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This article originally provided by Science and Technology

June 6, 2006

Biodiversity aids biological life

The role of biodiversity in stabilising ecosystems is a reality to be believed, says S Ananthanarayanan.

Prof David Tilman along with others at the University of Minnesota put current beliefs about the importance of biodiversity to the test, as reported recently in the journal Nature. Ecologists have long suspected that biodiversity affects various aspects of ecosystem functioning, but there has been a great debate among academic researchers because we lacked rigorous experimental tests, Prof Tilman said in a communication.

Many species growing together help each other to maintain nutritional value of the soil as well as in combating parasites. If a plantation consists largely of a species that draws more of particular nutrients from the ground, those nutrients are likely to be depleted. But if there is a variety of species, then the mix of nutrients drawn would stabilise after some time. And the mechanisms that replace the nutrients, like nitrogen fixation and seepage, can keep the level of nutrients adequate for the different dependant species.

If one species is higher in number, the particular nutrients it used most, would begin to reduce and the growth of the species would slow down. The mix of species in the forest would thus remain stable so long as conditions like rainfall, sunlight, soil quality remain unchanged. Nutrition apart, a specialised bio-system, like a plantation of only teak, is a sitting duck for parasites. A natural forest has the diversity of mutually supporting species, co-operating both against predators as well as in sustaining the nutritional viability of the land.

A parasite that selectively attacks teak would find the presence of other species as an impediment to undisturbed multiplication. Similarly, the teak would protect other species against parasites that threaten those species. If one species increased out of proportion, these very parasites would get active and keep down the population of the runaway species.

The mix of species in a natural forest is in continuous adjustment for optimum survival of the mutually co-operating species. Naturally occurring, wild strains of cereal and other food plants are thus likely to survive ups and downs of climate and pests. A cultivated strain, specialised through farming, on the other hand, has scant defence against a sudden epidemic, with the use of fertilizers or insecticides.

The study

Prof Tilman and his team at the Cedar Creek Natural History Area, Minnesotta, divided a seven hectare field into 168 plots of 9m x 9m each. The plots were seeded with 1,2,4,8 or 16 different perennial grassland species - the mix of seeding being repeated at different levels of diversity.

The experiment started in 1995 and data collection continued for 10 years. Strict controls were in place for the growth conditions. The plots were burned every year in spring before growth began and the biomass generated was assessed, with controls for statistical rigour, every August.

The study covered the short term and longer term effects of different conditions, of the presence of weeds and of periods of drought. The data collected has established that greater numbers of plant species lead to greater temporal stability of an ecosystems above-ground plant production. "Our results indicate that reliable and sustainable supply of some foods and biofuels can be enhanced by the use of biodiversity, says the report.


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