Grazing Animals May Reverse Man-Made Damage To Grasslands Around The World

By Shweta Iyer on March 9, 2014 2:18 PM EDT

pronghorn
As humans tamper with the world's natural grasslands, added fertilizers and other nutrients cause certain grasses to overpopulate, leading to less biodiversity. In an effort to increase biodiversity, researchers have found that allowing wild animals to graze unhindered might help. (Photo: Oregon Department of Fish & Wi)

Grasslands occupy more than one-quarter of the Earth's land area, spreading over six continents. As bio-diverse ecosystems, they support the growth of several indigenous plants and animals. But unfortunately, overuse of fertilizers in the past decades have accelerated ecological decline throughout the unique landscapes. Now, a team of scientists have come up with a solution to conserve and protect the biodiversity of the grasslands: Allowing animals to graze and chew out the overgrown grass.

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More than 50 scientists who study grasslands worldwide, known as the Nutrient Network (NutNet) conducted a five-year comparative study on the grasslands, and came up with the solution. The study, which will be published in the journal Nature, was carried out at 40 different sites around the world. "This study has tremendous significance because human activities are changing grasslands everywhere," said study co-author Daniel S. Gruner, associate professor of entomology at the University of Maryland, in a statement. "We're over fertilizing them, and we're adding and subtracting herbivores. We have a worldwide experiment going on, but it's completely uncontrolled."

The grasslands are dotted with a number of native plants, however, human activities, such as burning fossil fuels, adding fertilizers, and dumping farm animal manure into the land have added extra nitrogen and other nutrients into the grassland soil. This excess in nutrients has led certain grasses to overtake others, as they sap the nutrients faster and grow bigger — leading to not only fewer nutrients but also less sunlight. In addition to these factors, grasslands around the world are also being converted into pasture land for domestic animals, which have begun to outnumber other wild grazers like elk and antelope.

Allowing grazers to eat the grass, thus reversing the effects of over-fertilizing, had been theorized but never tested. To test the theory, the NutNet scientists created four test plots in each of the 40 sites. The first test plot was fenced, to prevent animals from grazing. The second was sprayed with fertilizers to replicate current farming practice — using fertilizer — with animals being allowed to graze. The third plot was both fenced and fertilized, and the last one was left alone.

In all the test sites the population of animals was unmonitored. So, in some places there were a lot of native grazers while others had more domestic animals.  Some sites had been previously grazed by cattle but were now abandoned.

The researchers found that the fertilized plots that didn't have grazing animals also had a lower diversity of plants, while fertilized plots with grazing animals showed an increase in plant diversity. The most improvement was observed in plots where large, wild and domestic animals were allowed to graze. These animals included cattle, pronghorn, and elk on North America's Great Plains; wildebeests and impala on Africa's Serengeti; and horses, sheep, and ibex in rural India. Places where only small animals like rabbits, voles, and gophers grazed did not show much improvement.

With these results, the researchers proved that grazing animals improved biodiversity by increasing the amount of light reaching the ground. "Where we see a change in light, we see a change in diversity," said lead author Elizabeth Borer, of the University of Minnesota, in the statement. "Our work suggests that two factors, which humans have changed globally, grazing and fertilization, can control ground-level light. Light appears to be very important in maintaining or losing biodiversity in grasslands."

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