March 11, 2014
Grazing Animals May Protect Biodiversity In Over-Fertilized Grasslands
April Flowers for redOrbit.com - Your Universe Online
The Nutrient Network has released a new, comparative study of grasslands on six continents, to be published in an upcoming issue of Nature, that suggests it might be possible to counteract the human-made overdose of fertilizer that threatens to permanently alter the biodiversity of the world's native prairies.
The key finding of this study, carried out at 40 sites globally, is simple: let grazing animals crop the excess growth of fast growing grasses that can out-compete native plants in an over-fertilized world. According to the researchers, the herbivores—grazing and browsing animals—will feed on tall grasses that block sunlight from reaching smaller plants closer to the ground.
"This study has tremendous significance because human activities are changing grasslands everywhere," said Daniel S. Gruner, associate professor of entomology at the University of Maryland. "We're over-fertilizing them, and we're adding and subtracting herbivores. We have a worldwide experiment going on, but it's completely uncontrolled."
Gruner has been a member of the Nutrient Network (NuNet) since its founding in 2006. He helped to plan the worldwide study and analyze the data. Gruner worked with Elizabeth Borer of the University of Minnesota and more than 50 other international scientists on the study.
Between one- and two-fifths of the planet's land area is grassland, according to the UN Food and Agricultural Organization estimates. These grasslands are also home to one-tenth of the human population of the planet. Like all plant communities, however, grasslands are suffering from an overabundance of fertilizer.
Humans introduce extra nitrogen and other nutrients into the soil, air and water when they burn fossil fuels, dose crops with chemical fertilizers, and dispose of manure from livestock. This nutrient excess creates challenges for the grasslands, where many plants, like annual wildflowers and others, have adapted to low nutrient levels. Grasses that use the extra nutrients to grow faster and bigger are providing competition and beating out the low nutrient level plants.
Another challenge is the fact that grasslands worldwide are being converted to pastureland for domestic animals. Native grazers, such as elk and antelope, are giving way to domesticated animals, such as cattle and sheep.
Gruner said that current ecological theory suggests that native grazers can counteract the effects of over-fertilizing in most cases. This theory has never been broadly tested, however. To achieve this, the NuNet scientists marked off test plots in groups of four at each of 40 sites worldwide. At each site, one plot was fenced to keep grazing animals out, one was treated with a dose of fertilizers—to mimic the effects of excess nutrients—but was not fenced. The third was both fenced and fertilized, while the fourth was left natural.
The animal populations at the test sites were not altered. At some of the sites, the native animals were abundant, while at others they had been mostly replaced by domestic animals like cattle, goats and sheep. Additional sites were former pastures where livestock had browsed, but were no longer present.
The team found that, in general, the variety of plants in the experimental plots decreased where fertilizer was added and grazing animals were kept out. Plant diversity generally increased in the plots where animals were allowed to graze and fertilizer was applied. According to the data analysis, biodiversity was improved by the grazers by increasing the amount of light reaching ground level.
A variety of strategies have been developed by the grassland plants to take advantage of a location where nutrients are in short supply and inconsistently available. Gruner explained that such plants might be ground-hugging, or ephemeral, or shoot up when they capture a nutrient pulse. The differences between these strategies create the diversity of the grassland ecosystem.
However, in a human-altered world with plentiful nutrients, plants that have evolved to grow tall to capture the sunlight have an advantage. These plants block most other species from gaining sunlight, limiting their growth and reproduction. The grazing animals help stop this process by cutting down the light-blocking plants and giving the others a chance to grow and bloom.
"Where we see a change in light, we see a change in diversity," said Borer. "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."
The greatest effect was observed where large animals, both wild and domesticated, grazed on the test plots. For example, 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. At sites where only small animals like rabbits, voles of gophers grazed, the effect was weak and variable.