Throughout its known range in the western part of the Northern Great Plains, P. leucopus is represented by small and disjunct populations associated with riparian deciduous timber. The known population nearest to Harding County is on the Black Hills to the south; next nearest are several isolated or semi-isolated populations along the Yellowstone River in eastern Montana. Presumably, this white-footed mouse spread northwestward into the western part of the plains region along river systems, in company with deciduous trees, in some post-glacial period when the climate was warmer and wetter than now. Subsequent drying altered substantially the distribution and perhaps composition of riparian forests, and isolated populations of P. leucopus evidently survived only in restricted areas, such as Deer Draw, many of them probably marginal habitat for the species.
Unverified species.—If the 10 species listed as of "unverified occurrence," it seems highly likely that as many as eight will be found to occur, or occurred within historic time, in Harding County. Among these eight are one steppe species (Vulpes velox), three with boreomontane affinities (Sorex cinereus, Gulo gulo, and Lynx canadensis), two (Sciurus niger and Urocyon cinereoargenteus) that are associated with the eastern deciduous forests, and two (Sorex merriami and Lagurus curtatus) that are Great Basin elements.
It is noteworthy that the last-mentioned faunal unit is not known to be represented in northwestern South Dakota.
Acknowledgments
For assistance in the field, we are especially grateful to the students who were enrolled in the Field Course in Vertebrate Zoology at The University of Kansas in the summers of 1961, 1965, and 1970, and to M. A. Levy, R. R. Patterson, and T. H. Swearingen. In 1965 and 1970, the summer field course was supported in part by grants (GE-7739 and GZ-1512, respectively) from the National Science Foundation; Andersen was supported in the field in 1968 by a grant from the Kansas City Council for Higher Education. Personnel of the U.S. Forest Service (Sioux Division, Custer National Forest), particularly District Ranger Timothy S. Burns, were most helpful to us in the field, as were Wardens Wesley Broer and Merritt Paukarbek of the South Dakota Department of Game, Fish and Parks. Robert Kriege, Federal predator control agent stationed in Buffalo, was most generous in sharing with us his knowledge of rodents and carnivores in the area, and many present or former residents, particularly Carl Cornell and Spike Jorgensen, also provided useful information and were helpful in other ways.
Ectoparasites reported here were identified by Cluff E. Hopla (fleas), Richard B. Loomis (chiggers), and Glen M. Kohls (ticks). Other than mammals housed in the Museum of Natural History, we examined only three, two in the U.S. National Museum (USNM) and one in the collection at South Dakota State University, Brookings (SDSU).
Literature Cited
Anonymous
1959. Building an empire: a historical booklet on Harding County, South Dakota. Buffalo Times-Herald, 108 pp.
Bailey, V.