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All landscape variables except SEMICID and ALLADJG varied significantly among regions (P < 0.05) (Table 1). Most measures of wetland basins and upland habitat indicated that study areas in the Red River Valley had the most disturbed and the Coteau the least disturbed conditions. The Red River Valley had the lowest TOTBASIN and BASAREA and also a high level of modification to basins (PCTNMOD, TOTDRAIN); however, neither of the latter variables differed between the Red River Valley and the Drift Plain, and TOTDRAIN actually was highest in the Drift Plain. PERCROP declined from the Red River Valley to the Coteau, whereas PERGRASS increased across these regions. ALLADJC did not follow the pattern for PERCROP as expected; it was higher in the Drift Plain than in the other two regions.

Estimated Breeding Pairs

Estimated numbers of breeding pairs also differed among regions each year, and differences were similar between years (Figure 2). Estimated numbers of all species combined (expressed as LSMEANS) were highest in the Coteau (1404 and 1233 in 1995 and 1996, respectively), intermediate for the Drift Plain (618 and 778), and lowest in the Red River Valley (35 in both years) (P < 0.01). Except for blue-winged teal, estimated numbers of pairs were usually twice as high in the Coteau than in the Drift Plain, while estimated numbers in the Red River Valley were <12% of estimated numbers in the Drift Plain (Figure 2). Similar patterns were found for γ, with the highest values in the Coteau and the lowest values in the Drift Plain (Figure 3).

Figure 2.  Least squares means (LSMEANS) for estimated numbers of breeding duck pairs among 45 study areas in the Coteau, Drift Plain, and Red River Valley regions of North Dakota, 1995-1996. LSMEANS within species for each year having a different letter label are significantly different (P > 0.05).

Figure 3.  Least squares means (LSMEANS) for gamma (γ) coefficients among 45 study areas in the Coteau, Drift Plain, and Red River Valley regions of North Dakota, 1995-1996. LSMEANS within species for each year having a different letter label are significantly different (P > 0.05).

PERCROP was a significant factor affecting estimated numbers of pairs for most species and years, but it often interacted with other factors (Table 2, Figure 4). It was not a significant factor for estimated numbers of pintails (both years), blue-winged teal (1996), or shovelers (1996). PERCROP was the single landscape variable affecting estimated number of pairs in 1995 for all species combined, blue-winged teal, and shovelers. Estimated numbers for these species were consistently higher with low PERCROP than with high PERCROP (Figure 4).

Figure 4.  Effects of high or low levels of cropland, as a percentage of total upland habitat (PERCROP), on estimated numbers of breeding pairs in 45 study areas in North Dakota, 1995-1996. Median value of PERCROP was 71.45 (1995) and 73.18 (1996); study areas having less or more PERCROP than the median value were classified as Low or High, respectively. LSMEANS within species for each year having a different letter label are significantly different (P > 0.05; see also Table 2).

Interactions were detected between PCTNMOD and PERCROP for mallards (both years) and between PCTNMOD and TOTDRAIN for pintails in 1996 (Table 2, Figure 5). Estimates of mallard pairs were higher with high PCTNMOD and low PERCROP in both years, but there was no difference by PERCROP when PCTNMOD was low (Figure 5a). The interaction of landscape factors was more complex for pintails (Figure 5b). In 1996, estimates of pintail pairs were higher when TOTDRAIN was low and PCTNMOD was high but the pattern was reversed for high PCTNMOD.

Figure 5.  Interactions between percent of modified wetlands (PCTNMOD) and (A) percent of total upland habitat (PERCROP) for number of mallard pairs and (B) between PCTNMOD and total number of wetlands drained (TOTDRAIN) for number of pintail pairs. Median values used to determine high and low categories of landscape variables were: 0.063 for PCTNMOD; 71.45 (1995) and 73.18 (1996) for PERCROP; and 7409 for TOTDRAIN. LSMEANS within species for each year having a different letter label are significantly different (P > 0.05; see also Table 2).

Interactions also were detected between PERCROP and TOTDRAIN for all species combined (1996), mallards (1995), gadwalls (both years), and shovelers (1996) (Table 3). Estimated numbers of pairs of all species combined was lowest with high PERCROP and low TOTDRAIN, and highest with low PERCROP. Estimates of mallard and gadwall pairs were highest with low PERCROP and low TOTDRAIN but TOTDRAIN did not affect estimates when PERCROP was high.

Only PERCROP affected γ, and the results were inconsistent among years and species. The effect of PERCROP was significant for mallards and gadwalls in 1996, blue-winged teal in both years, and shovelers in 1995. No interactions were significant. Values of γ were consistently higher when PERCROP was low (Figure 6).

Figure 6.  Effects of high or low levels of cropland, as a percentage of total upland habitat (PERCROP), on gamma (γ) coefficients, on 45 study areas in North Dakota, 1995-1996. Median value of PERCROP was 71.45 (1995) and 73.18 (1996); study areas having less or more PERCROP than this median value were classified as Low or High, respectively. LSMEANS within species for each year having a different letter label are significantly different (P > 0.05; see also Table 2.).

We examined various regression models to predict number of breeding pairs or γ using four landscape variables (PERCROP, PERGRASS, PCTNMOD, and TOTDRAIN) for each year separately. Most models yielded R² < 0.20 (Table 4). Models for estimated numbers of breeding pairs of all species yielded R² of 0.20 - 0.22. The strongest model for any single species was that for shovelers in 1995 (R² = 0.38). Model results for breeding pairs were inconsistent between years except for gadwall. Model results for γ were more consistent, with PERGRASS predominating as the single factor contributing to all models except for mallards in 1995.