This piece is part of the Slate 90, a series that examines the multibillion-dollar nonprofit sector. Read all the Slate 90 stories here, and view the Slate 90 nonprofit rankings here.

The treeless, pond-specked plains of Iowa, Minnesota, and the Dakotas may not look like much to you. To a duck, however, this is more than flyover country. This turf is the start of the Prairie Pothole Region, the 275,000 square miles of land stretching from Des Moines to northern Alberta, Canada. Waterfowl enthusiasts have a more colorful name for it: the Duck Factory. Nearly 3 in 4 North American ducks are raised here, and droughts in the PPR can be closely tied to booms and busts in duck populations. As the Prairie Pothole Region goes, so goes the northern pintail.

Which is why one of the largest conservation groups in the United States, Ducks Unlimited (No. 10 on the Slate 90’s Environment and Animals list), has focused so much time and attention on a region not exactly synonymous with natural majesty, to say nothing of tourism. With revenues of more than $209 million in fiscal 2015, DU is a powerhouse in conservation. Since its 1937 founding, DU has conserved more than 14 million acres in North America—an area roughly equivalent to the state of West Virginia, covering wetlands north and south of the border. (Ducks do not respect borders, but conservation work has to: There are sister Ducks organizations in Canada and Mexico.)

Unlike the other mega-nonprofits that dominate American environmentalism, such as the Nature Conservancy or World Wildlife Fund, Ducks Unlimited’s membership—more than 700,000 people—is composed overwhelmingly of hunters. The more ducks there are, the more ducks there are to shoot. DU has company. In the U.S., there are fishermen’s conservation groups like Trout Unlimited (founded in 1959, with 300,000 members) and Salmon Unlimited. And there is Pheasants Forever (founded 1982, with 149,000 members), and its Quail Forever subsidiary. These are people who are more passionate about their prey than their weapons. “Hunters hunt because they love their quarry,” Jared Wiklund, the spokesman for Pheasants Forever, told me.

“In the eyes of hunters and anglers, conservation is wise use, and wise use is limits,” argues Nick Wiley, DU’s chief conservation officer. The organization does not advocate on bag limits, seasons, and other government-imposed restrictions on hunting. “It’s about habitat, and wetlands, and making a difference there. Ducks is our calling card and our focus, and we’ve got a strong support base from people who love ducks.”

The nexus between hunting and conservation is as old as environmentalism itself, a dormant strain of the movement focused less on wilderness and more on the active, managerial use of land. “It’s not the sensibility of leaving alone a pure and untouched nature,” explains Benjamin Heber Johnson, a historian of the environmental movement. “It’s managing and interacting with the nature you use very self-consciously, and seeing that it’s a sustainable relationship in the long run. Sure, some places you just set aside. But the mark of an environmentally conscious person was actually knowing how to use nature.”

This spirit was the impetus for founding Ducks Unlimited and other hunting and fishing groups—more to push back against irresponsible practices than to open up the landscape for more hunting. Early sportsmen’s associations were elitist groups, Johnson says, and their ire was less likely to be directed at fussy nature-lovers than at, say, the coal miner fishing with dynamite or the immigrant hunter shooting and eating songbirds. Conservation levies like the Duck Stamps and the 1937 Pittman–Robertson Act, which put an 11 percent tax on hunting rifles and ammunition, were self-imposed.

Today, DU spends 83 cents on each dollar of revenue on education and conservation, which can mean any number of things: restoring damaged landscapes, hammering out conservation easements with big landholders, helping farmers switch to rotational grazing, even building water control infrastructure. The organization also engages in the kind of awkward alliances that often characterize big nonprofits. In 2017 it awarded its inaugural “Corporate Conservation Achievement Award” to ConocoPhillips, the Houston-based oil company that has fought greenhouse gas regulations.

It can be hard to declare victory when your very name promises a kind of national blizzard of ducks. But Ducks Unlimited has been remarkably successful. In an age of competing uses for land in its target areas—agriculture, oil drilling, wind farms, suburban subdivisions—Ducks Unlimited conserved 248,000 acres in its most recent fiscal year.

Combined waterfowl populations have increased by 37 million in the Prairie Pothole Region since 1990, even as forest birds like the Kentucky warbler and grassland birds like the bobolink have been in prolonged decline. “One of the reasons waterfowl are faring pretty well is because hunters, DU included, have done a fantastic job recognizing the needs of those organisms and implementing successful conservation programs,” says Ron Rohrbaugh, the assistant director for conservation science at the Cornell Lab of Ornithology.

It can be hard to declare victory when your very name promises a kind of national blizzard of ducks.

The benefits go beyond ducks. “Some people look at hunters selfishly wanting to increase the number of ducks so they have something to hunt, and it is that,” says Rohrbaugh, a bowhunter himself. “But they’re doing that through improving wetlands that provide flood mitigation, clean water. The ecosystem services those enhanced wetlands provide go far beyond making sure we have enough pintails.” Thanks to hunters, data on duck populations is among the best in ornithology.

The “wise use” doctrine that fuels Ducks Unlimited is in many ways a more practical framework for environmentalism in the Anthropocene age. Radical preservation aims like the Harvard biologist E.O. Wilson’s “Half-Earth” theory—half the earth for us, half for animals—are hardly compatible with the boundless spill of greenhouse gases and microplastics. “Wise use,” however, allows for an environmentalism that encompasses marginal spaces and private, working lands—an especially important objective for dealing with migratory birds whose territory, in the case of the red knot, can stretch from Tierra del Fuego to the Arctic Circle.

“When you’re working with birds, you have to go where they are and what they need and sometimes those aren’t the grandest of iconic landscapes. And that’s OK,” says David O’Neill, chief conservation officer at the National Audubon Society. One tactic that’s become particularly crucial is the conservation easement, which allows private property owners to dedicate a portion of their land to conservation without selling it—and take a fat tax break for their trouble. “That’s perhaps the most important new land protection tool in our toolkit,” O’Neill says.

It has become a big part of the DU model as well. Nearly 20 percent of the foundation’s revenue in fiscal 2017 came from donated conservation easements. It’s a lot easier and cheaper to string together easements than to buy property. A $25-million conservation easement on U.S. breeding grounds, donated last year, was the largest single gift in DU history.

The practice is booming—conservation deductions tripled between 2012 and 2014 to more than $3 billion a year—and has been criticized as a tax shelter for investors, who reap lucrative rewards for setting land aside. (The loophole even turned Donald Trump into an environmentalist; before he was elected president, Trump generated more than $100 million in write-offs from easements, including a $39 million deduction at his Bedminster, New Jersey, golf course.) But the abuse of the tactic is a problem for the IRS. For charities trying to convince big landowners not to build out their holdings, the 40-year-old conservation easement is a windfall.

“We rely heavily on working landscapes,” Wiley, the DU conservation chief, explains. This is essential in the Prairie Pothole Region, where agricultural and real estate interests are rapidly draining the land of its little lakes, forcing nesting ducks into fewer places, where they are vulnerable to coyotes and other predators. Adds Wiley: “Without that private landowner interface, we’d be in deep trouble.”

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