Grassland Biodiversity can be Maintained by Grazing Herbivores

Fertilizing plots of land can limit the diversity of plants that grow there as species less-able to tolerate a lack of sunlight are literally overshadowed by plants that react quickly to the fertilizer. But a new study has found that by allowing herbivores to graze on the land, they can offset the loss of plant biodiversity.

In a study that compared vegetation data across 40 sites on six continents, a research team led by Elizabeth Borer and Eric Seabloom of the University of Minnesota found that biodiversity across grasslands can be shaped by fertilizing and controlled grazing.

The research plots contained by fertilized and unfertilized soils, were both fenced and unfenced, and were spread areas diverse enough that grazing by sheep, deer, zebra and kangaroos were all considered. These plots were established in 2005 and since then, researchers have been measuring the amount of plant material grown, the amount of light reaching the ground and the number of species growing in the plots.

Fertilizer adds nutrients like nitrogen or phosphorous to the soils, and some plant species respond to the nutrients by increasing in size quickly, while others absorb the nutrients, but still grow slowly. If the slow-growing plants do not keep up with the taller ones, they can be cut off from the sunlight they need to grow.

The researchers found that on both fertilized and unfertilized plots of land plant biodiversity increased when the plots were grazed upon by herbivores. Additionally, this held true across all locations and was not dependent of the type of herbivore grazing the land.

"Biodiversity benefits humans and the environments that sustain us. Understanding how human actions control biodiversity is important for maintaining a healthy environment," said Borer. "What this suggests is that these two impacts, which are ubiquitous globally, dovetail with changes in light availability at the ground level, and that appears to be a big factor in maintaining or losing biodiversity in grasslands. In short, where we see a change in light, we see a change in diversity."

The research sheds light on how humans are impacting biodiversity across a variety of landscapes. Through demonstrating how fertilization, grazing, and biodiversity are linked, the researchers said new steps can be taken to ensure the world's grassland ecosystems remain healthy and thriving.

"Global patterns of biodiversity have largely defied explanation due to many interacting, local driving forces," said Henry Gholz, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the coordination of the research. "These results show that grassland biodiversity is likely largely determined by the offsetting influences of nutrition and grazing on light capture by plants."

Borer and her colleagues published their work in the journal Nature.