Too much of a good thing (nutrients or water) actually decreases the diversity of species in an ecosystem while it increases the productivity of a few species, according to a grassland experiment conducted by University of Minnesota researchers.
The reduction in species diversity occurs because increasing the amounts of limiting resources, such as nitrogen and water, makes an ecosystem more homogeneous and consequently reduces the number of opportunities for competing species to coexist. Put another way, it reduces the number of niches, allowing a few species to dominate.
The study, conducted by David Tilman, Regents Professor of Ecology, and Stanley Harpole will be published March 25 in the online version of the journal Nature. Harpole, who is now a postdoctoral associate at the University of California, Irvine, was a graduate student at the University of Minnesota when the research was carried out.
"In essence, the data in the article strongly supports a new explanation for why the world contains so many species," said Tilman. "It shows that plant diversity is directly related to the number of limiting factors (such as soil moisture, nitrogen, phosphorous, potassium and water)."
It also helps explain why grasslands, lakes and rivers that are polluted with nitrogen and phosphorous (usually from agriculture) have fewer species. The reduction of species where the Mississippi River empties into the Gulf of Mexico is one of the best known examples of this phenomenon.
The findings are based on experiments carried out at the University of California's Sedgwick Reserve in the Santa Ynez Valley, where the researchers applied combinations of nutrients and water to plots of grassland. Plots that received all of the resources had the fewest species and highest productivity. They combined this with analysis of the 150 year old Rothamsted Park Grass Experiment. Both supported their hypothesis.
"Our results show that the loss of plant species from a habitat due to nutrient pollution can persist for more than 100 years," Harpole said. "Thus human actions that simplify habitats can lead to long-term loss of biodiversity."