Northern Prairie Wildlife Research Center
We estimated that about 3.1 million wetland basins covering about 28,490 km² (Table 7) were in the prairie pothole region of the United States. Most basins were in North Dakota (2.0 million), and more were in South Dakota (0.8 million) than in Minnesota (0.2 million). Our sample in Montana (48,800 wetland basins) represented only a small area of that state.
The estimated area of wetland per km² was similar among the states except in Montana where the area was about half of that in other states. The estimated density of wetland basins varied among states. The highest density was in North Dakota. The estimated mean wetland-basin sizes were 2.7 ha in Minnesota, 1.2 ha in Montana, 0.6 ha in North Dakota, and 1.1 ha in South Dakota. The distribution of wetland basins by size class was highly skewed to the smaller sizes (Fig. 5). This distribution was biased because parts of some wetland basins on the plot boundary included only the area inside the plot. The bias was greatest in areas of large lakes. The estimated wetland area per km² was greater on service easements than on private land and was much greater on service-owned land than in any of the other landownership classes because the service tended to buy large wetland basins, especially for refuges. Estimated wetland-basin density was highest on service easements (Table 7) because easements were taken in areas of high wetland-basin density and the wetland basins were small.
Fig. 5 Cumulative distribution of wetland basins by size and wetland-basin class in the prairie pothole region of the United States, base on photographs from the early 1980s. Wetland-basin classes include lake basins (Lake), semipermanent wetland basins (semipermanent), seasonal wetland basins (seasonal), and temporary wetland basins (temporary).
|U.S. Fish and Wildlife Service||7.5||5.0||10.1||435.7||402.0||469.4||24.5||16.3||32.7||54.7||50.5||58.9|
|U.S. Fish and Wildlife Service||0.6||0.0||2.2||81.2||31.4||131.0||13.5||0.0||45.4||64.8||25.0||104.5|
|U.S. Fish and Wildlife Service||15.6||9.4||21.8||685.8||541.5||830.2||24.2||14.5||33.9||41.1||32.5||49.8|
|U.S. Fish and Wildlife Service||2.9||1.9||3.9||257.1||194.1||320.1||15.3||9.9||20.8||52.9||39.9||65.8|
|U.S. Fish and Wildlife Service||26.7||19.7||33.6||1,459.8||1,291.2||1,628.4||22.4||16.6||28.3||47.5||42.0||53.0|
The estimated area of upland-habitat classes (Table 8) varied by state and landownership class. Cropland was the dominant land-cover class on private land and on easements in all four states. The distribution of area of land-cover classes on easements and on private land was similar. The small amount of cropland on service-owned land was the result of tillage on some refuges and in waterfowl production areas where tillage was used to rejuvenate planted cover or to provide food plots. These tilled areas appeared as cropland on aerial photographs.
Grassland was the second most abundant component of the landscape in all states and its abundance increased from east to west. The percent of grassland was lowest in Minnesota (6.7) and lower in North Dakota (16.3) than in South Dakota (26.0) and in Montana (33.9). The distribution of grassland-wildlife (Table 8) was partly an artifact of our definition. No grassland was shown on service-owned land and no grassland-wildlife was shown on easements and on private land. The occurrences of hayland and other land were minor but were important duck nesting habitat (Cowardin et al. 1985). Planted cover was uncommon except on service-owned land where it was a major habitat component.
Annual Change in Ponds
Pond density varied from 4.4/km² in 1987 to 0.8 in 1990 (Fig. 7). The area of ponds (ha per km²) varied from 7.2 in 1987 to 2.7 in 1990 (Fig. 8). Climate in the prairie pothole region occurs in cycles of wetness and drought (Kentrud et al 1989). Near-average water conditions were present in 1987 and followed by drought in 1988, slight recovery in 1989, and severe drought in 1990. The highest density of ponds was on easements, especially in 1987 (Fig. 7). The area of ponds per km² was greater on service-owned land than in the other types of landownership (Fig. 8). Although these large basins with semipermanent water regimes responded more slowly to drought conditions, surface water declined steadily throughout the period (Fig. 8). The area of ponds per km² on easements and on private land fluctuated similarly. The average pond size (Fig. 9) increased slightly as the amount of surface water declined because small wetland basins dried up entirely. The average pond size in wetland basins on service-owned lands increased during 1987-88, contracted as small ponds appeared in 1989, and increased again in 1990 as the small ponds again disappeared. The average pond size was less in 1990 than in 1988 because even the larger ponds were becoming smaller because of severe drought.