In accordance with the policy of "compensatory mitigation," adopted as part of an amendment to the Clean Water Act, builders who wish to fill in wetlands are obliged to restore or create other wetlands in their stead. But a large percentage of the replacement wetlands do not perform well, owing to the incompetence of the developer, the inadequacy of the site or the lack of follow up. And even good faith efforts by skilled practitioners may fall short of their goals, given that we still don't understand exactly how natural wetlands function.
A restoration project in the Sweetwater Marsh National Refuge near San Diego provides a good example. To offset the loss of marshland from the widening of Interstate 5, the California Department of Transportation was ordered in 1984 to establish a wetland habitat for two endangered birds, the light-footed clapper rail and the least tern, and an endangered plant, bird's beak. The ambitious criteria were met for the tern and bird's beak, but not for the clapper rail. The main problem is that cordgrass, intended to serve as nesting sites for the birds, failed to grow to the birds' preferred height of 90 centimeters. Researchers blame the stunted grass on the site's sandy soils, which are low in nitrogen and organic matter. The undertaking was difficult because the site was so badly disturbed - crisscrossed by Interstate 5, a major power line and an abandoned railroad, and buried under dredged materials from San Diego Bay. Wetland wildlife is so depleted in southern California, says University of Wisconsin ecologist Joy Zedler, "that if you build a new marsh, where will the animals come from?"
Conditions were more favorable for a restoration effort in northern Illinois, the Des Plaines River Wetlands Demonstration Project. "Being connected to the river, a major corridor for small and large animal movement, helps a lot," notes project head Donald Hey.
Since 1985 twelve experimental marshes have been built on the 600 acre site - a riverine floodplain that offers refuge to several birds on the state's protected list, such as the yellow-headed blackbird, least bittern and common moorhen, plus a range of animals including frogs, turtles, coyotes and mink. "We sculpted the land to hold water and let the marshes 'self-design' from there," says Hey. At first, cattails took over, but then muskrats trimmed back the cattails, creating space for a diverse array of plant and animal life. "Now we keep out of the way and let the muskrats and beavers do all the work."
More than half of the wetlands in the United States have been lost to human activities and as much as 90 percent of the wetlands in states such as California and Ohio have been wiped out.