I loved to walk in the woods when I was a kid. My father took me there deer hunting, but it was the richness of animals and plants that captured my love. We call it biodiversity these days — the complex assembly of trees and shrubs, of deer and squirrels and raccoons and all the other creatures that live in a community like a forest.
It has always been a given with me that biodiversity is a good thing. But my strong, deeply-held feeling is one thing, a proper scientific judgement quite another. Is biodiversity actually good for a forest? Does having lots of different species make a forest community more stable, more able to return to normal after a disruption?
This is the sort of question that ecologists love to argue about. Theoretical ecologists (those that juggle equations rather than get their feet wet) had proposed all sorts of reasons why diverse communities should be more stable. Then in the early 1970s, prominent ecologist Robert May argued just the reverse, that diversity has no consistent effect on community stability. Who is right?
Six years ago, a massive field test seemed to settle the issue. David Tilman of the University of Minnesota, St. Paul, working with what must have been a small army of students, carefully marked off hundreds of small plots of grassland, each about a square meter. They then counted the number of prairie plant species in each plot. Some were diverse (that is, species-rich), others not. Now comes the important part. Every year for eleven years they repeated the counts, and estimated the total biomass of all the plants in each plot.
Were the more diverse plots more stable? Yes. Plots with more species showed less year-to-year variation in biomass, were less affected in drought years, and were more resistant to invasion by new species. Importantly, increased biodiversity also seemed to lead to greater productivity. In short, Tilman concluded, biodiversity is good for ecosystems.
Although Tilman’s experiment was widely acclaimed at the time, an outspoken group of ecologists has been critical of its validity and relevance. The Tilman experimental design is critically flawed, his result a statistical artifact, they claim. The error, well known in statistics, is called “sampling effect bias” — the more species you add to a mix, the greater the probability that you will by chance add a highly productive one. To show a real benefit from diversity, these critics argue, experimental plots would have to exhibit “overyielding” — the productivity of a diverse plot would have to be greater than that of the single most productive species grown in isolation.
A new study has been carried out in an attempt to resolve these problems. Called BIODEPTH, it took two years and involved 34 scientists from eight European countries. They found that in artificially constructed plots containing up to 32 species, productivity rose with diversity, just as in Tilman’s experiment. However, few plots exhibited overyielding without sampling effect, and the few instances of legitimate overyielding were in plots to which researchers added legumes (which fertilize the soil). “There’s no evidence from this experiment that 200 species is any better than 50 species,” concludes critic John Huston of Oak Ridge National Laboratory.
Experiments are now underway by Tilman and others that will attempt to demonstrate overyielding with many species, not just a few. I will be delighted if Tilman is proven right. In the meantime, ecologists continue to argue the issue heatedly. Science is most healthy when most contentious, it is said.
Ecology must be healthy indeed. While researchers argue about the merits of these experiments, outright war has broken out among ecologists over a pamphlet distributed last fall by the Ecological Society of America to Congress and federal agencies touting the benefits of biodiversity. The pamphlet, which Tilman helped write, describes his experiments and conclusions, with little mention of the objections raised to its design.
When are scientific data strong enough to form the basis of policy decisions? Clearly the established scientists running the Ecological Society of America judge Tilman’s case to be a strong one, and probably wanted to avoid diluting the force of their message to Congress. On the other hand, the objections to his experiments were — and are — serious. Ecologists will soon sort this out in the field, experimentally. In science, there is nothing like a good fight to move things along.
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