The Ecological Basis for the Conservation of Migratory Birds in the Mississippi Alluvial Valley
By: Charles Baxter
and Cynthia Brown
and David Pashley
In 1994, the Mississippi Alluvial Valley (MAV) Migratory Bird Planning Group began to devise a conservation plan that integrates habitat objectives for three groups of bird species—wintering waterfowl, migratory shorebirds, and breeding and wintering forest birds. The goal of the plan was to develop habitat conservation objectives based on population goals for each species group—objectives that could be met simultaneously through integrated and complementary management activities. The objectives were to be set within a large ecosystem that originally was heavily forested and defined and driven by hydrofluvial processes, but which now is highly altered and functionally impaired. The planning process that resulted required a regional analysis of how changes in ecosystem structure and process have affected the ability of the MAV to function as bird habitat.
This planning process is an explicit effort to better coordinate the activities of the North American Waterfowl Management Plan (NAWMP), Partners in Flight (PIF), and the Western Hemisphere Shorebird Reserve Network. The Lower Mississippi Valley Joint Venture staff in Vicksburg spearheaded this effort with strong support from elsewhere in the U. S. Fish and Wildlife Service, and from wildlife agencies from the seven states of the MAV (see Figure 1), academia, private landowners, and non-government organizations such as The Nature Conservancy, American Bird Conservancy, and the Tennessee Conservation League.
Figure 1. Lower Mississipi River Basin.
(click on this image to view a larger version)
Historic structure and processes of the MAV landscape
The MAV once was a 10 million ha forested wetland system—the largest in North America—created and maintained by regular backwater and headwater flooding, and by localized ponding on poorly drained soils. The headwater or mainstem flooding resulted from rainstorms and snow melt in the watersheds of Mississippi River tributaries and produced great spring floods that characterize the MAV. Backwater flooding is a phenomenon in which high water stages on the Mississippi create a damming effect, preventing tributary drainage into the mainstem and at times reversing tributary flow upstream. This situation results in long-duration flooding, accompanied by sediment and nutrient deposition, throughout tributary watersheds.
Concomitant with these flooding mechanisms were the hydrofluvial processes associated with meandering river systems. The high energy inherent in the Mississippi River and its tributaries originally sculpted the MAV landscape, producing a surface geomorphology consisting of natural levees, meander scar lakes, point bars, and ridges and swales. These hydrofluvial processes created primary successional substrate for the establishment of pioneer communities that eventually were replaced by later seral communities distributed according to varying flood regimes. Site conditions within the MAV ranged from permanently flooded areas supporting only emergent or floating aquatic vegetation, to high elevation sites supporting complex hardwood forests.
The historic dominant natural communities of the MAV were various types of bottomland hardwood forests. The distribution of types and stages of bottomland hardwood communities is in part determined by the timing, frequency, and duration of flooding. Elevational differences of only a few inches result in great differences in soil saturation characteristics and thus plant distribution. As a result, components of a bottomland hardwood ecosystem range from bald cypress/tupelo swamp communities in saturated or inundated situations, to a cherrybark oak/pecan community where inundation is infrequent and temporary. Between these distinct types are transitional overcup oak/water hickory, elm/ash/hackberry, and sweetgum/red oak communities.
With time and sediment deposition, succession in the MAV proceeds from pioneer communities dominated by black willow or cottonwood (depending on soil drainage characteristics), to a red oak- and finally white oak-dominated climax community (Hodges 1994). Disturbances, both nature- and human-induced—including windfall, beaver activity, hydrologic alteration, and silvicultural practices—greatly influence the rate and direction of succession. There is emerging thought that the dynamic nature of this water- and sediment-driven system, coupled with frequent disturbance, generally precludes the development or long-term viability of a closed canopy of senescent trees commonly thought of as old-growth (Meadows 1994). The 24 million acres of presettlement forests in the MAV likely consisted of a shifting mosaic of small, even-aged patches, themselves of differing ages, further defined by minute differences in elevation.
Historic habitat function of the MAV
The diversity of landforms and community types on the historical landscape provided extraordinary habitat for a range of species, including more than 200 species of fish, 50 species of mammals, 45 species of reptiles and amphibians, 37 species of mussels, and approximately 60 percent of all bird species in the conterminous United States (Fremling et al. 1989; Sparks 1992, USACE 1988).
