Northern Prairie Wildlife Research Center
Prior to settlement by Europeans, the northern plains were a vast grassland; trees were scarce or absent (e.g., Bragg and Steuter 1995). Early reports indicate that trees were largely restricted to river floodplains, east- or north-facing bluffs along streams, and prominent hillsides (Stewart 1975:4, Bragg and Steuter 1995). Grasslands were maintained by periodic drought; fires, especially to the east; and, especially in the west, grazing by large herds of herbivores such as bison. These forces created mosaics of habitat ranging from heavily grazed to undisturbed (England and DeVos 1969).
In the eastern portion of the northern plains, innumerable depressions were left when the Wisconsin glacier retreated about 10,000 years ago. These wetland basins, called prairie potholes, contain water for various lengths of time in most years (Stewart and Kantrud 1971). The most ephemeral wetlands may hold spring runoff or summer rains for only a few days. At the other extreme are lakes, which almost never go dry. In between are seasonal wetlands, which in a typical year contain water from early spring until mid to late summer, and semipermanent wetlands, which in most years are wet throughout the frost-free season. Less common are alkali wetlands--large, shallow basins with such high alkalinity that salts are blown out when the wetland is dry, and where no emergent plants grow when it is wet. Another unusual wetland type is the fen, characterized by floating or quaking mats of vegetation caused by groundwater seepage. Different wetland types support different kinds of vegetation and, in turn, different animal communities.
Critical to understanding the prairie is recognizing its dynamic nature, particularly as driven by recurring droughts. Prairie occurs primarily under semi-arid conditions. Precipitation is generally inadequate for growth of most woody vegetation, and the herbaceous vegetation favored fires and supported large herds of grazing herbivores, both features that further discouraged woody growth. Drought is essential to wetlands as well as uplands. The periodic drying of wetland basins facilitates nutrient cycling and results in high productivity when water returns (Murkin 1989).
Much of the terrestrial grassland habitat has been cultivated for crops. This conversion is nearly total in the eastern portion; tallgrass prairie is one of the most threatened habitats in the northern plains, with only scattered fragments remaining (Samson and Knopf 1994, Noss et al. 1995). Less mixed-grass prairie has been cultivated, largely because the terrain is rougher and precipitation is lower and less predictable. Irrigation has in many places rendered lands more suitable to cultivation, however. More shortgrass prairie remains, although much of it is intensively grazed by domestic livestock. Small grains such as wheat, barley, and oats are common crops in the western plains; in the east, row crops such as corn, soybeans, sunflowers, and potatoes are also planted.
Settlement of the northern plains by Europeans brought major increases of woodland. Tree claims were planted to protect farmsteads from the ever-present winds, and shelterbelts were established along field borders to reduce soil erosion, especially after the drought of the 1930's. Also, inadvertent increases of woody vegetation resulted from fire suppression by settlers (McNicholl 1988).
Prairie wetlands likewise have been altered in a number of ways. Drainage of basins to facilitate cultivation was very common, especially in the eastern prairies. Sometimes several small wetlands were drained into a larger one, which eliminated the smaller wetlands and altered the hydrology of the receiving wetland. Losses of wetland from settlement to 1980 were 27 percent in Montana, 35 percent in South Dakota, 49 percent in North Dakota, and 42 percent in Minnesota (Dahl 1990). Smaller, more temporary wetlands were more susceptible to drainage than were the larger, more permanent basins. Losses of some wetlands were partially offset by the creation of others. Stock-watering dams and dugouts have been constructed in the northern prairies, usually along intermittent streams. Several mainstem dams on rivers have created large reservoirs, although their value to breeding birds is limited.
A landscape perspective requires consideration of broader-scale issues than does a local perspective. Diversity in a regional sense is more important than local species diversity (Knopf and Samson 1994). Maximizing the species richness of an area (species packing) is not a goal; maintaining viable populations is (Johnson et al. 1994b). This requires an understanding of each species' habitat needs and how different habitats relate to one another.
Species that forage in one habitat but nest in another illustrate connections between different habitats. Dabbling ducks feed in wetlands but commonly nest in upland grassland. Certain shorebirds, such as Wilson's phalarope, willet, and marbled godwit, likewise require both terrestrial and aquatic habitats. American bitterns and northern harriers will nest either in emergent wetland vegetation or in dense upland vegetation. Red-winged and yellow-headed blackbirds nest in wetlands but often forage in terrestrial habitats. Certain species take advantage of the dynamic nature of the prairies, settling in wetland cover as available, but using normally terrestrial vegetation during unusually wet periods. For example, the Le Conte's sparrow, which usually nests in wet swales, will nest in high numbers in upland grass-forb plantings during wet periods (Igl and Johnson 1995a).
Not any patch of habitat, even preferred habitat, will suffice; size of the patch may be influential. Several grassland species are area sensitive. For example, Herkert (1994a) found that transects in larger grassland blocks were more likely to contain grasshopper sparrows, bobolinks, and savannah sparrows than were comparable transects in smaller blocks. Studies of birds in Conservation Reserve Program fields have indicated that many of the larger-bodied species such as northern harrier, short-eared owl, Wilson's phalarope, marbled godwit, and willet rarely occur in small habitat blocks (D. H. Johnson, in prep.). Marsh size and isolation influenced occupancy by wetland birds in Iowa (Brown and Dinsmore 1986). Even habitats that are used may not be effective in maintaining viable populations. Habitat patches may consistently attract breeding birds that fail to reproduce (sink habitats: Pulliam ). Such areas occur in the northern plains for waterfowl (Klett et al. 1988, Greenwood et al. 1995). Although less is known about the population dynamics of passerines, cultivated fields (Rodenhouse and Best 1983) and especially hayfields (Bollinger et al. 1990, Frawley and Best 1991) likely operate as sink habitats.
Features in a landscape may affect bird use of habitats at some distances. Occupancy of grassland habitat can be influenced by the nearby presence of woody vegetation; Johnson and Temple (1986) reported nest densities of grasshopper sparrows were lower near trees than farther away, whereas the reverse held for clay-colored sparrows and western meadowlarks. Trees also may provide perch sites from which raptors can hunt and brown-headed cowbirds can seek host nests in which to lay their eggs. Johnson and Temple (1986) found that nest success of several grassland bird species was significantly higher for nests located far from a field-forest edge. Burger et al. (1994) reported similar results for artificial nests in grassland. Birds that nest in wetlands, woody areas, or human developments often forage in nearby grasslands, and may compete with grassland-dependent birds.