USGS - science for a changing world

Northern Prairie Wildlife Research Center

  Home About NPWRC Our Science Staff Employment Contacts Common Questions About the Site

Duck Populations as Indicators of Landscape
Condition in the Prairie Pothole Region

Results


Landscape Variables

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.

Table 1.  Analysis of variance results and least squares means (LSMEANS) estimates of landscape variables 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 the same letter superscript are not different (P > 0.05).
Variable F P LSMEANS
Coteau Drift Plain Red River
PERCROP (%) 4.29 0.02 53.5a 69.9b 78.7b
PERGRASS (%) 14.29 <0.01 42.0c 22.4b 7.9a
TOTDRAIN* (m) 5.63 0.01 3416a 11848b 7931ab
PCTNMOD (%) 3.63 0.04 0.038a 0.103b 0.122b
TOTBASIN (no.) 15.93 <0.01 507.0a 764.4b 128.6c
BASAREA (ha) 7.97 <0.01 334.8b 412.9b 127.7a
TEMPCID* 6.37 <0.01 0.14a 0.40b 1.18a
SEASCID* 5.84 0.01 0.39a 0.51a 0.17b
SEMICID* 0.38 0.69 0.37 0.43 0.30
ALLADJC (%) 11.46 <0.01 34.1a 62.4b 43.0a
ALLADJG (%) 23.37 <0.01 57.4 26.9 20.1
* Back-transformed LSMEANS.


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).

bar chart
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).

bar chart
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).

bar chart
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).

Table 2.  Analysis of variance results (P values) of the effect of three landscape variables on estimated numbers of breeding pairs and gamma (γ) coefficients for 45 study areas in North Dakota, 1995-1996.
Response variable Year PCTNMOD PERCROP TOTDRAIN PCTNMOD*
PERCROP
PCTNMOD*
TOTDRAIN
PERCROP*
TOTDRAIN
Estimated number of breeding pairs
All species 1995 0.53 <0.01 0.86 0.72 0.20 0.15
1996 0.10 <0.01 0.92 0.23 0.34 0.05
Mallard 1995 0.15 <0.01 0.17 0.02 0.35 0.05
1996 0.04 <0.01 0.58 0.03 0.49 0.11
Pintail 1995 0.46 0.41 0.33 0.86 0.34 0.11
1996 0.24 0.10 0.45 0.29 0.03 0.22
Gadwall 1995 0.40 0.03 0.18 0.14 0.13 0.04
1996 0.13 <0.01 0.15 0.16 0.19 0.05
Blue-winged teal 1995 0.78 0.01 0.20 0.66 0.66 0.94
1996 0.26 0.11 0.40 0.74 0.94 0.20
Shoveler 1995 0.93 <0.01 0.75 0.62 0.06 0.10
1996 0.42 0.09 0.77 0.37 0.21 0.04
Gamma coefficients
Mallard 1995 0.48 0.51 0.09 0.18 0.43 0.72
1996 0.72 0.04 0.17 0.12 0.64 0.52
Pintail 1995 0.49 0.66 0.18 0.79 0.74 0.39
1996 0.87 0.11 0.55 0.35 0.50 0.85
Gadwall 1995 0.84 0.09 0.13 0.21 0.43 0.12
1996 0.27 <0.01 0.11 0.11 0.28 0.13
Blue-winged teal 1995 0.88 0.03 0.18 0.14 0.67 0.37
1996 0.62 <0.01 0.34 0.67 0.78 0.34
Shoveler 1995 0.36 0.02 0.59 0.50 0.45 0.70
1996 0.55 0.08 0.86 0.29 0.49 0.22
PCTNMOD = proportion of basins modified and/or drained, an indicator of partially or wholly drained basins. PERCROP = cropland as a percentage of total upland habitat. TOTDRAIN = total linear drainage length, an indicator of drainage intensity.

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.

bar charts
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.

