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Management of Northern Prairies and Wetlands for the Conservation of Neotropical Migratory Birds

Effects of Habitat Management

Publicly owned native grasslands are managed in several ways, but grazing and prescribed burning are the two most common active practices. Many public grasslands are left idle for long periods, however, permitting encroachment of woody vegetation and excessive build-up of litter. Further, woody species are frequently planted in grasslands.

Managers do not always state their grazing objectives, but objectives may include returning the vegetation to an assumed pristine state, favoring certain plant species or communities, reducing the accumulation of organic matter, and encouraging vegetation that supports desired wildlife species. Other objectives are economic returns and good relations with neighboring landowners who want to graze the lands. Grazing often is controversial (e.g., Fleischner 1994) because objectives often are not clearly defined and progress toward objectives may not be measured with a rigorous monitoring program (Kirby et al. 1992). In addition, there are numerous grazing regimes whose impacts on wildlife and habitats vary according to the type of grassland, soils, kinds of grazing animals, precipitation patterns, and other influences. It is sometimes argued that grazing by cattle represents an ecological replacement of bison grazing, but diets of the two species differ considerably (Peden et al. 1974, Schwartz and Ellis 1981) as do their mobility and ability to capitalize on vegetation growth over wide areas (McNaughton 1993). Moreover, the replacement of bison by cattle, and the associated fencing, reduces the heterogeneity of grazing effects and resulting habitats for some birds (Knopf 1996a).

Although grassland birds evolved with grazing animals (Knopf 1996b), the effects of grazing on birds are variable and depends on the region (Kantrud and Kologiski 1983), grazing regime, precipitation and other environmental conditions, and the species. Further, short-term effects may differ markedly from longer-term ones. Heavy grazing favors species—including burrowing owl, horned lark, and chestnut-collared longspur (Kantrud 1981)—that use shorter vegetation, but greatly reduces numbers of Sprague's pipit, sedge wren, bobolink, savannah sparrow, Baird's sparrow, Le Conte's sparrow, and common yellowthroat (Maher 1973; Owens and Myres 1973; Kantrud and Kologiski 1982, 1983; Dale 1984; Lingle and Bedell 1989). Prescott and Collister (1993) suggested that heavy grazing may reduce suitability of habitat for loggerhead shrikes in southeastern Alberta. Light grazing enhances the habitat for Baird's sparrow and clay-colored sparrow; light to moderate grazing supports higher densities of Sprague's pipit, savannah sparrow, and vesper sparrow (Kantrud and Kologiski 1983). Berkey et al. (1993) suggested that short-term grazing in North Dakota likely was beneficial also to the ferruginous hawk. They deemed grazing detrimental to American bittern, northern harrier, upland sandpiper, short-eared owl, dickcissel, grasshopper sparrow, Le Conte's sparrow, and bobolink, species that prefer taller and denser grassy vegetation.

Under certain grazing systems, livestock are rotated through a series of pastures, which receive intense grazing pressure for short periods. This practice provides a patchwork of grasslands at any one time ranging from heavily grazed to idled for an entire growing season. Managers recommend these systems over season-long grazing both to enhance livestock production and to offer a variety of habitats for birds. Berkey et al. (1993) concluded that, as an alternative to season-long grazing, short-term rotational grazing would benefit species that favor taller and more robust vegetation, such as northern harrier, Baird's sparrow, lark bunting, and others.

Prescribed burning has similar biological, but fewer economic, objectives as grazing. Burning is made more difficult by unsafe or unsuitable weather conditions (e.g., high winds, rains), air quality concerns, personnel and equipment needs, and unfavorable attitudes by neighbors.

Burning has an immediate effect on grassland birds, although Kruse and Piehl (1986) found that 69 percent of active ground nests survived fires in mid-June. Recently burned areas are favored foraging areas for a number of species, including marbled godwits and willets. As growth resumes following a burn, habitat succession favors a sequence of species, beginning with species such as horned lark, chestnut-collared longspur, and vesper sparrow (Maher 1973, Huber and Steuter 1984, Pylypec 1991). Until the vegetation on a burned area is fully restored, the habitat is less suitable for species such as savannah sparrow, clay-colored sparrow, grasshopper sparrow, and bobolink (Tester and Marshall 1961, Halvorsen and Anderson 1980, Pylypec 1991, Herkert 1994b). Fire was a natural phenomenon in the northern plains and maintained the prairie. It may be an essential tool for managers who want to continue to maintain prairie and support true grassland birds. Fire and grazing occurred together naturally, and studies are needed of the interactive effects of these practices.

Often trees and other woody species are planted in grassland areas. Reasons for such plantings include aesthetic considerations, creation of habitat for game species such as deer and pheasants, and increased local species diversity (Cable et al. 1992). Even without an active planting program, encroachment by woody vegetation in grassland is favored by leaving the land idle, especially by protecting it from fire (Knopf 1994).

Grasslands invaded by woody species typically contain more bird species than those without (Arnold and Higgins 1986). But these species tend to be edge or generalist species, such as brown thrasher, gray catbird, song sparrow, American robin, and common grackle. Such species have plentiful habitat elsewhere, and their populations are robust. Meanwhile, the addition of trees may reduce the quality of habitat for true grassland species, such as Sprague's pipit, Baird's sparrow, and short-eared owl. These species have much more restricted habitats or breeding ranges and require maintenance of prairie for their viability.

