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Estimated Areal Extent of Colonies of Black-tailed
Prairie Dogs in the Northern Great Plains

Results and Discussion

We estimated that 329.6 km² 72.3 SE, 139.9 21.6 km², 576.0 81.4 km², and 1,332.3 149.8 km² of active colonies of black-tailed prairie dog occurred in Nebraska, North Dakota, South Dakota, and Wyoming, respectively, for a total of 2,377.8 186.7 km², and similarly, we estimated the area of inactive colonies in those states to be 122.9 26.8 km², 27.8 11.2 km², 152.9 49.2 km², and 256.8 68.5 km², respectively, for a total of 560.4 89.2 km² (Table 1).

Table 1.  Estimates of coverage (km²) for active and inactive colonies of black-tailed prairie dogs in the northern Great Plains.  Two estimates () are presented: average-density estimate (ADE) and ratio estimate (RE).
State and stratum Survey area (km²) Length flown (km) Number of transects Active colonies, C hat (SE) Inactive colonies, C hat (SE)
    High 2,840 2,018 287 193.1 (16.8) 203.2 (15.5) 38.7 (5.8) 53.4 (7.9)
    Low 138,262 9,908 35 126.4 (70.6) 144.7 (81.7) 69.5 (25.6) 68.5 (23.1)
North Dakota
    High 1,048 777 129 89.5 (10.4) 103.3 (9.5) 18.2 (5.9) 11.6 (2.9)
    Low 49,726 3,379 19 36.6 (19.4) 23.0 (11.4) 9.5 (9.5) 7.6 (7.7)
South Dakota
    High 3,570 2,525 252 308.1 (27.2) 339.6 (24.0) 34.2 (5.8) 38.7 (5.4)
    Low 107,380 7,748 27 236.4 (77.8) 261.3 (83.4) 114.2 (48.9) 126.3 (53.0)
    High 3,254 2,357 175 583.6 (44.7) 598.8 (37.0) 19.5 (3.7) 26.3 (4.5)ADE
    Low 62,831 4,352 19 786.3 (216.6) 733.5 (145.1) 177.3 (53.9) 230.5 (68.4)

State agencies estimated the area of active colonies as 240-323 km² in Nebraska during the mid-1990s (M. Fritz, Nebraska Game and Parks Commission, pers. comm.), 85 km² in North Dakota in 1990 (Williams 1999), 745 km² in South Dakota in 1987 (Tschetter 1988), and 530-825 km² in 1987 (Oakleaf et al. 1996) and 1,465 km² in 1998 in Wyoming (R. Reichenbach, Wyoming Department of Agriculture, pers. comm.). However, previous estimates cannot be used to indicate trends in colony area because previous estimates are often cursory and incomplete and do not allow valid comparisons with our data. State agency estimates are based on limited aerial surveys, review of available aerial photographs, and collective visual estimates from weed- and pest-control staffs. Nevertheless, our estimates and state agency estimates concur that unlike popular belief thousands of square kilometers of black-tailed prairie dog colonies do not remain in the northern Great Plains.

Our survey results indicate only a weak correlation between length of line flown and length of colonies intercepted in the low-density stratum of each state, except for Wyoming (Table 2). That correlation was true for active and inactive colonies. In contrast, correlations were strong and positive in the high-density stratum of each state, except for inactive colonies in North Dakota (Table 2). Because of this disparity between strata, the ratio estimator of active is preferred in the high-density stratum of each state and the low-density stratum of Wyoming. The average-density estimator is recommended for the low-density stratum in all states except Wyoming. Accordingly, we computed a composite estimator, which for each state except Wyoming is the sum of average-density estimates for the low-density stratum and the ratio estimate for the high-density stratum. For Wyoming, the composite estimator is the sum of both ratio estimates. Variances of the composite estimates were calculated as the sum of the variances of the 2 constituent estimates. Composite estimates capitalize on correlations between length of line flown and length of colony intercept when they are strong but do not use those correlations when they are weak. Accordingly, composite estimates, along with their standard errors, are best estimates of the area of active colonies of black-tailed prairie dogs in the surveyed portion of each state.

Table 2.  Pearson correlation coefficients between length of line flown and line length that intercepts colonies of black-tailed prairie dogs (by stratum and state).
State Active colonies Inactive colonies
High Low High Low
Nebraska 0.48 0.18 0.51 0.06
North Dakota 0.64 -0.33   -0.04   -0.15  
South Dakota 0.60 0.24 0.40 0.18
Wyoming 0.63 0.62 0.44 0.54

In the area of inactive colonies, the correlation between length of colonies intercepted and length of flight line was minimal, except in the high-density strata of Nebraska and South Dakota and in both strata in Wyoming (Table 2). Use of ratio estimates for those strata, and average-density estimates for the other strata, led to our general estimates of inactive colony area of black-tailed prairie dogs (Table 1).

Stratification was successful in distinguishing between areas with relatively abundant active colonies of black-tailed prairie dogs and those without colonies. Percentage of high-density strata covered by colonies ranged from 6.9% in Nebraska to 17.9% in Wyoming. In contrast, values for the low-density strata ranged from only 0.1% in North Dakota to 1.3% in Wyoming. These comparisons were based on average-density estimates, but analogous conclusions were true for ratio estimates.

The low-density strata contributed substantially to the total estimated area. Colonies in those strata were sparse, but because of the great extent of the low-density strata, colonies in them constituted an appreciable area. Estimates for the high-density strata were more precise, often markedly so, than those for the low-density strata (Table 1). That disparity was due mainly to the higher sampling intensity in the high-density strata (transects were 0.86 km apart there versus 13.85 km separating transects in the low-density strata). Most of the uncertainty, as reflected in the standard errors, derived from the low-density strata.

