CEDAR CREEK, Minn. -- Ecologists have voiced concern in recent years about the disappearance of plants and animals around the globe. But controversy has raged over whether Earth's diversity of species is fundamental to the stable functioning of the planet's ecosystems.
Now, a study has produced strong evidence that biodiversity does increase the health and productivity of an ecosystem. And, in a sneak peek at what the world could be like in 2050, researchers have demonstrated that preserving more species could provide a greater natural cushion against environmental insults.
A team led by Peter B. Reich of the University of Minnesota focused on a previously unexplored relationship between species diversity and steep increases in nitrogen and carbon dioxide. Both are rising largely as a result of fossil fuel consumption and chemical use by farmers. Since the industrial revolution, carbon dioxide in the atmosphere has doubled and continues to increase about 5 percent annually. Nitrogen, which naturally cycles between the atmosphere and the living tissues in plants and animals, has also doubled.
We've gotten away with putting short-term economic considerations ahead of future environmental well-being for a long time. And that's going to be costly to us in the end
Peter B. Reich
University of Minnesota
In a field experiment outwardly resembling a kind of high-tech Stonehenge, Reich grew 16 native grasses and herbaceous plants in various combinations inside six large circular plots at the Cedar Creek Natural History Area, a 5,000-acre oak savanna near Minneapolis. The plots are ringed with white plastic "vent tubes" rising vertically on the perimeters. The tubes add carbon dioxide to the air within the circles and are regulated by a computer that adjusts for wind and other variables. Nitrogen fertilizer is also applied to the soil to help achieve a composite approximation of the enriched environment the researchers believe plants will grow in 50 years from now.
As expected, all of the groups grew better with increased nitrogen and carbon dioxide, both of which are essential to plant life. But the groupings that included all 16 species were significantly more productive than any combination of fewer species. More important, the most diverse plantings outpaced the most productive single species when they were grown alone -- an outcome called "overyielding" that plant ecologists have long considered the elusive Holy Grail in such biodiversity experiments.
"The interpretation of similar data by critics of previous experiments has been that a single, super species inevitably gets included in the most diverse plots and then dominates," Reich said. "We've shown that no individual species dominates any of our plots and that different species combine to increase overall productivity."
Reich likened the diversity effect to the difference between a standard basketball team and a squad made up only of lumbering centers. "The team with an assortment of player sizes and skills will be the better one," he said.
Plants in the most diverse groupings complement one another by using resources in different ways and at different times, and there are also "positive species interactions" among different plants, Reich said. These could range from complex nutrient exchanges that are not yet well understood to something as simple as taller plants providing needed shade for shorter ones.
Andy Dobson, an ecologist at Princeton University, called the study "a beautiful demonstration of the importance of biodiversity."
"This is big science," Dobson said. "We've learned more from Cedar Creek about how our planet works that is pertinent to us than we've learned from all the space shuttle flights put together."
Reich's work is a continuation of studies at Cedar Creek by David Tilman, also of the University of Minnesota and a co-author of a report on the findings in the April 12 issue of the journal Nature. Tilman's experiments showing that diverse plant communities are more resistant to environmental stresses such as drought have been at the center of a long-running feud among ecologists.
Michael Huston of the Oak Ridge National Laboratory, who has been the chief critic of the Cedar Creek work, said Reich's experiment is a big improvement over the earlier ones. But the results really show only that a few dominant species, primarily weeds, account for most of the productivity gains, Huston said.
"There's clearly a diversity effect here," Huston said. "But it does not show that you need a lot of diversity, just a few really highly productive species."
Huston insisted that the emphasis on productivity, which is simply a measure of the total plant mass in each grouping, is misplaced. In nature, he said, diversity does not equal productivity.
"I'm solidly in favor of preserving biodiversity," Huston said. "I'm just not convinced this experiment makes a strong case for it. What this says is that if our sole aim is productivity, we should plant just a few weeds and fertilize them."
But Reich's findings confirm what most ecologists already believe, said Joy Zedler of the University of Wisconsin. "It's hard to think that the results could have been any different," she said. "Diversity has to be important, and it's manifest in this experiment."
Zedler shares the concern that several of the plants that did well in the experiment were in fact "aggressive weeds."
"That's not surprising either," Zedler said. "It's a little scary, though." Many ecologists see a future Earth dominated by opportunistic "weedy species" of plants and animals that can rapidly adapt to changing environmental conditions.
The findings also point to an intertwining of cause and effect, especially with respect to carbon dioxide. Recent satellite data have confirmed the role carbon dioxide plays in global warming, and other evidence links global warming to declining biodiversity. Thus plants, which absorb nearly one-third of all carbon dioxide emissions, are at the same time at risk from carbon dioxide emissions.
"Nature is in effect 'scrubbing' carbon dioxide from the atmosphere for us," Reich said. "But we don't know if there's a saturation point, when suddenly all of the carbon dioxide we produce will stay in the air. And now we have learned that a less diverse biosphere will be less efficient at carbon dioxide absorption."
Reich said that the study proves an important principle but that further work is needed to show how it applies in natural systems. "We've tested a basic theoretical question," he said. "The magnitude of the effect may not be the same in nature, but I think we're likely to see a similar relationship. And that should be a concern.
"We've gotten away with putting short-term economic considerations ahead of future environmental well-being for a long time. And that's going to be costly to us in the end."
© 2001 The Washington Post Company