According to a study published in today¿s issue of Nature, preserving the earth¿s biodiversity is not just a nice idea¿it is necessary to ensure ideal functioning of the planet¿s ecosystems. Michel Loreau of the Ecole Normale Superieure in Paris and Andy Hector of Imperial College in London analyzed data collected from BIODEPTH, an international experiment run on more than 500 grassland sites located in seven European countries, to calculate the relative importance of complementary interactions between plant species.

In the mid-1990s, researchers planted BIODEPTH sites with varying numbers and types of plant species and functional groups¿ classifications used by ecologists to describe the role species play in an ecosystem¿to mimic the effects of species extinction. Loreau and Hector, borrowing techniques from evolutionary genetics, devised a new equation to isolate the effects of two competing mechanisms to explain the effects of species diversity on ecosystem productivity. The first, the "sampling," or "selection," effect, states that as the number of species increases, the probability that a random sample will include a more productive species also increases. In other words, certain individually productive species are what's important. The second effect is based on species complementarity¿it says that the more species present, the more likely it is that cooperation between species will lead to the most efficient use of resources.

The new approach, christened the Loreau-Hector equation, showed that complementarity effects are more important. In the analysis of BIODEPTH data from 205 sites, the average value for the selection effect was zero. The average complementarity effect, however, was significantly positive, suggesting that the specialization of different plant species to different roles fundamentally affects the way that ecosystems work.

"Previous justifications for conserving biodiversity have taken in aesthetic and ethical reasoning: that we like some of it and that it is wrong to let it go extinct," Hector says. "Here we suggest, along with the findings of other ecologists, that there is another, complementary reason to preserve diversity¿it plays a role in determining the way the environment works."

In an accompanying commentary, Osvaldo E. Sala of the University of Buenos Aires cautions that because the grasslands sampled in BIODEPTH were necessarily disturbed on a regular basis, they may be less affected by the loss of species than other environments. "Species complementarity may act even more strongly," he writes, "in ecosystems that have been disturbed less often and have a longer evolutionary history."