Biodiversity Helps Keep Ecosystems Healthy

Article excerpt

For years, prominent biologists and conservationists have campaigned for the preservation of biological diversity, despite little proof of their assertion that reducing the number of plant and animal species upsets nature's balance. Now, two experimental studies illustrate the detrimental effects of species loss.

More than 10 years ago, ecologist David Tilman of the University of Minnesota in St. Paul and his colleagues began investigating how as many as 250 kinds of plants could thrive in midwestern grasslands, even though the flora competed for limited resources. They did not expect to address the question of the value of biodiversity, Tilman says.

For this experiment, the researchers created 207 4-meter-square plots distributed among one native prairie and three abandoned fields of different ages.

Each season, they clipped a different 0.33-square-meter section in each plot and analyzed its species composition and biomass--the weight of leaves, stems, and flowers combined. They left some plots alone and added specific amounts of nitrogen fertilizer or other nutrients to others.

In 1987, less than 300 millimeters of rain fell, down from an average of 450 mm. The next growing season brought just 200 mm. The more diverse the plant community, the less its productivity declined during these dry years and the faster it rebounded. These changes "provided data that have not been available before," Tilman says.

During the dry spell, plots with nine to 23 species in the clipped sections produced half as much as normal. With fewer than nine species, the plot's productivity declined precipitously. Those with just one or two types of plants dropped to one-eighth normal, note Tilman and John A. Downing of the University of Montreal.

The plots with many species regained their productivity a year later; those with five or fewer species took more than four years to recover, the researchers report in the Jan. 27 NATURE.

"[This work] demonstrates clearly that species diversity can make a difference in ecosystem4evel characteristics," comments Peter M. Vitousek, an ecologist at Stanford University.

Under the right conditions, a field with just one species can produce as much biomass as one with many plants. This observation had led some to suggest that the existence of lots of kinds of plants doesn't add much to the health of an ecosystem because those species are "redundant. …