Trying to Show the Door to a Marsh Munching Immigrant from South America

Wildlife managers are trying to rid U.S. wetlands of a large, South American rodent

  • T. Edward Nickens
  • Dec 01, 1999
At first glance, nutria seem harmless enough. Like their slightly larger cousins, the beavers, nutria are water-loving mammals with big incisors, prominent whiskers and cloaks of dense, warm fur. But Glenn Carowan has a different view of the nonnative, plant-eating creatures.
"Picture a pack of voracious brown Pac-Men with a taste for precious marshland," says Carowan, manager of the Blackwater National Wildlife Refuge on the Chesapeake Bay in Maryland, "and you have a fairly good concept of nutria."

It´s easy to forgive Carowan for his harsh assessment. Nutria numbers at Blackwater have exploded from fewer than 150 in the 1960s to as many as 50,000 today. The animals have turned 12 square miles of marsh into open water in recent decades, destroying crucial wintering habitat for waterfowl.

The story is much the same wherever nutria are found in North America. Introduced from South America by fur farmers and government officials in the first half of this century, the prolific and adaptable nutria are now found in 15 states from coast to coast. With demand for their beaverlike fur down and native predators unable to keep them in check, nutria have munched their way through many U.S. marshes at an alarming rate. But scientists and resource managers in Maryland and elsewhere are fighting back. Enlisting the help of such far-flung allies as Louisiana chefs and British scientists, they hope to repel the nutria´s invasion of the U.S. wetlands.

It would be difficult to imagine creatures more damaging to fragile coastal ecosystems such as those at Blackwater. Nutria (the Spanish word for "otter") use their powerful front legs to dig into the dense mat of muck and bulrush roots that form Blackwater´s marshes. Consuming as much as 25 percent of their body weight per day, nutria literally devour the marsh, leaving behind large "eat-outs" of exposed marsh sediments. Their travel lanes erode into canals, funneling brackish water into the depressions. As the water evaporates it becomes saltier, killing even more plants. The nutria-dug ponds coalesce into large bays, which are further eroded by tidal surge and storms. The destruction occur with awesome speed. "I´ve seen 40-acre chunks of marsh lost in a few months," says Maryland biologist Robert Colona.

Scientists are quick to point out that marsh losses occurs even without nutria. But the rodents exacerbate such losses. "They ignite the process," says Colona, "like throwing a match into a can of gas."

Or a fistful of matches. In addition to prodigious appetites, nutria are amazingly prolific. A female matures at six months of age and can get pregnant just two days after giving birth, so she can produce 10 or more young a year. With few natural enemies outside of the South (where alligators prey on the rodents), nutria survivorship is high. The only limiting factor to the rodents in northern areas seems to be cold weather, but they have learned how to cope. When the thermometer plummets in Maryland, nutria will burrow into muskrat dens or sleep in masses of 40 and 50 individuals. Animals on the outside of the clump might freeze to death, but their bodies insulate the survivors.

But no such brutal weather whittles nutria numbers down in Louisiana. As many as 20 million nutria may call Louisiana home, the descendants of 20 animals imported from Argentina in 1938. Last year alone, they ate away 100,000 acres of marsh, and Louisiana received part of a $2 million federal grant to help control their numbers.

Louisiana scientists contend that eliminating so many invaders would be nearly impossible. Instead, nutria control in the state is based in part on making the noxious critters less noxious. Desirable, even. In 1998, Louisiana Department of Wildlife and Fisheries (LDWF) officials launched a project to turn the muck-dwelling nutria into gourmet fare. They enlisted Louisiana Culinary Institute chef Philippe Parola to develop recipes, such as Culotte de Ragondin la Moutarde (nutria in mustard sauce).

The plan isn´t as farfetched as it sounds, explains Greg Linscombe, a LDWF biologist: "Ragondin is a high-end product in Europe, with millions of pounds sold for the table during the 1980s." The state has sent nutria-bearing staff to food shows in Asia, so far with only limited success.

Maryland scientists, however, don´t want to turn their furry lemons into lemonade. They want them eradicated. It´s a harsh position, but hanging in the balance is the fate of Blackwater refuge and other marshes along the Chesapeake Bay. Most of the refuge´s wetlands are dominated by the three-square bulrush, a plant that grows as tall as three feet and is relished by animals from muskrats to ducks and geese. Three-square bulrush plants form dense mats atop what Robert Colona calls a "floating ooze" of tidal sediments. These mats are the structural underpinning of Blackwater´s tidal marshlands and host tens of thousands of wintering waterfowl.

Scrambling to save this imperiled habitat, Blackwater managers and Maryland officials have asked Congress for $2.9 million to help study ways to evict nutria from coastal marshes and restore the damaged bulrush mats. Congress approved the plan in late 1998, but the money has not yet been appropriated.

The plan calls for a three-year study to find out how the South American herbivores make a living off Chesapeake Bay marshes. "We need to do the basic natural history and behavioral research to identify the weak links in their armor," says Colona. Collaring animals with radio transmitters will help biologists understand how nutria establish territories in marsh too dense for human travel. Intensive trapping will give managers clues to how the animals change behaviors as populations decline.

Twenty-two state, federal and nongovernmental groups have signed on to the program, bolstering hopes for its success. Eradicating tens of thousands of animals from vast stretches of impenetrable marsh will be "a monumental effort, unprecedented in North America," says Colona. But it can be done, and indeed it has, in England.

In the 1980s, Great Britain extirpated its entire nutria population with a six-year trapping campaign. The chief researcher for that project, Morris Gosling, has assisted Maryland researchers with their plan, but not all of the Old World methods are easily exported. For example, Great Britain´s open marshes allowed trappers to access most nutria habitat by boat--something that is impossible in the Chesapeake Bay marshes. In Maryland, says Colona, nutria exploit different habitats and have adapted in different ways. "Any eradication program," he cautions, "has to be based on the biology and behavior of the animals right here."

Still, if Maryland does succeed in eradicating the rodent, its efforts could serve as a template for other states that are watching nutria munch their marshes to oblivion. Just as nutria were once imported to bolster fur supplies, Maryland could then export a related product: a nutria solution.

Writer T. Edward Nickens lives in North Carolina. This is his first article for National Wildlife Magazine.

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