The area of inactive colonies divided by the sum of active and inactive colonies was 27.2% for Nebraska, 19.9% for North Dakota, 21.0% for South Dakota, and 16.2% for Wyoming. Several years may pass before an inactive colony is no longer discernible from the air (Uresk and Schenbeck 1987). Inactive colonies likely are sites recently poisoned and fumigated or affected by plague epizootic, factors that can eliminate populations of black-tailed prairie dogs (Cully 1993; Fagerstone and Ramey 1996). Recreational shooting of black-tailed prairie dogs usually only limits rather than eliminates populations (Vosburgh and Irby 1998).

Because any colony of black-tailed prairie dogs known to us was included in a high-density stratum, colonies estimated for the low-density strata represent previously unknown colonies. For that reason, if no other, any comparisons of areas of colonies estimated at an earlier time to those included here could be misleading. However, our survey revealed that the general distribution of colonies of black-tailed prairie dogs in the 4-state area was similar to that reported by others (North Dakota Game and Fish Department 1990, M. Fritz, pers. comm., Oakleaf et al. 1996, Tschetter 1988).

In North Dakota, the black-tailed prairie dog largely occurs on Standing Rock Indian Reservation, which borders the Missouri River and lies south of the Cannonball River and extends into South Dakota, and in the badlands in the southwestern part of the state. The intervening area is largely cropland. Regions where colonies of black-tailed prairie dogs occur are largely grassland with a significant element of public (Little Missouri National Grassland and Theodore Roosevelt National Park) and tribal ownership. However, control efforts have eliminated vast areas of colonies of black-tailed prairie dogs in the southwestern area (Bishop and Culbertson 1976). Only 1,157 ha of colonies exist on the 462,705-ha Little Missouri National Grassland, although 296,000 ha are potential habitat for black-tailed prairie dogs (United States Department of Agriculture, Forest Service 2001). Colonies likely persist on tribal lands due to changing policies and a lack of resources to poison and fumigate the species.

The distribution of colonies of black-tailed prairie dogs in South Dakota follows a similar pattern as in North Dakota. Most colonies occur on and in the vicinity of public and tribal lands. From the airplane, the boundary of areas rich in colonies of black-tailed prairie dogs and poor in colonies often was signaled by passage into and out of tribal lands and public lands such as Buffalo Gap National Grassland and Badlands National Park. Surprisingly, large areas of South Dakota dominated by privately owned rangeland such as the region west of the Standing Rock and Cheyenne River Indian Reservations, harbor very few colonies of black-tailed prairie dogs.

Nebraska, has very little public land and no tribal land within the range of the black-tailed prairie dog. Colonies were found primarily in the western panhandle region and the southwestern region. However, colonies occurred throughout the surveyed area. Colonies were observed on small patches of grassland surrounded by cropland for miles, and near housing developments in eastern and central Nebraska. Even in the heavily cropped Platte River valley, an occasional colony occurred, attesting to the adaptability of the species. In central and western Nebraska, colonies were observed more commonly in a landscape of cropland and rangeland, such as in the vicinity of the cities of Alliance and Scottsbluff, than in large areas of contiguous rangeland in the Nebraska panhandle. We observed few colonies in the extensive Sandhills region, a large area of nearly uniform grassland. Very sandy substrate probably accounts for few colonies of black-tailed prairie dogs. Moreover, many valleys between dunes have been converted to hay fields.

Many colonies of black-tailed prairie dogs in Wyoming occur on Bureau of Land Management public land and the Thunder Basin National Grassland. However, substantial areas of colonies occur on private land and in the vicinity of roads and other developments.

Although substantial areas of grassland have been converted to cropland in the northern Great Plains (Samson and Knopf 1994), we observed vast areas of suitable habitat for colonization and expansion of black-tailed prairie dogs on public, private, and tribal lands. This visual impression is consistent with analyses of National Grasslands (United States Department of Agriculture, Forest Service 2001) and analyses in Montana (Proctor 1998; J. Proctor, Predator Conservation Alliance, pers. comm.), where hundreds of thousands of hectares and millions of hectares, respectively, are viewed to be potential habitat for colonies of black-tailed prairie dogs. Little need probably exists for habitat restoration, but rather a need to reestablish colonies of black-tailed prairie dogs. Given the negative attitude of many people toward black-tailed prairie dogs (Reading et al. 1999), expanding populations of black-tailed prairie dogs on public lands should be a priority in any conservation strategy.

Our survey provided the only unbiased estimates of the areal extent of colonies of black-tailed prairie dogs over an extensive area. The survey also suggests a sampling plan for future monitoring of the species. Conducting the survey described herein required 600 h of flying. Future monitoring could repeat a fraction of the original survey every year. For example, surveying 20% of the region each year for 5 years could provide reasonably current estimates of the extent of colonies and detailed information about where changes are occurring.

Standardized survey techniques are important as Great Plains states implement conservation strategies for the black-tailed prairie dog (Van Pelt 2000). Working groups comprising private landowners, state and federal agencies, tribes, and others have been established throughout the Great Plains to plan for conservation of the species. Monitoring is an important facet of conservation planning. Although millions of black-tailed prairie dogs and thousands of colonies probably exist, the ongoing threats of plague and eradication efforts only can be assessed only by adequate monitoring throughout the Great Plains.

Remote sensing may prove valuable in monitoring colonies of black-tailed prairie dogs. However, aerial photography larger than the 1:24,000 scale recommended for black-tailed prairie dog monitoring (Best et al. 1983; Cheateam 1973; Dalsted et al. 1981; Schenbeck and Myhre 1986; Tietjen et al. 1978) was not available for the study area. Resolution of satellite imagery at the time of the aerial survey was not adequate, although new orbital space sensors now provide adequate resolution.

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