Study: U.S. commodity farmers imperil biodiversity for ever-lower yields

In less than a decade, U.S. corn, soybean, and wheat fields wiped out an expanse of native grasslands and other ecosystems larger than the state of Maryland, according to a new analysis, destroying crucial wildlife habitat and spewing carbon dioxide into the atmosphere. And the new fields produced lower crop yields than existing farmland.

Farmers long ago plowed up the prairie’s richest soils — in what we now think of as the Corn Belt — but mostly avoided sloped or wet areas and stretches with poorer soils. Starting in the mid-2000s, however, farmers began cropping that “marginal” land as well. Using satellite data validated with farmer surveys and high-resolution aerial photography from the USDA, Tyler Lark, an agriculture researcher at the University of Wisconsin-Madison, and colleagues analyzed the expansion’s impact on farmers and wildlife.

From 2008 to 2016, more than 10 million acres of this marginal land were cropped for the first time in at least six years, the researchers found; much of it hadn’t been touched in decades. Ground zero for this expansion is a vast, wetland-strewn area known as the “prairie pothole” region, which stretches from northern Iowa through much of the Dakotas into northeastern Montana and southern Canada. Other hot spots include western Kansas, western Kentucky, and the North Carolina foothills.

The American Plains teem with biodiversity. A mix of hundreds of species of native grasses and flowering plants support a wealth of insects, birds, and other wildlife. Over millennia, these grasslands have soaked up carbon and stored it underground. Although the land is used — much of it as cattle pasture — its biodiversity and carbon have remained remarkably intact.

Lark’s team measured the impact on wildlife in several ways. Every year, monarch butterflies come to the Plains seeking milkweed, the only plant its caterpillars can eat. Lark’s team found that cropland expansion wiped out more than 200 million milkweed plants, potentially making it one of the top causes of the butterfly’s well-documented decline.

An estimated 138,000 potential nesting sites for ducks and other birds were also lost, the researchers found; the prairie pothole region has historically produced half of the nation’s waterfowl.

Though little noticed, this takeover of American grasslands represents one of the world’s most dramatic recent incursions into nature, comparable to the destruction of Brazil’s Cerrado savanna. “Temperate grasslands are the least protected, most converted biome on the planet,” says Martha Kauffman, managing director of the Northern Great Plains program at the World Wildlife Fund. 

What’s more, the newly planted land was worse than existing farmland for growing crops, Lark and his colleagues reported in their paper, published this month in Nature Communications. Yields on the converted land were, on average, nearly 7 percent lower than those on existing farmland.

“As far as I know, this is the first time someone has quantified what that loss of productivity is,” says conservation biologist Patrick Lendrum of the World Wildlife Fund, who was not involved in the study but who has also mapped grassland conversion. The message, says Lendrum, is that conservationists “should be focusing more on doing a better job of maximizing production on already converted land.”

Biodiversity isn’t the only thing at stake. When soils are plowed, much of their stored carbon is released as carbon dioxide, which is responsible for most global warming. In a separate study, Lark and his colleagues found that since 2008, cropland expansion has put as much carbon dioxide into the air each year as 31 million cars.

Several factors are driving farmers to plant marginal lands, experts say. The biofuels mandate, passed by Congress in the mid-2000s to boost corn ethanol, caused crop prices to skyrocket. Subsidy rates for crop insurance have also ballooned in the past two decades, making crop farming much less risky; the federal government now covers more than half the cost of a typical commodity grower’s insurance. And the USDA’s Conservation Reserve Program (CRP), which pays farmers to remove land from production, shrank while incentives to plant crops increased.

The confluence of the above factors and others has shifted the economic incentives strongly toward crops and away from cattle across a wide swath of the country. Some landowners in Kentucky, for example, can now rent land to commodity farmers for $200 an acre, versus $60 an acre for cattle, according to Chad Lee, an extension professor at the University of Kentucky.

Other, less obvious, factors have also played a role. Many American cattle farmers are aging out and selling or renting their land to crop farmers. The rise of no-till planting makes it easier to convert grasslands to crops, says farmer Lyle Perman. “You can apply Roundup on native grass and start no-till farming it literally the same year.”

To Perman, a fourth-generation farmer and rancher who owns Rock Hills Ranch in Lowry, South Dakota, the answer to the loss of grasslands is simple: more cows and fewer crops. When grazed at high intensity for short periods, cows eat the tops of grasses and other plants, spurring them to grow and keeping invasive grasses from crowding out native species. As important, cows give landowners and farmers a financial reason not to convert grasslands. “If you cannot figure out how to keep cows on the landscape, you’re in trouble,” Perman says. “You lose the cows, you lose water quality, you lose air quality, you lose species diversity.”

Some recent developments could help grasslands start to hold their own against crops. The 2018 farm bill partially restored CRP funding. The Ecosystem Services Market Consortium, an industry-NGO partnership, is building a marketplace to reward farmers for environmentally beneficial practices, including conserving grasslands.

But to convince farmers to stay off remaining grasslands and return marginal farmland to nature, more incentives are needed, Lark says.

“They really are the most endangered ecosystem in the world,” he says. “We need to stop the bleeding.”

For more on FERN’s coverage of grasslands and biodiversity, click here and here.