20 January 2006
Birds Ain't Doing It, Bees Ain't Doing It, And Biodiversity Is The Victim
By Rusty Rockets
Biologists who have just concluded analyzing years of detailed and painstaking observations of flora and fauna have released alarming findings concerning the likely future of biodiversity on our planet. The findings show a widespread decrease in pollinators such as birds, bees and flies, which means that plants in species-dense areas are not getting enough pollen to reproduce.
The study's team leaders, Jana Vamosi, Susan Mazer and Tiffany Knight, believe that if the current state of affairs continues in species-rich hotspots, plant extinctions are unavoidable. The researchers proffer a number of possible reasons for the current parlous state of biodiverse hotspots, but as yet they are still unsure as to whether this is a recent phenomenon or whether they are simply witnessing something that has been occurring for millions of years; a situation that reflects the lack of existing knowledge in this area.
Vamosi, Mazer, Knight performed an exhaustive global analysis of more than 1,000 pollination studies that included 166 different plant species. Their study, appearing in the Proceedings of the National Academy of Sciences, found that plants suffer lower pollination and reproductive success in areas where there is considerable plant diversity. The analysis shows that ecosystems with the greatest number of species - including the jungles of South America and Southeast Asia and the rich shrub land 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, a professor of biology at the University of California, Santa Barbara. "Our detection of global patterns required the simultaneous analysis of many studies conducted independently by plant ecologists all over the world." Mazer said their meta-study analyzed 482 field experiments on 241 flowering plant species conducted since 1981. The work 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.
A typical field study compared plants that were naturally pollinated to those to which pollen was added by hand. If the plants that received human intervention showed increased fruit, then it was clear that the naturally pollinated flowers were not getting enough pollen to achieve maximum fruit production. "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."
For some plant species, this reduction in fruit and seed production caused through lack of pollination could drive them towards extinction. The team found this pattern to be especially true for species that rely heavily on pollinators to assist with outcrossing (seeding a flower's stamen with pollen sourced from another flower of the same species) for reproduction, because individuals of the same species tend to be separated by large distances when species diversity is high. This separation means that pollinators have to fly long distances to deliver pollen, and when they do arrive, they may deliver lots of unusable pollen from other plant species.
Not being able to outcross means that extinctions are a real likelihood. While it is possible for plants to self-pollinate (selfing), this alone does not progress or strengthen the species, as, like any other living organism, a plant needs genetic variation in order for the species to survive as a whole. In short, selfing does not deliver the genetic variation that may increase the fitness of a plant's progeny.
The new study does not bode well for life globally, as many of the so-called biodiverse hotspots are home to many valuable organic compounds, used for medicines and other applications. "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 are also very fragile. They already suffer from low pollen receipt, and future perturbations of the habitat may exacerbate the situation," said Knight. That's a scenario that would not auger well for human progress, but Knight's comment also implies another explanation for this drastic state of affairs.
It seems that we humans like to shoot ourselves in the foot every so often, as many of the biodiverse hotspots also happen to be areas where habitat is being destroyed either directly or indirectly through human intervention. "Pollinators are on the decline globally because of habitat loss and destruction, pesticide use, invasive species, and extinction of vertebrates," said co-researcher Tia-Lynn Ashman. "The concern is that we are losing habitat really rapidly globally, especially in tropical areas, and losing pollinators there as well," added Knight.
Despite the gloomy outlook, Mazer cautioned that it is not yet possible to determine whether the problem of high-level pollen limitation observed in species-rich areas is a new phenomenon or a long-standing one. 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, commented Mazer. "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. So it would appear that arguing over whether plants and animals in biodiverse hotspots are victims of human destruction or the cruel whims of nature is a moot point. As far as the research stands currently, it seems that the birds and the bees, the flowers and the trees are facing both, with one exacerbating the other.