MINNEAPOLIS / ST. PAUL--In seven years of experiments, University of Minnesota ecologist David Tilman and colleagues have pried into the hidden interactions among plant species. After three years, experimental results suggested that land planted with many species produced more vegetation than land with one or a few species. Greater vegetative production means higher returns from land and expands its ability to store carbon as biomass, rather than atmospheric carbon dioxide. Their early results were controversial, however. Some scientists have asked whether the association between diversity and productivity might be a "sampling effect"; for example, plots with more species have a higher chance of containing plants that naturally produce well. Such questions led to a major debate about biodiversity and whether the effects of biodiversity loss were relevant to public policy. In work to be published in the Oct. 26 issue of Science, Tilman and his colleagues present evidence that such "sampling effects" are not significant, and diverse ecosystems likely do better because differences among species allow such ecosystems to utilize resources better.

The researchers found that no plant species growing by itself could do at all as well as some mixtures of several species. This disproved the "sampling effect" conjecture, which said that effects of diversity came solely because there would be a species which, although growing alone, could perform as well as all diverse mixtures of species, Tilman said.

"Some thought that if diversity was having its effects for simple reasons, like the sampling effects, then a manager could do just as well with a single species," he said. "In fact, however, when many species are together, they do better as a system than any species does alone. As the number of species goes up, more and more plots do better than the single species that does best in monoculture."

These results suggest that managed systems like hayfields, p
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Contact: Deane Morrison
morri029@umn.edu
612-624-2346
University of Minnesota
25-Oct-2001

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