A new analysis shows that restored wetlands store less carbon and host a less diverse group of plants and animals than untouched ones.
Before the Revolutionary War, George Washington had a professional interest in wetlands: He invested in a company that planned to drain the Great Dismal Swamp of Virginia and turn it into farmland. For centuries, Washington's attitude was considered the only reasonable one regarding swamps, marshes, peatlands, floodplains, mangroves, fens, potholes, bogs, and other places of muck and slime: They should be avoided or drained for better uses. Only in the past few decades have citizens decided that these areas—what we now call wetlands—did more than sog up perfectly good farmland.
Even though they cover only 1.5 percent of the earth’s surface, some experts estimate that wetlands provide 40 percent of renewable “ecosystem services”—jobs like water filtration and carbon sequestration. And although at least part Washington's particular "dismal swamp" survived, the United States has lost more than half of the wetlands that covered the continent when the first European settlers arrived. Similar losses have been documented in Europe, Australia, and New Zealand.
To mitigate losses, governments in the U.S. and elsewhere have supported wetlands restoration projects and begun requiring developers who destroy wetlands to offset the losses by creating new ones. But ecologists have found that restored wetlands are not as ecologically valuable as the originals—a new analysis of 621 wetland sites shows that, on average, restored wetlands regained only about three-quarters of their original biological performance. In restored wetlands, plants, insects, and animals do not reach their former abundance, density or diversity. Wetlands are also excellent carbon storage facilities: They hold at least a quarter of the world's land-based carbon, according to the World Resources Institute’s Millennium Ecosystem Assessment. But restored wetlands hold less carbon—on average, 23 percent less than untouched wetlands, according to the analysis.
It’s not news to ecologists that revived wetlands are a shadow of their former selves, but the study’s lead author, David Moreno-Mateos, a postdoctoral fellow at University of California, Berkeley, was surprised by how definitive the pattern was. “It was clear it's happening all over the world and all sorts of wetlands,” he says. Most of the wetlands included in the study are located in the United States, but Moreno-Mateos was able to include examples from all over the world. A few wetlands had been restored 50 or 100 years ago, but even they don’t perform as well as the ones they replaced.
This doesn’t mean that wetlands can never recover to their original state, but that a drained and developed wetland cannot be replaced by another constructed or restored wetland in a reasonable time period. Part of the problem is that restored wetlands might be judged on cosmetic fixes: If there are enough plants and animals around, it’s considered a job well done. But just because a wetland appears to have recovered doesn’t mean it’s performing the same functions—like carbon storage—it once was.
“You must worry about the function that you're losing,“ Moreno-Mateos says. “It's going to take centuries to grow back.”