MORE than 100 years ago, Charles Darwin had concluded his book The Origin of Species by declaring that the stability of an ecosystem 'was related to the diversity of life forms present therein. According to him, a grassland would yield a higher quantity of grass if more varieties of grass rather than a single species, were to be sown. Scientists from the Universities of Minnesota in the US and Toron1o in Canada, under the leadership of David Tilman, an ecologist working with, the former, now claim to have confirmed Darwin's theory. The researchers, having studied experimental prairie grasslands, found that the greater the number of species a plot held, the more was the biomass produced and the better was the retention of nitrogen - an essential plant nutrient. Though conservationists had long believed in this theory, there was very little scientific data to support their stand.

The team working with over 50 people burned, plowed, planted and tended 147 plots at the Cedar Creek Natural History area in Minnesota. The 100-sq foot plots were sown with upto 24 native prairie species each. A year later, each plot as examined to ascertain the ' growth, yield and nitrogen- consumption pattern. It was found that the plots planted with more species, yielded greater amounts of biomass and retained more nitrogen in the process of growing. The grass in the plots with fewer species showed-poor growth and leached large amounts of nitrogen from the top soil. As a result, the roots of the plant suffered from nitrogen deficiency, and on the whole, the plants lacked sustained growth. Tilman and colleagues confirmed their findings by studying, natural grasslands too.

As to the question of how exactly biodiversity leads to greater productivity and sustainability, once again Darwin's hypothesis has proved correct. He had explained it by saying hat competition between various species is avoided because different plants utilise resources in different ways, resulting in greater quantities of plant mass.

But the contentious issue that remains to be solved is the number of species which are actually required to do the job. Precisely how much is 'many'? According to some, relatively few would do. Tilman's study found that there was a dramatic increase in productivity till the different plants numbered 10, but after that, the addition of each new plant resulted in lesser productivity. Even though this trend supports the 'few-would-do' theory, Tilman insists that there was an increase with each, new addition even if it was not very significant. There are others like Peter Karieva of the University of Washington in Seattle, us, who, support the use of many species. According to Katieva, an ecosystem has to perform many functions and more plants may prove helpful to achieve the same.

Although studies conducted by others in this field have not been as conclusive as this one - either because of doubts about the plot's size or the levels of nitrogen retained - even this latest experiment has not been very enthusiastically received by some.' Says ecologist, Peter Vitousek at the Stanford University in Palo Alto, California, us, "I think it is a building block, but not the final answer. The application of this theory in practice is yet to take shape 4s the researchers work out which hypothesis would best help in the management of multiple agricultural crops or forest species."