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Figure 1.   Range of the black-tailed prairie dog (dark and light shading) and the region (dark) surveyed by aircraft to estimate areal coverage of black-tailed prairie dog colonies (CO = Colorado; KS = Kansas; MT = Montana; ND = North Dakota; NE = Nebraska; NM = New Mexico; OK = Oklahoma; SD = South Dakota; TX = Texas; WY = Wyoming).

Line-intercept sampling. — We used line-intercept sampling to estimate the area covered by colonies of black-tailed prairie dogs in the 4-state area. Line-intercept sampling often is used to estimate vegetative canopy coverage and relies on noting points along a transect line where canopy begins and ends (Bonham 1989; Daubenmire 1959; Elzinga et al. 1998; Heady et al. 1959). Lengths of canopy intercepts are divided by length of the line and the resultant is applied to the study area for an estimate of total canopy cover. The accuracy of line-intercept sampling is comparable with other sampling techniques such as quadrats and point interception (Daubenmire 1959; DeVries 1979; Floyd and Anderson 1987; Lucas and Seber 1977).

We flew an aircraft along a series of transect lines and used a global positioning system receiver to record locations where transects intersected boundaries of colonies of black-tailed prairie dogs (Sidle 1999). To reduce sampling error and increase precision, we stratified the survey area into high-density and low-density strata (Caughley and Sinclair 1994; Norton-Griffiths 1978; Thompson et al. 1998). Colonies of black-tailed prairie dogs did not occur randomly in the study area because soil types, slope, and land-use characteristics were heterogeneous and defined habitat features to which black-tailed prairie dogs were sensitive (Campbell and Clark 1981; Proctor 1998; Reading and Matchett 1997; Reid 1954; Stromberg 1975). Moreover, in North Dakota and South Dakota, most colonies occurred on and near large areas of public lands where poisoning was limited, or on tribal lands where limited resources and regulatory constraints limited poisoning and fumigation.

Stratification was based on areas of recently known colonies of black-tailed prairie dogs (high-density stratum) and areas where no colonies were known (low-density stratum). For the high-density stratum, locations of colonies reported by public agencies during 1988-1996 were entered into a geographic information system (GIS) using the Map and Image Processing System (MIPS™, MicroImages, Inc., Lincoln, Nebraska; The use of company names, software, or trademarks does not imply endorsement by the United States Government). All legal sections of land (2.59 km²) containing known colonies were digitized to create the high-density stratum area of 10,712 km² (Table 1). An array of north-south lines uniformly distributed at 0.86-km intervals intersected the high-density stratum polygons for a total of 7,677 km. The low-density stratum consisted of the remainder of the study area, totaling 358,199 km². North-south transect lines in the low-density stratum were 13.85 km apart, and lines extended across the range of the black-tailed prairie dog in the study area, excluding the high-density stratum polygons, for a total of 25,387 km.

Aircraft operations. — Colonies of black-tailed prairie dogs could be seen from the air because of the conspicuousness of most burrow entry mounds, which measure 2-3 m in width, were barren of vegetation, and often consisted of lightly colored subsoil (Cincotta 1989; Hoogland 1995). Moreover, graminoid herbivory by black-tailed prairie dogs caused significant zonation and other changes in plant cover near burrows (Bonham and Lerwick 1976; Cincotta 1985; Garrett et al. 1982; Gold 1976; Koford 1958; Whicker and Detling 1993). Bare ground and erosion increase, vegetative structure decreased, and colony appearance therefore differed markedly from adjacent unmodified grassland (Munn 1993; Whicker and Detling 1993). Areas of gopher (Geomys bursarius and Thomomys talpoides) activity lacked the grazed definition of colonies of black-tailed prairie dogs, and mounds of dirt pushed to the surface by gophers were smaller than mounds in colonies of black-tailed prairie dogs and contained no entrance hole. Mounds of harvester ants (Pogonomyrmex occidentalis) were distinguished from mounds of black-tailed prairie dogs by a ring of vegetation around the mound, absence of a burrow hole, and lack of a grazed appearance.

Low-density stratum lines were flown in Nebraska, North Dakota, and South Dakota in 1997. In Wyoming, low-density stratum lines and all high-density stratum lines were flown in 1998. Single-engine, high-wing aircraft were used at an altitude of 152 m and a speed of 145-160 km/hr. A global positioning system receiver with ±10 m accuracy and loaded with waypoints was used to navigate to and along high- and low-density strata lines. A computer displayed the aircraft position and high- and low-density strata lines on a map and recorded global positioning system coordinates at 1-s intervals (Sidle 1999). Global positioning system input to the computer was annotated to indicate portions of lines that intersected colonies and activity. We judged colonies to be active if we saw black-tailed prairie dogs, fresh burrow excavations, lack of vegetation on burrow mounds, or extensive bare ground. Colonies with heavily vegetated burrows and adjacent ground were classified as inactive.

Analysis. — We developed 2 estimates of coverage (C) of colonies of black-tailed prairie dogs occurring in each stratum (with area A) based on ratios (r_i) of lengths of intercepts (y_i) of colonies of black-tailed prairie dogs and lengths of lines flown (x_i). Lines were indexed by i; i = 1, ... n, where n is the number of lines. One estimate, is based upon a simple average of the individual line ratios . The variance of  _A was estimated by . The ratio estimator was where . The variance of was estimated by (_R ) = A²(_R), where (_R) = [V(y) + _R²V(x) - 2_R cov(y,x)] / [n²], cov(y,x) was the sample covariance between y and x, and was the mean of x (Cochran 1977:155, equation 6.13). Because we ignored the finite population correction, variance estimates were biased to be high. Estimates were developed for each stratum individually, as well as combined, and the variance of that sum was the sum of the variances of estimates for each stratum. The ratio estimator is likely to be suitable for situations in which a strong positive correlation between the length of colony intercept and the length of line flown (Cochran 1977). Without such a strong correlation, the average-density estimator will have better properties than the ratio estimator.