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Grazers mitigate biodiversity loss caused by fertilization
07 March 2014
Swiss Federal Institute for Forest, Snow and Landscape Research WSL
Grazers help to counteract the loss of biodiversity caused when fertilizing grassland ecosystems. By feeding mainly on tall-growing plant species, herbivores promote the growth of small-growing ones as the light supply increases. These findings were just published in the scientific journal Nature by an international team of researchers including staff from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and the University of Zurich.
Fertilization is responsible for a considerable biodiversity loss in grassland ecosystems worldwide. Fertilizers promote fast- and tall-growing herbs and grasses at the cost of all other plant species, as competition for the typically limited nutrients decreases. The study published in the scientific journal Nature shows that fertilization reduces the competition for nutrients in the root layer, but increases the aboveground competition for light, since with fertilizer sunlight becomes the most important factor that limits plant growth.
A single plant functional type benefits
Fertilization primarily favors fast- and tall-growing plants. At the WSL research site Val Müstair (Grisons) an example for such a species is the blue monkshood (Aconitum). This tall-growing plant shades all smaller-growing species, thus reducing their light supply and therefore causing their dieback. "A sharp decline in biodiversity is the result. This is a worldwide phenomenon that does not only affect grasslands but without exception all ecosystems, including marine and freshwater systems", said Anita Risch from WSL. Once an ecosystem is dominated by tall-growing plants it depends on permanent nutrient and water supply. Changing environmental conditions, for example droughts, become a problem, as the single dominant plant functional type cannot adapt to these changes. Consequently, such ecosystems lose their stability and might even collapse during extreme environmental events.
Grazers may help
The study shows that grazers can mitigate biodiversity loss in fertilized systems, as they generally prefer to feed on nutrient-rich, tall-growing plants. Cropping of these plants results in improved light availability for smaller-growing species, which facilitates their co-existence with the tall-growing ones and stabilizes the grassland ecosystems despite the high level of available nutrients.
An international research network
The Nutrient Network is a grassroots ecological experiment. Research teams from five continents collect data, e.g. in African savannahs, North American prairies and high-alpine pastures. Two of the five experimental sites in Europe were set up in Switzerland. Anita Risch and Martin Schütz from WSL sampled their data in the Val Müstair; Yann Hautier from the Institute of Evolutionary Biology and Environmental Studies at the University of Zurich at Zugerberg near Zug. "It is highly motivating for a scientist to collaborate with more than 70 teams of enthusiastic researchers in a worldwide project, particularly when minimal efforts are needed for coordination and logistics", said Martin Schütz from WSL.
In theory, two plant species growing together compete for the same limited resources above- and belowground. While their roots compete for water and nutrients (especially nitrogen and phosphorus) in the soil, the aboveground plant shoots compete for light. Light quantity may be manipulated in artificial systems such as greenhouses, whereas the quantity of sunlight can not be altered in natural systems. Nutrients, however, do increase worldwide in all ecosystems, either intentionally by using agricultural fertilizers to enhance food or forage production or unintentionally by nutrient emissions from agriculture, industry and burning fossil fuels.