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Evaluations of Duck Habitat and Estimation of Duck Population Sizes with a Remote-Sensing-Based System

Habitat Base

The amount of land in the sample universe was 315,980 km². Most land was in North Dakota (42%), and more land was in South Dakota (29%) than in Minnesota (25%) and in Montana (5%). These values reflected the proportions of the prairie pothole region in each state except in Montana where our data were taken from only one waterfowl management district in the northeastern part of the state and in Iowa a state that was not included in the survey. Most of the land in each state was in private landownership (Table 6). The difference among states reflected differences in the acquisition of easements rather than variation in the amount of wetland. Only about 1% of the land in the prairie pothole region is in service landownership. This land, including wetland and upland, has the greatest potential for intentional modification of habitat to benefit ducks because the service controls management.

Table 6. Land ownership (%) by state in the prairie pothole region of the United States based on 1986 data.

State Easementa FWS Privateb
Minnesota
0.83
 1.03 98.14
Montana
1.82
 0.87 97.30
North Dakota
14.77
 1.26 83.97
South Dakota
9.96
 0.53 89.51
All States
9.37
 0.97 89.65

aIncludes all land in the easement tract. Only the wetlands are under easement.
bIncludes government land not owned or under easement by the U.S. Fish and Wildlife Service.
Wetland Habitat

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.

GIF - Wetland basin distribution

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).

Table 7. Estimated number of basins, area of wetland, density of basins, and area of wetland/km² by land-ownership class in the prairie pothole region of Minnesota, North Dakota, South Dakota, and extreme eastern Montana, based on photographs from the early 1980s.

Number of Basins
Area of Wetland
Basins/km²
Wetland ha/km²
State Ownership N/1000 95% CIa km² 95% CI N. Basins 95% CI Hectares 95% CI
Minnesota Easement 8.6 7.5 9.8 136.1 114.5 157.7 34.7 30.1 39.3 21.1 17.8 24.5
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
Private 226.8 188.9 264.7 5,900.3 3,723.8 8,076.9 7.7 6.4 9.0 7.8 4.9 10.6
Total 243.0 204.3 281.7 6,472.1 4,303.6 8,640.6 8.1 6.8 9.4 8.4 5.6 11.2
Montana Easement 3.1 1.8 4.4 44.4 22.4 66.4 31.0 18.4 43.6 17.0 8.6 25.5
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
Private 45.0 26.5 63.5 449.3 146.2 752.4 8.4 4.9 11.8 3.2 1.1 5.4
Total 48.8 29.5 68.0 574.9 210.4 939.3 8.8 5.3 12.3 4.0 1.5 6.6
North Dakota Easement 449.5 388.9 510.1 2,387.1 2,123.1 2,651.1 59.4 51.4 67.4 12.2 10.8 13.5
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
Private 1,524.1 1,358.5 1,689.7 9,475.7 8,172.2 10,779.3 35.4 31.6 39.3 8.5 7.3
 9.7
Total 1,989.2 1,780.1 2,198.4 12,548.7 11,118.7 13,978.6 38.8 34.8 42.9 9.5 8.4 10.5
South Dakota Easement 144.8 117.0 172.4 1,139.9 905.3 1,374.5 41.1 33.2 49.0 12.5 9.9 15.1
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
Private 662.4 563.7 761.2 7,396.9 4,821.9 9,972.0 20.9 17.8 24.1 9.0 5.9 12.2
Total 810.1 693.5 926.7 8,793.9 6,243.8 11,344.1 22.9 29.6 26.2 9.6 6.8 12.4
All States Easement 606.1 539.4 672.7 3,707.5 3,353.0 4,062.0 53.0 47.1 58.8 12.5 11.3 13.7
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
Private 2,458.4 2,261.0 2,655.7 23,222.3 19,594.7 26,849.9 22.5 20.7 24.3 8.2 6.9 9.5
Total 3,091.1 2,847.8 3,334.4 28,389.6 24,731.3 32,047.9 25.3 23.3 27.3 9.0 7.8 10.2

a + or - 2 standard errors.
The distribution of wetland-basin classes varied by landownership class (Fig. 6). The distributions of wetland-basin classes on easements and on private land were similar, except more seasonal wetland basins and fewer lakes were on easements than on private land. Temporary and seasonal wetland basins were greatly underrepresented and lakes were overrepresented on service-owned land most of which is on refuges, which were historically purchased as resting areas for migrating birds and not as waterfowl production areas.

GIF - Wetland basin distribution

Fig. 6 Distribution of wetland-basin classes by landownership class in the prairie pothole region of the United States, based on photography from the early 1980s. Wetland-basin classes include lake basins (Lake), semipermanent wetland basins (semipermanent), seasonal wetland basins (seasonal), and temporary wetland basins (temporary). Landownership classes include land owned by the U.S. Fish and Wildlife Service (service) and privately owned land (private).

Upland Habitat

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.

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