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Michael Haramis USGS Patuxent Wildlife Research Center, Laurel, Maryland 20708-4015
Robert Colona, Maryland Department of Natural Resources, 4220 Steele Neck Road, Vienna, Maryland 21869

See related Fact Sheet entitled "South American Nutria Destroy Marshes" (need Acrobat Reader to view file--if you need to download Acrobat Reader tool, go to: )

Abstract: Introduced in the 1940's to bolster Maryland's Eastern Shore fur industry, the South American nutria (Myocastor coypus) has been implicated in the loss of emergent marsh, especially that dominated by Olney 3-square (Scirpus olneyi) along the Blackwater River in Dorchester County. Nutria are basically herbivorous and marsh loss has coincided with the introduction and expansion of the population. Marsh loss was detectable in this region from photographs as early as the 1950s and has accelerated to the present. Also implicated in marsh loss are rising sea level, a cumulative result of global warming and regional land subsidence, and annual marsh burning, a cultural tradition in the region now practiced on a broad scale by marsh managers. The perceived effect of nutria foraging activity on marsh loss has escalated over the past 2 decades by a decline in the fur industry and a resultant overpopulation of nutria in lower Eastern Shore marshes.

The overabundance of nutria and the alarming loss of marsh in this region has prompted state legislation proposing a 10-year nutria eradication program. This program was outlined following recommendations from British researcher L. M. Gosling who conducted a similar nutria eradication effort in Britain. The relationship between nutria foraging activity and marsh loss, however, remains unclear and a long-term eradication effort has been postponed until evidence of cause and effect can be obtained. To address this issue, a collaborative partnership between the Maryland Department of Natural Resources and the Patuxent Wildlife Research Center was established in 1995. A formal peer-reviewed study plan was developed outlining the use of exclosures to isolate the effect of nutria foraging activity on marsh loss. The objective of the study is to demonstrate whether exclusion of nutria from emergent marsh habitats can stabilize or recover marsh vegetation. We predict that exclusion of nutria within these habitats will result in expanded vegetative cover or decelerated vegetative loss, while marsh loss will continue on unprotected sites. Use of exclosures in this study implies certain assumptions: 1) that nutria are the primary herbivore in this habitat and that muskrats, deer, waterfowl and other possible grazers have negligible effect on vegetation (or will be controlled), and 2) that the physical influence of fenced enclosure will have negligible effect on the measured vegetative response.

Research investigating the role of nutria in marsh loss is of major concern to the State of Maryland and the Blackwater National Wildlife Refuge. It is also of special interest to the Chesapeake Bay Program's Wetland Workgroup whose major goal is to achieve "no net loss" of wetlands within the Chesapeake Bay watershed. In 1994, the workgroup recognized the potential adverse effects of exotic species on Bay wetlands and importantly has adopted an objective to address the problem of exotic species management.

At present, 2,160 m (7,000 ft) of heavy gauge vinyl-coated wire has been entrenched in the marsh at Blackwater NWR to establish 18 30m x30m randomly selected exclosures. A set of 18 paired as well as 18 randomly selected control plots (unfenced) also have been established to control for site variation, especially differences in nutria density. Measures of vegetative cover and species compositional changes will be monitored by aerial photography and ground plot measurements over time. The study will monitor vegetative changes through two annual cycles with projected completion in 1998.

Aerial Photo of Nutria Marshland Habitat Photo of Nutria--Nutria are large (8-18lb) beaver-like rodents introduced from South America.
First noticeable in the 1950s, marsh loss along the Blackwater River in Dorchester County, Maryland, has accelerated at an alarming rate. What was once continuous marshland (above) now appears as fragmented remnants in an apparently irreversible conversion to open water. Nutria are large (8-18lb) beaver-like rodents introduced from South America. They have become overpopulated because of the decline in the fur industry and a lack of any other apparent market for the animals.

Photo of Nutria Tracks Photo of Foraging Eatout Areas Photo of Nutria Loafing Platform
Nutria sign is everywhere in this marsh, including abundant tracks (left), foraging eatout areas (center), and loafing platforms (right). All are testimony to the abundance of nutria in this marsh.

Photo of Evidence of Excavation of Root Matt Material in Marsh Photo of 30m x 30m exclosures: These will eliminate foraging pressure by nutria and measure the ability of the vegetation to recover.
The ability of this large mammal to excavate the root matt of the marsh, in both creating swim canals and foraging sites, makes the nutria directly instrumental in marsh loss. Large 30m x 30m exclosures will eliminate foraging pressure by nutria and measure the ability of the vegetation to recover.

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