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Some have estimated that black-tailed prairie dogs once occupied between 400,000 and 1 million km² of the Great Plains before European settlement and have declined by 98% (Mac et al. 1998; Miller et al. 1990; Mulhern and Knowles 1997). Merriam (1902) stated that 1 colony in Texas covered about 65,000 km². Conversion of grassland to cropland, poisoning and fumigation programs, and sylvatic plague (Yersinia pestis) have decimated populations of the black-tailed prairie dog (Cully 1993; Cully et al. 2000; United States Fish and Wildlife Service 2000). Most remaining colonies are <40 ha in size and are isolated from other colonies (J. G. Sidle, in litt.). Colonies of black-tailed prairie dogs declined to 6,000 km² in the Great Plains by 1960 (Anderson et al. 1986; Marsh 1984) and are estimated to occupy 3,120 km² today (P. Gober, United States Fish and Wildlife Service, pers. comm.). Most of those estimates were based on imprecise and cursory information.

The United States Fish and Wildlife Service (1999) determined that a petition to list the black-tailed prairie dog as threatened under the Endangered Species Act presented substantial scientific information, and the agency later concluded that listing of the species as threatened is warranted (United States Fish and Wildlife Service 2000). Because of the concern about the status of the black-tailed prairie dog, the Forest Service of the United States Department of Agriculture and United States Geological Survey began planning an aerial survey of the northern Great Plains in 1996. We report the results of that survey.