Geology topics

H. Thomas Sklebar

Publications and source records attributed to H. Thomas Sklebar.

3 recordsLinked to original sources

USGS2003

Differences in distribution of modified basins and ducks relative to roadside transects

Wetland basins in the Prairie Pothole Region of the U.S. are commonly modified by excavation (e.g., roadside ditches, stock dugouts), partial drainage (ditching), and diking. Differences in the distribution of modified wetlands may affect the predictive accuracy of waterfowl survey data if such wetlands are not distributed randomly in the landscape and if waterfowl are not distributed equally among them. We used data collected on thirty-eight 40-km2 plots in North Dakota to examine the distribution of modified basins relative to roadside transects and their use by five species of dabbling ducks in 1995. The 800-m-wide transects were subdivided into an inner 400-m transect, centered on the road, and the remaining outer transect area. We compared the distribution of modified and natural wetland basins among three sample areas: 1) the inner 400-m wide roadside transect area, 2) the outer transect area, and 3) the remaining area within the 40-km2 plot that was outside of the transects (outer plot). Duck use was compared between the two transect areas. The plots contained 20,582 basins, of which 88.5% were unmodified, 7.5% were excavated, 3.7% were partially drained, and 0.2% were diked. Nearly all excavated temporary (89%) and seasonal (90%) basins occurred in the inner transect area, reflecting the high proportion of basins that would be defined as roadside ditches. Excavated semipermanent basins were more evenly distributed among the outer plot and two transect widths; these basins often were dugouts but also included roadside ditches. Partially drained and diked basins also were fairly evenly distributed among the three sample areas. Semipermanent basins had greater use by mallards (Anas platyrhynchos) and northern pintails (A. acuta) when they were partially drained than when they were excavated or unmodified; pintails also had greater use of partially drained seasonal basins. Use of wetland basins by gadwall (A. strepera), blue-winged teal (A. discors), and northern shovelers (A. clypeata) did not differ among water regimes or modification. We found no evidence to indicate that duck numbers determined from standard 400-m-wide roadside transects were biased relative to the larger landscape. However, pond counts derived from such transects were biased. Correlations of duck numbers to pond counts that exclude ditches or temporary basins would poorly reflect the response of ducks to available water.

Wetlands↗

USGS2001

Duck populations as indicators of landscape condition in the Prairie Pothole Region

The Prairie Pothole Region of the northern Great Plains is an important region for waterfowl production because of the abundance of shallow wetlands. The ecological significance of the region and impacts from intensive agriculture prompted the U.S. Environmental Protection Agency to select it as one of the first areas for developing and evaluating ecological indicators of wetland condition. We examined hypothesized relations between indicators of landscape and wetland conditions and waterfowl abundance on 45 40 km2 study sites in North Dakota for 1995-96. Landscape condition was defined a priori as the ratio of cropland area to total upland area surrounding wetlands. Measures of waterfowl abundance included estimated numbers of breeding pairs (by species and total numbers) and , a species-specific correction factor which effectively adjusts breeding pair estimates for annual or area-related differences in pond size. Landscape indicators and waterfowl measures varied among regions. Results indicated that most areas in the Coteau region are of much higher quality for ducks than those in the Drift Plain, and areas in the Red River Valley are of the poorest quality for ducks. Regression models demonstrated the impact of agricultural development on breeding duck populations in the Prairie Pothole Region. The most consistent landscape indicators of waterfowl abundance were percent of cropland and grassland. Models were inconsistent among years and species. The potential biotic indicators of landscape and wetland condition examined here would be appropriate for temporal trend analyses, but because of inherent geographic variability would not be appropriate for single-year geographic trend analyses without more extensive evaluations to improve explanatory models

Environmental Monitoring and Assessment↗

USGS2000

Effects of roadside transect width on waterfowl and wetland estimates

Strip transects located along roads are commonly used to estimate waterfowl populations and characterize associated wetland habitat. We used data collected in May and early June, 1995, on forty-five 40-km 2 plots in North Dakota to evaluate bias of 800-m and 400-m wide roadside transects for sampling wetlands relative to a larger (40-km 2 ) scale and to compare duck abundance at the two widths. Densities of all basins combined and of seasonal basins considered alone were biased high for both transect widths, but mean bias did not differ from zero for temporary or semipermanent basins. Biases did not occur when excavated seasonal and temporary basins (i.e., road ditches) were excluded from the sample. Mean basin density was higher for the inner (400-m) transect width than for the outer transect width (area remaining of the 800-m transect, outside of center 400-m width) for all basins combined and for seasonal and temporary basins. We detected an area-related bias in the occurrence of basins in transects: smaller basins (0.08–1.6 ha) were over-represented in transect samples by 2.9–6.5%, and larger basins (≥11 ha) were under-represented in 800-m transects by 7.3% and in 400-m transects by 16.3%. We compared the distribution of ducks relative to water conditions in the inner and outer transect widths to evaluate whether they were affected by proximity to the road. Mallards ( Anas platyrhynchos L.), northern pintails ( A. acuta L.), and gadwall ( A. strepera L.) responded to water conditions equally in the inner and outer transect widths, but northern shovelers ( A. clypeata L.) and blue-winged teal ( A. discors L.) responded more strongly to wetlands on the inner than the outer transect width, indicating that estimates of these species would be higher from a 400-m wide transect than from an 800-m wide transect. Differences in an adjustment index, used to account for the portion of basin obscured from view, were highly variable between inner and outer transect widths but did not indicate that use of wider transects was hampered by visibility. Biases of transect sampling need to be carefully considered when extrapolating wetland basin or duck densities from transects to larger landscape scales.

Wetlands↗