By Mark Kinver
Science and environment reporter, BBC News
Excess nutrients have been found to have an adverse impact on diversity
Scientists have identified why excessive fertilisation of soils is resulting in a loss of plant diversity.
Extra nutrients allow fast growing plants to dominate a habitat, blocking smaller species' access to vital sunlight, researchers have found.
As a result, many species are disappearing from affected areas.
A team from the University of Zurich, writing in Science, warned that tighter controls were needed in order to prevent widespread biodiversity loss.
Estimates suggest that the global level of nitrogen and phosphorous available to plants has doubled in the past 50 years.
Looking at grasslands, the researchers said it was widely recognised that an increase of chemical nutrients in an ecosystem led to a loss of diversity, but the mechanism of how it was occurring had been difficult to determine.
"You would think that more [nutrients] would lead to more biodiversity," said co-author Andrew Hector, a researcher at the University of Zurich's Institute of Environmental Sciences.
"Yet it is considered to be one of the main threats to biodiversity this century."
'Winner takes all'
Professor Hector explained that there were two main hypotheses: "One is that the presence of more resources led to a general increase in the strength of competition among plants.
The study showed that understory lighting halted plant diversity loss
"The other is a little bit more mechanistic," he told BBC News.
"When you get an increase in fertilisation, you get an increase in productivity, leading to increased plant biomass and increased shading.
"This shifts the idea to light being the critical resource, with shorter species being shaded out by taller species, resulting in a loss in diversity."
Professor Hector's team, led by PhD student Yann Hautier, fitted lights to the understory of grass in boxes containing fertilised soil.
"Additional understory light compensated for the increased shading caused by the greater above-ground biomass production," they explained.
The supplementary light "prevented the loss of species and maintained
levels of diversity".
The findings led the team to conclude that it was the lack of access to light that affects diversity, not an increase in the strength of competition.
"We have done the critical experiment that has been asking to be done for the past 35 years," said Professor Hector.
"If it all depends on light levels, then if you put the light back then you should prevent a loss of biodiversity."
However, he added that their findings did not offer a "magic bullet" for conservationists.
"What our research shows is that competition for light is very asymmetric.
"So if a plant can get between the sun and its competitors, not only can it get all the light it needs but it can also block its competitors' access to light.
"Because this competition for light is such a 'winner takes all', it emphasises how important it is that we control nutrient enrichment."