Dispersal of Richardson Ground Squirrel in Colorado

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Richardson ground squirrels (Citellus richardsonii) have dispersed southward central Colorado at about one and one-half miles per year since 1930. These squirrels may be reinhabiting areas formerly occupied during the Pleistocene. The present rate of dispersal of Richardson ground squirrels Colorado is favorably affected by: (1) lack of absolute physical barriers, (2) absence of natural competitors, (3) natural vegetative conditions, and (4) the ability of Richardson ground squirrels to readily occupy and inhabit burrows of other large fossorial rodents. Their dispersal is a natural consequence of past geologic events and not caused by modern man. The first record of occurrence Colorado for the Richardson ground squirrel (Citellus richardsonii) was reported by Vernon Bailey (1893). He recorded it from along Fish Creek . . . in extreme northern part of Colorado. The record is most likely from Fish Creek, Routt County, since two specimens from the U. S. National Museum were listed from this locality by Howell (1938). The distributional limits were not well known until Warren (1910) published his first edition of Mammals of Colorado. He listed the occurrence of Richardson ground squirrels from the following counties: Larimer, Grand, Routt, Moffatt, Rio Blanco, Eagle, and Summit. He stated the most northeastern record was at Fish Creek, Larimer County. Since he did not list any specimens examined, it is not definitely known if he was giving reference to Bailey's specimens from Routt County or whether he Ihad bona fide records from Fish Creek, Larimer County. Richardson ground squirrels occur at both localities and were probably inhabitants of both areas at the time of Warren's publication. Warren listed the westernmost record from about 10 miles west of Lay, Moffatt County; and the most southern records were at Minturn, Eagle County, and Dickey, Summit County. New locality records and known instances of dispersal were published between 1893 and 1931 (Warren, 1910, 1912, 1913; Burnett, 1912, 1913, 1916, 1918, 1919, 1920a, 1920b, 1923, 1924, 1929, 1931; Cary, 1911; Howell, 1938). However, no new information on dispersal has appeared the literature since Burnett's publication of 1931. This study reports new records of dispersal of the Richardson ground squirrel and compares the past history of distribution with the present known limits. New records of occurrence are primarily based upon specimens now the collection at Colorado State University. Acknowledgments.-I wish to thank M. W. Cummings, former District Agent, Branch of Predator and Rodent Control, U. S. Fish and Wildlife Service, Denver, and field personnel directly under his supervision for information on the present distribution of Richardson ground squirrels areas not visited by me. I also wish to thank R. R. Lechleitner, Colorado State University, for collecting a specimen from Taylor Park.