1930년대 이후 옥수수와 콩이 미국의 초원을 쓸어버렸다

America’s prairies are shrinking. Spurred on by high commodity prices and a biofuels rush, farmers are digging up grasslands in the northern Plains to plant crops at the quickest pace since the 1930s. While that’s been a boon for farmers, the upheaval could create unexpected problems.

Map showing the percentage of grasslands that were converted into corn or soybean fields between 2006 and 2011.

A new study by Christopher Wright and Michael Wimberly of South Dakota State University finds that U.S. farmers converted more than 1.3 million acres of grassland into corn and soybean fields between 2006 and 2011, a period of soaring crop prices and biofuel mandates (right). In states like Iowa and South Dakota, some 5 percent of pasture is turning into cropland each year.

It’s a big transformation in the heart of the country: The authors conclude that the rates of grassland loss are “comparable to deforestation rates in Brazil, Malaysia, and Indonesia.” And those changes are already having plenty of impacts.

For one, farmers are now growing crops on increasingly marginal land. In Nebraska, North Dakota and South Dakota, corn and soy are planted in areas that are especially vulnerable to drought. But farmers take the risk because corn and soy have become so lucrative — and, in part, because the federal government offers subsidized crop insurance in case of failure. (The study also finds evidence that many farmers are no longer enticed by federal conservation programs that pay for grassland cover.)

The loss of pasture itself could also have big environmental impacts. Studies have foundthat grasslands hold carbon in their soil better than cropland does. So there’s a climate-change angle here. A 2008 paper in Science argued that fuels like corn ethanol and soy biodiesel lose a portion of their carbon advantage over gasoline if farmers are simply digging up virgin grassland to grow the crops.

There’s a wildlife angle, too: The Prairie Pothole Region, traversing Minnesota and the Dakotas, is one of the continent’s key breeding grounds for ducks and other ground-nesting birds. Tall grasses in the area help sustain a number of species and shield birds from predators. But corn fields are now encroaching on the habitat, and bird populations are dropping.

In recent years, some environmental groups have argued that it doesn’t make sense for the federal government to keep subsidizing this push into the prairies. A recent report (pdf) from the Environmental Working Group, for instance, argues that Congress should scale back crop insurance for farmers who move into the country’s grasslands and wetlands. Farm groups, for their part, say the insurance is vital for their work — instead, Congress should expand conservation programs.

And what about biofuels? Groups like EWG have criticized ethanol mandates for pushing up corn and soybean prices and driving the crop boom. There’s a lot more hope for next-generation cellulosic biofuels grown from switchgrass or other plants with a much smaller environmental footprint. Or biodiesel made from algae, say. But until those become viable, the crop rush continues.

Further reading:

–The full peer-reviewed study on grassland loss in the northern plains, published in theProceedings of the National Academies of Science.

–A longer dive into the debate over federal crop insurance.

–My colleague Steve Mufson explored some of the hurdles still facing cellulosic biofuels. And David Biello looked at attempts to make corn ethanol sustainable.