In just five years, Minnesota has lost 312 square miles of valuable wetlands and the natural vegetation that surrounds them — an area about 5.7 times the size of Minneapolis — reducing a resource that provides vital habitat for waterfowl, minimizes floods and keeps agricultural chemicals out of rivers and streams.

Between 2008 and 2012, rural landowners nationwide plowed up 11,300 square miles of wetlands and highly erodible land, most likely because of the high payouts that come with federally subsidized crop insurance, according to an analysis of land use trends released Tuesday.

The loss of wetlands was by far the greatest in the Dakotas and Minnesota — a total of 1,142 square miles — according to the Environmental Working Group (EWG), a national watchdog group that uses data to sway federal policy.

It is the latest in a series of studies to find that record prices for corn and other commodities are driving farmers to convert natural lands like prairie and wetlands at a rate not seen since the 1930s.

But EWG also implicated federal farm policy, noting that the highest rates of land conversion closely tracked the highest insurance payouts to landowners.

“We can’t prove cause and effect, but it’s very striking,” said Craig Cox, the group’s senior vice president for agriculture and natural resources. “We think it is more evidence that oversubsidized crop insurance is facilitating the conversion of these risky lands.”

An Environmental Working Group study shows that Minnesota is among the states that have converted the most wetlands to farmland, a practice that disturbs natural habitat and affects water quality.

Tom Zacharias, president of National Crop Insurance Services, said in an e-mailed statement that high crop prices, not taxpayer subsidized insurance, are driving the change. “It appears that both commodity prices and world demand for food and biofuel will remain high, which will send continued market signals to farmers to expand area to meet demand,” he said.

Congress debating subsidies

The EWG report was made public at a time when Congress is writing a new farm bill and debating whether subsidized insurance should be tied to protecting native prairies, wetlands and other sensitive lands. Historically, agricultural subsidies have been provided as long as farmers agree to protect the land by using conservation practices. But that has not been true of crop insurance, which in recent years has become the most widely used agricultural subsidy program. Now, the bills pending in Congress would eliminate virtually all other agricultural subsidies, and rely solely on subsidized crop insurance to help farmers manage risk.

Conservationists argue that the historic partnership between farmers and taxpayers should be part of the new farm bill.

Without it “there is an incentive for farmers to convert those lands,” said Eric Lindstrom, regional government affairs representative for Ducks Unlimited, a wildlife advocacy group.

The current House version of the farm bill does not tie insurance support to conservation practices. The Senate version would require participating farmers to leave wetlands alone, or replace them if they are drained for cropland. Similar rules would protect highly erodible land and native prairies.

At currents rates of conversion “half of the remaining prairies will be gone in 30 years,” Lindstrom said.

‘Duck factory’

EWG analysts used federal satellite images and other mapping technologies to analyze parcels converted to cropland. A comparison of 2008 to 2012 showed that 1.9 million acres of wetlands, including 500 feet of buffer vegetation surrounding each one, and 5.3 million acres of highly erodible land had been converted to crops, primarily wheat and corn.

Wetlands loss was “particularly dramatic” in South Dakota, North Dakota and Minnesota — the Prairie Pothole region known as the “duck factory” because it provides habitat and nesting grounds for more than half of North America’s migratory water birds. Those states accounted for 39 percent of wetland loss.

During that same period, EWG found, the average crop insurance payout nationally was $2.3 million per county. But in the 71 counties that lost more than 5,000 acres of wetlands and wetland buffers, the average payout was $10.1 million — more than four times the average.

Some cropland was converted back to its natural state, but far less than was turned into crops, Cox said.

The trends were parallel to those found in some other recent reports, including one published January in the Proceedings of the National Academy Sciences, that found startling rates of change on the landscape. Researchers said that in Minnesota, most of the converted land was wetlands.

And in May, the Minnesota Department of Natural Resources issued the first in a series of wetland tracking reports in Minnesota. Using a sample of 5,000 plots, it estimated that from 2006 to 2011 the overall wetlands area had increased by 2,000 acres. But there was a troubling trend: The increases were largely ponds or wet areas on farm fields that are farmed during dry periods. While these provide some environmental benefit — habitat for migratory waterfowl and reduced soil and nutrient losses — true wetlands, with natural vegetation and buffers around them, had declined by about the same amount.

To read the EWG report, go to www.startribune.com/a2409.