Earth’s wild grazers can’t keep up with effects of nutrient pollution

Human activity is transforming natural environments everywhere on Earth. Nutrients – such as nitrogen and phosphorus – that are produced through burning fossil fuel, using fertilizers in agriculture, and other activities are boosting plant growth in grasslands and other ecosystems. The additional plant growth from nutrient pollution can lead to increased fire risk, loss of native species and invasion of non-native species. Wild herbivores eat some of the plant growth, but not enough to counteract these negative impacts according to a new study from the University of Minnesota published in Nature Communications.

The study was based on a globally replicated experiment conducted at 58 sites on six continents led by Elizabeth Borer, a professor in the College of Biological Sciences and co-founder of the Nutrient Network global ecological research collaborative. Working with scientists from around the world who are contributing to the Nutrient Network, the research team was able to conduct exactly the same experiment at each site involved in the experiment.

At the same time as humans are adding nutrients, people are also changing the abundance and types of herbivores in landscapes, with shifts on every continent that often displace and reduce wild herbivores such as zebras, guanacos and reindeer while increasing domestic grazers like cattle, goats and sheep.

“Our goal is to measure the impacts of two of the most important impacts humans have had on the Earth’s ecosystems – increasing supplies of limiting nutrients, such as nitrogen and phosphorus, and changing the density of grazing animals,” said Borer. “By building a collaborative network of scientists, we have been able to conduct globally relevant research, generating new insights across continents.”

“While all of these changes are happening on nearly every continent, they are not happening at the same rate in all locations,” said Borer, who notes that the question at the heart of the study is whether environments around the world will respond in a similar way to these changes.

The study shows that wild grazing animals around the world can partially reduce some of the impacts of excess nutrients by eating some of the extra plant growth, but they cannot keep up completely. Consequently, unintentional nutrient pollution will continue to have negative impacts on grasslands around the world.

Not every location is experiencing the same effects, though. The results also suggest that the capacity of wild grazers to control excess plant growth will decline as regions become wetter or more fertile, with likely negative consequences for biodiversity and wildfires.

“Our results suggest that grazing by sheep, cattle and other domesticated animals may counteract the impacts of nutrients on grasslands,” said Eric Seabloom, a coauthor of the study and co-lead of the Nutrient Network. “but we’ll need to do more work to understand the role of domestic grazers and how to use them to improve the health of grassland ecosystems.”

Funding for the study was provided by the National Science Foundation, the Institute on the Environment and the College of Biological Sciences at the University of Minnesota.

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