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Effects of Vegetation Manipulation on Breeding Waterfowl in Prairie Wetlands — A Literature Review

Introduction


The natural forces of climate, grazing, and fire were once the major factors controlling the abundance and species composition of vegetation in prairie wetlands. Breeding and migrant birds that used the wetlands evolved successfully under these influences, as evidenced by numerous accounts of large numbers and varieties of water birds present under pristine conditions.

Although wetland drainage has received the most publicity, other activities of European man had greatly changed prairie wetlands by the end of the 19th century. Domestic animals confined within fences sometimes grazed wetlands almost year-round. Wetlands near farmsteads often became highly eutrophic from barnyard and feedlot runoff water. Prairie fires, feared by both farmers and cattlemen, were suppressed whenever possible, which allowed dead vegetation to accumulate in many wetlands. In agricultural areas, bottom soils of the shallowest, least permanent wetlands were regularly cultivated, even during wet years. In some years, wetlands with moderate water-retention ability could also be cultivated. During drought years, the bottom soils of more permanent water bodies were used to raise crops. The vegetation in all or part of some wetlands was mowed as often as possible for hay or bedding for livestock. Some wetlands were burned in the fall to reduce the amount of snow trapped in the basin or to discourage the spread of weeds; these wetlands could sometimes be cultivated the following spring.

In recent decades, cultivation of steep slopes, use of row crops, and the practice of summer fallowing have caused much topsoil to move into the basins of countless prairie wetlands, further changing their vegetative species composition and abundance. Dissolved salts and residues from agricultural chemicals probably have moved into many prairie wetlands. Irrigation practices have also altered the hydrology and vegetation of prairie wetlands. Finally, both herbage and woody vegetation have increased greatly in many wetlands in the eastern portion of the prairie pothole region. In this area, much livestock raising has been discontinued; thus, many formerly grazed or hayed wetlands that remain undrained now lie idle.

Although these land-use practices have undoubtedly affected the value of prairie wetlands to waterfowl and other birds, especially on privately owned lands, the effects have been only slightly less severe on many wetlands owned or managed by conservation agencies.

Research has emphasized bird habitat use, behavior, food habits, and recruitment of prairie-nesting waterfowl. Techniques used to manage upland nesting cover and, to a lesser degree, to control rates of hen and egg predation at upland sites are now fairly well developed. Yet little is known about practices that can rejuvenate vegetatively degraded prairie wetlands and restore their attractiveness to breeding waterfowl and other marsh and aquatic birds.

Wildlife problems associated with vegetation in wetlands and the response of wetland vegetation and animal populations to fire and grazing by domestic livestock are reviewed in this paper.

Weller (1978) stated that the theoretical basis for present marsh management techniques for wildlife is weak because of poor experimental design and inadequate evaluation of results; he encouraged the adoption of community-oriented management systems based on natural successional patterns that give benefits for a longer time and at lower cost than artificial systems. He identified burning and grazing as the systems most in need of study. Murkin (1979) also urged that the natural processes involved in the marsh cycle be studied; he stressed the importance of determining if semi-open marsh can be maintained in a productive state.

The natural fluctuation of water levels is probably the most important cause of vegetative change in prairie wetlands. Control of water levels has been extensively used to manipulate vegetation on many areas in the United States, but is not discussed here because such control is possible on only a small portion of the publicly owned wetlands in the prairie pothole region. Artificial management with costly herbicides, explosives, and sophisticated mechanical devices is also not considered here.


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