Table 3.  Interactions between total drainage length (TOTDRAIN) and percentage of total upland habitat (PERCROP) for number of breeding pairs. Median values used to determine high and low categories of landscape variables were 71.45 (1995) and 73.18 (1996) for PERCROP; and 7409 for TOTDRAIN. LSMEANS within species for each year having the same letter superscript are not significantly different (P > 0.05). Only those species and years in bold had significant interactions (see Table 2). Sample sizes noted for All species remained the same for individual species.

Interaction PERCROP TOTDRAIN 1995 1996
LSMEAN SE (n) LSMEAN SE (n)
All species Low Low 1215b 205 (13) 1223c 174 (14)
High 846b 245 (7) 824bc 213 (8)
High Low 269a 233 (8) 351a 201 (9)
High 559a 183 (13) 710ab 166 (14)
Mallard Low Low 299b 38 233b 31
High 158a 47 159b 38
High Low 95a 44 76a 36
High 122a 33 113a 30
Pintail Low Low 136a 31 110a 19
High 47a 39 68a 23
High Low 52a 36 49a 22
High 74a 27 59a 18
Gadwall Low Low 275b 48 302b 49
High 89a 63 116a 60
High Low 41a 56 47a 57
High 82a 46 77a 47
Blue-winged teal Low Low 409b 110 436a 92
High 575b 141 388a 112
High Low 89a 125 140a 106
High 236a 105 357a 88
Shoveler Low Low 135b 23 141b 24
High 100ab 28 93ab 30
High Low 23a 27 39a 28
High 73a 21 103ab 23

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).

bar chart
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.

Table 4.  Regression models to explain number of breeding pairs and gamma coefficient (γ) based on four landscape variables (percentage of total upland habitat (PERCROP), percentage of total upland habitat (PERGRASS), total number of wetlands drained (TOTDRAIN), and percent of modified wetlands (PCTNMOD)). For each year, only the models yielding the highest R2 values are given.
Response variable Year Terms in model Unadjusted R2

Estimated numbers of breeding pairs

All species 1995

PERGRASS, TOTDRAIN, PERGRASS*TOTDRAIN

0.29
PERGRASS 0.29
1996 PERGRASS, PCTNMOD 0.22
Mallard 1995 PERGRASS, PCTNMOD, TOTDRAIN,
PCTNMOD, PERGRASS*TOTDRAIN  
0.32
1996 PERGRASS, PCTNMOD, PERGRASS*PCTNMOD 0.17
Pintail 1995 PERGRASS 0.10
1996 PERGRASS, PCTNMOD, TOTDRAIN,
PCTNMOD*TOTDRAIN
0.10
Gadwall 1995 PERGRASS, TOTDRAIN, PERGRASS*TOTDRAIN 0.29
1996 PERGRASS, TOTDRAIN, PERGRASS*TOTDRAIN 0.30
Blue-winged teal 1995 PERGRASS 0.16
1996 PERGRASS, TOTDRAIN, PERCROP*TOTDRAIN 0.11

Shoveler

1995 PERGRASS, PCTNMOD, TOTDRAIN,
PERGRASS*TOTDRAIN, PCTNMOD*TOTDRAIN
0.38
1996 PERGRASS, TOTDRAIN, PERGRASS*TOTDRAIN 0.15

Gamma coefficient

Mallard 1995 TOTDRAIN 0.12
1996 PERGRASS, PCTNMOD, PERGRASS*PCTNMOD 0.18
Pintail 1995 PERGRASS 0.08
1996 PERGRASS 0.07
Gadwall 1995 PERGRASS 0.25
1996 PERGRASS, PCTNMOD, PERGRASS*PCTNMOD 0.32
Blue-winged teal 1995 PERGRASS 0.10
1996 PERGRASS 0.32
Shoveler

1995

PERGRASS 0.36
1996 PERGRASS 0.22


Previous Section -- Methods
Return to Contents
Next Section -- Discussion

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://www.npwrc.usgs.gov/resource/birds/dpopland/results.htm
Page Contact Information: Webmaster
Page Last Modified: Friday, 01-Feb-2013 19:13:03 EST
Menlo Park, CA [caww55]