Woody vegetation can influence grassland birds in several ways. First, it reduces the area of grassland and fragments it; there is evidence that certain grassland birds, like some forest species, are area-sensitive. Second, it precludes certain species from using an area (Whitmore 1981, Kahl et al. 1985). Third, trees and shrubs provide perches for raptors and cowbirds, and travel lanes for mammalian predators. And fourth, species attracted to the woody vegetation may forage in adjacent grasslands and compete with prairie species.

Attempts are sometimes made to restore prairie after it had been cultivated (e.g., Thompson 1992). Although many native grasses and some forbs can be seeded with relative ease, prairie restoration is practical only for relatively small tracts of land. Use of restored grasslands by breeding birds is little-studied, but Blankespoor (1980) found that restored prairies in South Dakota supported breeding populations of grasshopper sparrows, dickcissels, common yellowthroats, and other species 2 to 4 years after planting. Grasshopper sparrows, Henslow's sparrows, bobolinks, and eastern meadowlarks colonized prairie restorations in southern Wisconsin and established sizable and apparently stable populations within 4 to 5 years (Volkert 1992).

Former croplands are often replanted with mixtures of native and introduced grasses and forbs, especially legumes. These planting are made for several purposes, including providing habitat for upland-nesting ducks, enhancing soil quality, and reducing soil erosion. For example, the Soil Bank Program of the 1960's and 1970's and the Conservation Reserve Program of the 1980's and 1990's resulted in the retirement of vast areas of cultivated land and their conversion to mixtures of grasses and legumes. Such programs do not recreate natural habitats, but they do provide productive habitats for a variety of grassland birds (e.g., Duebbert 1981, Higgins et al; 1984, Renken and Dinsmore 1987). Johnson and Schwartz (1993) found that many species, including lark bunting, grasshopper sparrow, western meadowlark, clay-colored sparrow, bobolink, and sedge wren were relatively common in CRP fields in the northern plains, whereas they occurred far less commonly in croplands of the type that CRP replaced.

The most important practice for wetlands is protection from drainage. This goal can be achieved by purchasing in fee title, by buying easements that prevent drainage, by legal proscription, by tax incentives, or by encouraging wetland owners in other ways (Johnson et al. 1994a). Wetland preservation obviously protects a number of wetland-dependent species. In addition, programs such as the U.S. Fish and Wildlife Service's wetland acquisition program also acquire uplands surrounding the wetlands, which protects and restores grasslands.

Many wetlands are restored, usually by undoing the ditch or tiling used to drain the wetland (Galatowitsch and van der Valk 1994). Wetland restoration is more readily achievable than prairie restoration, but the degree to which a restored wetland performs the ecological functions of a natural wetland varies from one situation to another. Wetlands can be created even where they did not occur previously. This practice is sometimes done to mitigate for wetland losses elsewhere. Restored or created wetlands should support the customary wetland avifauna if they develop the vegetative and invertebrate communities of natural wetlands. Delphey and Dinsmore (1993) suggested that the absence of low-prairie and wet-meadow zones in restored wetlands in Iowa may have contributed to reduced bird use compared with natural wetlands. Although considerable effort has been expended in restoring wetlands in the northern plains, relatively little attention has been paid to evaluating the restorations. An extensive interagency effort is underway to remedy that situation (N. H. Euliss, Northern Prairie Science Center, pers. commun.).

Wetland managers often sought to increase the permanency or depth of wetlands, with the thought of minimizing the effects of dry seasons and years. The unfortunate consequences of this practice include reductions in desirable emergent plant communities (Kadlec and Smith 1992) and invertebrate populations. Maintaining the natural dynamics of wetlands is key to maintaining their productivity and value as habitat for birds (Weller 1978).

Some wetlands have become choked with emergent vegetation, notably hybrid cattail (Typha glauca). This phenomenon typically occurs in semipermanent wetlands surrounded by cropland, where grazing and other disturbances have been eliminated (Kantrud 1992). Although dense emergent stands afford nesting habitat for species such as common yellowthroat and marsh wren, they render the wetland less suitable for waterfowl and some other species. For that reason, managers try to reduce cattail stands to create a "hemimarsh" situation (Weller and Spatcher 1965), considered the ideal condition for dabbling ducks (Kaminski and Prince 1981). Several methods of cattail reduction have been attempted, but currently the favorite is the application of the herbicide glyphosate in a patchwork pattern (Linz et al. 1992). Natural forces sometimes still work; heavy precipitation in 1993 and 1994 flooded out cattails throughout much of North and South Dakota.

Where water levels can be controlled in wetlands, managers attempt to achieve several goals. One is to attract birds by applying water before they arrive in spring, but drawing it down later in summer or fall. This procedure simulates the natural dynamics of seasonal wetlands, and increases productivity of plants and invertebrates. Moist soil management (e.g., Reid et al. 1989) is a well-studied management practice designed to mimic natural dynamics. Originally intended to benefit waterfowl, it enhances the value of habitat for breeding and migrating shorebirds and other wetland-dependent species as well (Fredrickson and Reid 1986).

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