Mark Kinver, BBC News 1 May 09;
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 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."
Why more fertiliser harms plant diversity
Helen Thomson, New Scientist 1 May 09;
The 35-year-old mystery of why fertilisers decrease biodiversity has finally been solved. The secret? They increase competition for sunlight.
Adding fertilisers to grasslands increases the productivity of plants, but leads to a major drop in biodiversity.
The amount of nitrogen and phosphorus available to plants has doubled in the last 50 years, but the reason why this has harmed diversity has not been easy to answer. The debate has centred on whether fertilisers increase competition above or below ground – for sunlight or soil resources?
To resolve the argument, Yann Hautier and Andy Hector from the University of Zurich, Switzerland built their own experimental plant community from scratch.
The light fantastic
Hautier's team grew 32 plant communities for four years, before transferring them to a glass house.
Each community comprised four different sets of six species. Half were fertilised, the others were left unfertilised. Half of each of these sets had light added, using a system of three fluorescent tubes that were raised as the canopy grew, while the other half were left to grow in normal light conditions.
After two years, the sets that were fertilised in normal light conditions showed a significant increase in productivity and biomass, but lost around one-third of their species diversity compared to the unfertilised groups. Those that were fertilised and given additional light showed no significant loss of diversity.
Underground competition
To uncover whether underground competition for root space had any influence on biodiversity, the team added two new species of plant at the beginning of the second year. The roots of half these plants were contained in plastic tubes, which prevented any below-ground competition: the other half were left exposed.
Removing below-ground competition from fertilised plots had no detectable impact on the mortality of the seedlings, compared to those that were exposed to full root competition, says Hautier.
"In the fertilised groups without additional light, there was no difference with or without the root-tube – they died both ways. Even if we remove competition below ground, these plants are unable to grow."
Drew Purves, a computational ecologist from Microsoft Research Cambridge is impressed with the team's findings.
"This is a rare example of a simple experiment providing an unambiguous answer to an important ecological question. If these results are general to temperate grasslands – which seems likely – then we can start to develop more targeted policies to offset one of the most important sources of diversity loss in grasslands."
Journal reference: Science, DOI: 10.1126/science.1169640