Migratory dabbling ducks historically have made significant use of the natural wooded wetland/slough complex in the winter, feeding primarily on acorns, seeds, and invertebrates. The bottomland hardwoods of the MAV provided the primary overwintering habitat for Mallards (Anas platyrhynchos) in North America. Diving ducks benefited from food sources characteristic of permanently flooded wetlands. These wintering habitats were critical not only to sustain ducks through the winter, but also to provide for nutritional demands that influence breeding success in the following spring (Yaich 1990).
The bottomland hardwood forests of the MAV also provided diverse habitat for a variety of forest birds. The continuous nature of the historical forest, along with its complex vertical structure and multiple seral stages, provided a haven for forest-interior species and niche specialists. Migratory forest birds used the bottomland hardwood forest in various ways, with many showing habitat preferences based on vertical structure, flooding regimes, and microhabitat features such as vine tangles, canebrakes, Spanish moss, and scour channels.
Use by migratory shorebirds was relatively low in the predominantly forested and shrub-scrub wetland types of the historical MAV. However, late summer evaporation from sloughs and backwater areas created some habitat. Also, sandbars and mudflats of the major river systems—including the Mississippi and Arkansas rivers—probably served an important function for many species of in-transit shorebirds, as have localized prairie habitats of eastern Arkansas and north central Louisiana.
The last two centuries have witnessed dramatic changes in the structure and processes of the bottomland hardwood system of the MAV, which have affected its ability to function as habitat for waterfowl, shorebirds, and migratory forest birds. A concerted flood control effort began in 1879 with the establishment of the Mississippi River Commission. Its flood control functions were assumed by the U. S. Army Corps of Engineers after the great flood of 1927 and the passage of the 1928 Flood Control Act (MacDonald et al. 1979). Since that time, one of the world's most comprehensive flood control systems has been developed along the Mississippi River and its tributaries, consisting of some 6,880 km of levees (Interagency Floodplain Management Review Committee1994). As a result, mainstem flooding has been virtually eliminated, and tributary flooding has been reduced by approximately 90% (Galloway 1980). In addition, channels have been cut and rivers straightened to improve drainage of the hydric soils that are characteristic of the vast majority of the landscape. These activities have reduced localized ponding due to rain by about 90% (Charles Baxter, pers. comm.). Continued rechannelization of streams that flow through agricultural lands, and general stream dredging and/or straightening still are carried out on most tributaries. This drainage has resulted in a gradual drying of the landscape. Flooding and ponding on poorly drained soils have decreased in both duration and frequency. Even in more flood susceptible areas where drainage has been less achievable, winter flooding is often less extensive and more ephemeral.
By the late 1930s, the elaborate system of levees and drainage projects had created increased opportunities for agricultural production. As a result, the bottomland hardwood forest has been reduced to only 1.8 million ha, or about 20 percent of its former extent. The remaining forest exists as fragmented patches of varying size and habitat quality. Recent satellite data show more than 35,000 discrete forest blocks of 1 hectare in size or larger (Mueller et al. this volume). Much of this remaining habitat is found in the wettest backswamp systems of the Yazoo River in Mississippi, the Tensas River in Louisiana, and the Cache/Bayou DeView/White River in Arkansas. Forests on drier ridges and higher terraces were cleared early in the history of human settlement in the MAV, as these better-drained soils provided optimal conditions for growing commodity crops.
Altered flow regimes along the Mississippi and its tributaries have threatened the structure and function of the remaining forested wetland ecosystem. Most forested wetlands outside the mainstem levees of the Mississippi River no longer are connected to floodplain processes that once shaped community age, composition, and distribution. Timing, duration, depth, and velocity of flood events have been severely altered, with the overall effect of eliminating the natural, annual process of overbank and backwater flooding in much of the historic floodplain, thus preventing the delivery of nutrient-rich sediments and water. Natural hydrofluvial processes, critical for establishment of early successional conditions, have been virtually eliminated from the floodplains. Only in a few places, such as the Lower Arkansas River, are processes such as bank erosion still relatively active.
