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Biodiversity: A Productive Way to GROW

November 1996

Burning rainforests, beleaguered spotted owls, and dying oceans--they are all symbols of the worldwide loss of species, charismatic totems of a biodiversity crisis that conservationists have been warning the public about for years.

Yet what good, really, are all these species? Do we really need the earth's great bounty of biodiversity?

The answer is a resounding "yes!" according to a recent study at the Cedar Creek Natural History Area in Minnesota, one of NSF's Long Term Ecological Research (LTER) sites. There, a team of researchers led by David Tilman, ecologist at the University of Minnesota, found that when it comes to the health of an ecosystem, the more, the merrier.

Their large-scale study was conducted on experimental prairies--individual plots with different numbers of species. Through the study, the researchers were able to correlate number of species with total plot production. The greater the number of species in an experimental plot, the greater the actual total mass of plant produced, that is, the higher the yield.

It was not only "plant biomass" or actual heft of plant produced that improved with increasing numbers of species. Researchers also found that plots with higher numbers of species were more effective in retaining nitrogen, the most crucial nutrient in Cedar Creek soils. These high-diversity plots retained the nutrients that might allow them to continue to be productive in future seasons as well.

A key feature of the study's design was its focus on the number of species in a plot, without regard to specific kinds of species. In fact, species selection was done randomly.

The study has verified the ecological importance of high species numbers and the dangers of low species numbers. Related research on native prairies has led to similar results. The work is being seen as a coup for conservationists looking for solid science to back up their claims about the importance of biodiversity. Just as significant, the study has been described as a breakthrough by research biologists hoping to learn more about natural habitats.

"It's a brilliant experiment and it's a first," says Samuel McNaughton, an ecosystem ecologist at Syracuse University in New York. "Nobody's ever done what they've done."

Peter Kareiva, an ecologist at the University of Washington in Seattle, agrees, saying, "This is really the first experiment that is unequivocal, clear, and striking."

The findings may also eventually be of interest to those managing ecosystems, such as agricultural fields or forests, for harvest. In the LTER study, researchers found that in prairies every additional species increased a plot's yield. However, in previous studies in agricultural systems, single-species plantings have often shown the greatest yields. So while the prairie study suggests intriguing possibilities, the jury is still very much out as to exactly when and under what conditions more species will lead to greater productivity.


Tilman says that, despite the broad implications of their work, it was the biodiversity crisis itself that set the new study in motion.

"It wasn't until we knew how rapidly species were going extinct that this issue really came to the forefront," says Tilman. "Ecologists, such as Stanford's Harold Mooney, started asking, would the number of species in an ecosystem affect its functioning?"

But while interest reawakened in this question--once pondered by Charles Darwin himself, the father of modern evolutionary biology--answers have remained elusive for some time.

More than a century ago, in his book The Origin of Species, Darwin suggested that more species would make an ecosystem more productive. Like Tilman and colleagues, Darwin based his conclusions on work related to grasslands. He referred to an unidentified study in which fields planted with several grass species yielded more than those planted with only one--a 100-year-old forerunner of the newest research.

But while many modern-day scientists, like Darwin, suspected early on that more species might make natural habitats more productive, researchers struggled to test the idea rigorously. The rigorousness of Tilman's field study is what sets it apart.


Some of the early studies were done under highly controlled conditions in the laboratory, bringing into question whether these findings would hold up under conditions in natural habitats. Other studies, some by Tilman's group, while very suggestive, were not designed specifically to test the effect of high or low species numbers and so could not speak to the issue directly.

The design of Tilman and colleagues' latest experiment was simple: Construct a large set of miniature prairies and plant each with anywhere from one to 24 species. Wait for them to grow and see which plots did best.

While simple in theory, carrying out this massive experiment required a back-breaking amount of work.

First, researchers had to burn, plow and plant 147 100-square-foot plots. Then, assisted by an army of well-trained undergraduates, researchers had to keep all of the plots weeded to maintain them as designed.

Balancing on boardwalks that allowed them to reach plots without actually stepping in them, field workers weeded throughout the summer months. Squinting at countless tiny sprouts of seedlings, these field hands had to distinguish between the many undesirable plants whose seeds were still in the soil or had been blown in from other plots and the desirable species that should be left in that particular plot.

So, while one plot was to have nothing in it but brown-eyed susans or bunch clover or yarrow, the plot next door might need to be entirely free of that particular neighboring species.


While researchers still don't know exactly why it is that more species make these prairie plots more productive, Tilman suspects that the answer has to do with a more efficient use of the available resources of sun and soil.

"Each species differs from the others in a variety of traits," says Tilman. "Some have high water requirements and grow well during the cool part of the year. Others grow well when it's really warm and dry. Each one in the system does what it's good at, if you will, but there's always something left to be done. There are always resources for other plants to use."

In this way, if some species don't do well, for any reason, when there are lots of species in the plot, one of the others should grow and fill the plot in. Researchers say that may explain why, in more diverse plots, at least some plants were likely to be growing lush and green whatever the circumstances.


While researchers expressed excitement over the new study, they noted that much work remains to be done.

"It's a building block," says Peter Vitousek, an ecosystem ecologist at Stanford University in Palo Alto, California, "but it's not the final answer."

With the experimental prairies only two years old, researchers say there may well be surprises to come as the plots grow into greater maturity.

While the miniature prairie plots provided a Herculean task of sowing and weeding, Tilman and others did not let this deter them from setting up an even more gargantuan experiment.

At the same site, they have planted a series of bigger plots, each some 14 yards on a side, the size of a comfortable, suburban backyard. Tilman says this should allow researchers to take a better look at the effects of yet other factors, including how plots with different numbers of species fare under attack by insects or disease.

"It's an enormous amount of work," says McNaughton of the continuing study. "A regular researcher with a three-year NSF grant could not have done this. This really shows the importance of long-term research and Tilman would not have been able to do this without NSF funding through the LTER program."

Tilman invites others to come take advantage of these plots--hard-won experimental systems now nicely established--as they can be used to search for answers to an endless number of questions, even those as yet unimagined.

"We are setting these up as a national resource for others to come in and pursue questions they find exciting," says Tilman. "It's like setting up a telescope or cyclotron. It's an investment that will lead to its greatest return when it is shared by many, many people."

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