Black-tailed prairie dog monitoring at Scotts Bluff National Monument: A comprehensive report 1995-2009

Morrison, Lloyd; Dunkle, Ashley; Peitz, David

Three black-tailed prairie dog (Cynomys ludovicianus) colonies have been in existence at Scotts Bluff National Monument, Nebraska, for some period since 1981: (1) The Main Colony was initially established in the northwest corner of the monument in 1981 by dispersers from outside the monument, and has persisted continuously. (2) The North Colony, first observed in 2002 and still in existence, is near the Main Colony, but separated by an irrigation canal. (3) The Saddle Rock Colony, located in the east-central area of the park, was first observed in 2002 and disappeared between the 2007 and 2008 visits. The Main Colony was monitored for 15 consecutive years (1995-2009) using consistent methodologies outlined by Plumb et al. (2001), to measure area occupied and to estimate density and total population size. The North and Saddle Rock Colonies have been monitored since 2004. Additional estimates of these population parameters obtained by other techniques between 1983 and 1995 for the Main Colony are also included here, although the estimates may not be directly comparable. From colonization in 1981, the Main Colony grew to 303 individuals by 1987, despite treatments with reproduction-inhibiting drugs and removal of individuals. The Main Colony declined rapidly from 1987 to 1989, and then declined generally more slowly to 1995. The Main Colony grew at an exponential rate from 1995 to 2003, increasing from 17 to almost 800 individuals. From 2003 to 2009, the Main Colony was highly variable, declining to 147 individuals in 2007, before rebounding to almost 700 individuals in 2009. Area occupied by the Main Colony revealed patterns similar to total estimated population size. The North Colony, surrounded on all sides by barriers to expansion, grew rapidly from 2002 to 2004, occupying a large proportion of the entire habitable area and reaching an estimated population size of >160 by 2004. The colony has varied in area occupied and population size since 2004. In 2008 and 2009, several individuals and active burrows were observed on neighboring private lands. The Saddle Rock Colony generally increased in area occupied and estimated population size from discovery in 2002 until 2007, before mysteriously disappearing between the 2007 and 2008 surveys. Illegal poisoning is the most likely reason for its sudden disappearance. Density varied greatly over time for all colonies. The overall prairie dog population at Scotts Bluff National Monument is relatively small and occupies only a small portion of the monument (i.e., 1.45% of the monument’s 698-ha of grassland). Although the prairie dog is known as a keystone species of grassland ecosystems, the Scotts Bluff population is not large enough to produce this aspect of community structure and function. Moreover, small population sizes generally increase susceptibility to random demographic events that may result in inbreeding depression or extinction. Mowing and burning treatments have been shown to have significant positive effects on colony expansion and could be employed to increase area occupied and total population size. Prairie dog colonies may also be re-established through translocations. The North Colony, surrounded by obstacles to expansion, represents an excellent source of colonists for such translocations.

Black-tailed prairie dog monitoring at Scotts Bluff National Monument: A comprehensive report 1995-2009

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