EFFECTS ON AVIAN POPULATIONS
Habitat loss and fragmentation and hydrologic alteration in the MAV have resulted in population declines in both overwintering waterfowl and migratory forest birds.
Populations of dabbling ducks have decreased in the past several decades. Little evidence suggests that factors such as disease, environmental contaminants, or predators are responsible for this decline. Rather, availability of foraging habitat has had the greatest influence on the abundance, distribution, and body condition of waterfowl in the MAV (Loesch et al. 1994). The NAWMP suggests that the foraging requirements for wintering waterfowl in the MAV include more than 2.8 million ha of naturally flooded bottomland hardwood forest. With only 1.8 million ha of this forest remaining, and with much of it hydrologically impaired, land managers have augmented habitat by mimicking seasonal ponding through winter water management on agricultural land. Unfortunately, the increased reliance on managed habitats has resulted in a shift in waterfowl diet from natural food sources to waste grain and weed seeds. Although agricultural grains and natural wetland foods are largely interchangeable as sources of energy, grains do not provide the same protein and nutrients. Furthermore, in early winter when the landscape is naturally dry, and in all seasons during years when precipitation is normal to below normal, there is a scarcity of foraging habitat, managed or natural, to meet waterfowl requirements (Loesch et al. 1994).
Populations of diving ducks, because of their dependence on permanently flooded wetlands that still are abundant in the MAV, and because of the establishment of catfish and crayfish ponds in the southern reaches of the floodplain, appear to be faring well.
Agricultural expansion has resulted in an increase in cleared habitat that potentially could be used by migratory shorebirds if it were not impaired by hydrologic alteration. Migratory shorebirds use the MAV predominantly in the early fall and spring, when they exploit flooded farm fields and moist soil units as foraging habitat at all latitudes. Spring habitat requirements for shorebirds are largely met in the MAV as a result of natural flooding and ponding. Winter waterfowl management can be a relatively passive process of damming runoff from typically abundant rainfall; this form of management generally is compatible with agricultural practices (Loesch et al. 1994). However, active water management is required to provide the summer/early fall shorebird habitat and early winter habitat for waterfowl. Late summer/early fall is the period of lowest rainfall, and water delivery during this time will require greater expense.
The decline in the extent, form, and composition of the bottomland hardwood forest has been mirrored by a decline in many species of migratory forest birds in the MAV. Fewer than 1% of the remaining forest patches are large enough to support source populations of the more area-sensitive species such as Cerulean Warblers (Dendroica cerulea), Swainson's Warblers (Lymnothlypis swainsonii), and Swallow-tailed Kites (Elanoides forficatus) (Mueller et al. this volume). Forest fragmentation has resulted in the creation of edge habitat along forest patch margins which encourages predation and brood parasitism. This and other factors associated with small patch size have had a negative impact on many bird populations in all but the largest remnant blocks of forest. Some species have been affected by changes in habitat quality separate from issues of forest loss and fragmentation. The demise of Bachman's Warbler (Vermivora bachmanii), for example, may have been caused by the eradication of the formerly abundant canebrake habitat (Remsen 1986), a community typically found on better drained soils that now support agricultural crops. Cerulean Warblers may be reduced in number because the tall trees that they favor, such as cottonwoods in early successional communities, have been largely eliminated from the landscape.
Ecosystem management requires “protecting or restoring the function, structure, and species composition of an ecosystem while providing for its sustainable socioeconomic use”(U. S. Fish and Wildlife Service 1994). PIF Bird Conservation Plans should follow this general guideline, but must be more specific regarding the status and conservation needs of birds. A first and necessary step toward this end is to develop a clear understanding of the ecosystem: The way it functioned prior to human alteration, the history of change, and structure and function as they currently exist.
Alterations to the ecological processes and structure characterizing the floodplain of the Mississippi River system have been widespread and thorough. Over 80% of the bottomland hardwood forest has been converted to non-forest conditions, and the natural hydrologic and geomorphologic processes that created and maintained this vast wetland system have been virtually eliminated. It is clear that one result of these changes is a reduced capability to support populations of birds. In this paper, we have identified the nature of the problem. The remaining papers in this section trace the thought processes that have been aimed toward ameliorating, if not actually solving, the problems that bird populations face in this degraded ecosystem.
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