May 16, 2012

European settlement shifted methane emissions from wildlife to livestock

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By Amy Stewart / ASAS Communications

In our current industrialized culture, it can be difficult to imagine how different North America used to look. There are nearly 90 million beef and dairy cattle in the contiguous United States today, but that was not always the case. Before European colonization, many more elk, bison and deer dotted the landscape. And as they wandered the wilderness, these wild ruminants added to methane emissions.

Alexander Hristov, an associate professor in the department of dairy and animal science at Pennsylvania State University and author of a paper published in the May issue of the Journal of Animal Science, determined that the different demographics of ruminants in the contiguous United States has an impact on greenhouse gas emissions.

“In the big picture of total U.S. greenhouse gas emissions, livestock is a relatively small player, anywhere between 2.5 and 3 percent of the total greenhouse gas emissions,” Hristov said in an interview. “But on a global scale, livestock could be responsible for as much as 18 percent of the total greenhouse gas emissions.”

Hristov wanted to see how methane emissions from animals have changed over time. He used several estimates of the historical wild ruminant population to determine that methane emissions from these wild ruminants added up to about 86 percent of the methane emissions from current livestock. Today, wild ruminants only emit 4.3 percent of the methane that domesticated ruminants emit.

Finding the population of ruminants before European settlement was a difficult task.

“Assumptions had to be made because there are no reliable data for much of the calculations we did,” Hristov said. “I used the best estimates available, in my judgment, and my own calculations based on the work we do with domestic cattle.”

The Plains bison population was especially difficult to estimate. Hristov used low, medium and high estimates of bison numbers to calculate methane emissions. The largest estimate, 75 million, was from naturalist and writer Ernest Thompson Seton (1860-1946). The smallest estimate, 30 million, was from biologist Tom McHugh, who estimated how many bison the ecosystem could support. Hristov chose 50 million as a medium population estimate.

After European settlement, the population of wild ruminants declined dramatically. The population of bison was about 50 million before the 15th century, but there are only about 500,000 bison alive today. Elk populations declined from 10 million to 1 million, and mule deer and ­white-tailed deer declined from 43 million to 29 million during this period.

Before this study, there were few estimates of greenhouse gas emissions from wild ruminants before European settlement in the United States.

“What actually triggered this study was a comment on one of our previous papers that we should also think about what greenhouse gases wild ruminants were emitting before the continent was populated,” Hristov said.

This study was titled “Historic, pre-European settlement, and present-day contribution of wild ruminants to enteric methane emissions in the United States.” It can be read at The Journal of Animal Science is published by the American Society of Animal Science.

Scientific contact:

Alexander Hristov /

Media contact:

Amy Stewart / (805) 708-8561 /