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For over a century, Dyke marsh along the Potomac River just south of Alexandria, VA, has been a favorite site for natural history studies. Despite earlier attempts at diking to create agricultural land and dredging for sand and gravel, the remaining marsh represents the last major tidal freshwater wetland on the upper Potomac River, and is now owned and maintained by the National Park Service as the Dyke Marsh Preserve. In the present paper historical data on physical properties and biota are compared and contrasted with more recent biological investigations to show functioning ecosystem components, interrelationships among the flora and fauna, and documented changes in biotic communities over the years. As a haven for migratory waterfowl and shorebirds, unique plant communities, and large, dynamic insect and vertebrate populations, the Preserve is constantly threatened by river pollution, nearby large-scale development projects, and bridge construction. The long-term stability of this unique Preserve depends largely on the vigilance of the Park Service, concerned citizens, and environmental groups to ensure the preservation of this valuable functioning ecosystem.