By Gail Gallessich

A new study of grasslands conducted at the Sedgwick Natural Reserve in the Santa Ynez Valley sheds light on how human activities can decrease biodiversity. The study shows that activities, such as adding water and nutrients, decrease species diversity in an ecosystem. The research was published in the March 25 online version of the journal Nature.

First author W. Stanley Harpole, a member of a working group at UCSB’s National Center for Ecological Analysis and Synthesis (NCEAS) and currently a postdoctoral scholar at UC Irvine, explained that the reduction in species diversity occurs because increasing the amounts of limiting resources, such as nitrogen and water, makes an ecosystem more homogeneous and reduces the number of competing species.

Put another way, the inputs reduce the number of niches, allowing a few species to dominate.

Jim Reichman, director of NCEAS, explained that the results are important scientifically because they reveal a distinctive biological mechanism to explain patterns of diversity in natural plant communities. “The results are particularly important in habitats like California grasslands where the delicate balance between available resources and the ability of plant species to obtain those resources have been disrupted by past human activities,” he said.

The findings are based on experiments in which combinations of nutrients (nitrogen, phosphorous, and cations) and water were applied to plots of grassland. Plots that received all of the resources had the fewest species and highest productivity.

The researchers compared this result with analysis of the 150-year-old Rothamsted Park Grass Experiment, the oldest ecological experiment in existence. Both supported their hypothesis.

“Our results show that the loss of plant species due to nutrient pollution can persist for more than 100 years.” Harpole said. “Human actions that simplify habitats can lead to long-term loss of biodiversity.”

Co-author G. David Tilman, professor of ecology at the University of Minnesota, explained, “It shows that plant diversity is directly related to the number of limiting factors (such as soil moisture, nitrogen, phosphorus, potassium, and water).”

Harpole said the data helps explain why grasslands, lakes and rivers that are polluted with nitrogen and phosphorous (usually from agriculture) have fewer species. The so-called “dead zone” where the Mississippi River empties into the Gulf of Mexico is one of the best-known examples.