Scientists may have discovered why excessive fertilizer use leads to a loss of species diversity in grasslands, according to a study conducted by researchers from the University of Zurich, Switzerland, and published in the journal Science.
Rain washes fertilizers off of the fields where they are applied and into neighboring ecosystems, leading to an increase in the nutrient content of those soils. For some time, scientists have been aware that this causes biodiversity of grasslands to plummet.
"You would think that more [nutrients] would lead to more biodiversity," said study co-author Andrew Hector. "Yet it is considered to be one of the main threats to biodiversity this century."
Researchers have put forward two main hypotheses to explain this seemingly paradoxical effect.
"One is that the presence of more resources led to a general increase in the strength of competition among plants," Hector said. "The other is a little bit more mechanistic. When you get an increase in fertilization, 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."
Hector and colleagues fertilized boxes containing various grasses, then fitted the understory of some of these boxes with lights. They found that adding these lights prevented the loss in diversity typically seen in fertilized fields.
This led them to conclude that fertilizers cause fast-growing grassland species to shade out other, more slow-growing species.
Hector noted that the study offered no solutions for the problem, other than the one scientists have known all along: reduce fertilizer use. "What our research shows is that competition for light is very asymmetric," he said. "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 emphasizes how important it is that we control nutrient enrichment."
