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Management of Agricultural Landscapes for the Conservation of Neotropical Migratory Birds

Recent Changes in Agricultural Habitats and Landscape Structure


Non-cropland Habitats

Pastureland and Rangeland. The general trend in the Midwest between 1982 and 1992, was toward less pastureland and rangeland; between 1967 and 1977 the trend varied geographically (fig. 1). Herkert (1995) reported a 53% decline in pastureland from 1964 to 1992 in Illinois, Iowa, Ohio, and Wisconsin. Pastureland is used primarily for production of forage plants for livestock and is typically managed by fertilization and reseeding, according to the instructions for reporting NRI data. Rangeland is land on which the climax or potential vegetation is grasses, forbs, and shrubs suitable for grazing; management does not usually include application of chemicals. Pastureland is more common in the eastern part of the Midwest and rangeland is more common in the western part. In Iowa, the amount of pastureland declined by 23% between 1958 and 1967 (Iowa Soil and Water Conservation Needs Inventory Committee 1970). Most of this converted Iowa land was classified as forest or cropland in 1967. At a national scale, too, much of the conversion to cropland prior to the 1980's was from rangeland and pastureland (Heimlich 1985).

Wetlands. Wetlands are a prominent part of the natural landscape in the Prairie Pothole Region of North Dakota, South Dakota, Minnesota, and northern Iowa. Many wetlands have been connected to ditches or underground pipes (tile lines) to facilitate rapid drainage of cropland in the spring. Loss of original wetland area in the Midwest ranged from 164,211 ha in Kansas to 2,816,802 ha in Illinois (Dahl 1990). The percentage of wetland area lost ranges from 35-48% in the Northern Plains states, 42-50% in the Great Lakes states, and 85- 90% in the Corn Belt. Wetlands are discussed further by Johnson (1966).

Woodlands. Woodlands on midwestern farms are mostly riparian areas, woodlots, and shelterbelts. They are more abundant in the east (Table 2). In Iowa, the amount of land classified as forest or woodland increased between 1958 and 1967 (Iowa Soil and Water Conservation Needs Inventory Committee 1970). Most of this land had previously been classified as pasture. Woodland habitat decreased in the Midwest from 1967 to 1977 but has been relatively stable since then (fig. 1). Below, we discuss shelterbelts as a form of strip cover under land-use practices. We do not consider riparian areas and woodlots further in this paper. Wooded, non-strip-cover habitats may be the only nesting habitats used by some woodland species, such as cerulean warblers (see other chapters, this volume) .

Strip cover. Strip-cover habitats (e.g., fencerows) have declined in recent decades as the average size of crop fields has increased. Rodenhouse et al. (1993) reviewed information on loss of fencerows in midwestern agricultural regions and found that 30-80% of fencerows had been removed since the 1930's.

Landscape structure. The habitats discussed here (small grains, corn, soybeans, sunflowers, hay, pastureland and rangeland, wetlands, woodlands, and strip cover) are arranged in landscapes that affect NTMBs (Freemark et al. 1995). Interspersion of habitat types, habitat-patch size, and other aspects of landscape structure can affect species composition, abundance, pairing and reproductive success, and population dynamics. Species richness and abundance of birds in farmland are greatest in grasslands, pasture, early-successional habitats, strip cover, and shelterbelts (Freemark et al. 1995). Recent declines in the areal extent of these habitats have led to a simplification, or reduction in diversity, of farmland landscapes in general (Warner 1994). In Ohio, for example, the percentage of farmland that was harvested increased from 45% to 67% between 1940 and 1982 (Barrett et al. 1990). The declines in these habitats also have been accompanied by a trend toward larger crop fields. As an indication of this trend, the average size of farms in the Midwest in 1970, 1980, and 1990 was 164, 183, and 206 ha, respectively (USDA 1971a, 1981, 1991).


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