post-doctoral research associate in the University of Calgary’s Department of Biological Sciences. “We’ve found that in areas where there is a lot of competition between individuals and between species, many plants aren’t getting enough pollen to successfully reproduce. If plants can’t survive, neither can animals. These biodiversity hotspots are important because they are where we most often find new sources of drugs and other important substances. They are also the areas where habitat is being destroyed the fastest.”
Mazer cautioned that it is not yet possible to determine whether the high level of pollen limitation observed in species-rich areas is a new phenomenon or a long-standing one. It may be a recent problem due to habitat fragmentation or destruction, or it may be a long-term phenomenon. Plant species in ecologically-complex biodiversity hotspots may be continually faced with new competitors and simply cannot evolve as rapidly as their environment changes. If this is true, then pollen limitation may be a chronic problem for species in biodiversity hotspots — a challenge with which they have coped for millions of years.
“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 Mazer.
She described the typical field study included in the meta-analysis as one that looked at the likelihood that a flower will develop into a fruit —— with or without supplemental pollination. Experimenters compared untouched plants, those that were naturally pollinated, to those to which pollen was added by hand. If the plants that received added pollen showed increased fruit production then it is clear that the naturally pollinated flowers were not getting enough pollen to achieve maximum fruit production.
She also noted that studies revealing the relationship between local species diversity and inadequate pollination at the global level can be detected only by looking at hundreds of species. “Fruit and seed production in some species may be especially vulnerable to low pollinator activity,” she said. “But it’s the global pattern that’s particularly important, not the individual species.”
Other authors of the PNAS article besides Vamosi and Mazer include Tiffany Knight at Washington University; Tia-Lynn Ashman and Janette Steets at the University of Pittsburgh; and Martin Burd at Monash University in Melbourne, Australia.
Study: Competition for sex is a ‘jungle out there’
Washington University in St. Louis
16-Jan-2006
Mother Nature could use a few more good pollinators, especially in species-rich biodiversity hotspots, according to a new study in the Proceedings of the National Academy of Science (PNAS).
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
These are two modified news releases from the University of California, Santa Barbara and Washington University in St. Louis.