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<br />Some of the mammals noted in [he Craig vicinity have been surprises. Accord- <br />ing [o Armstrong (1972), the red-backed vole (Clethrionomys gapperi) and <br />masked shrew (Sorex cinerus) are not known [o occupy Moffat County at [tie ele- <br />vations involved in [his survey. In total, 93 red-backed voles were captured <br />at the "Summit Site" (Pfap M19) on the Williams Fork Mountains, attesting to <br />the common occurrence of this species at high elevations on [his range. A[ <br />the same time, some of the expected mammalian species are absent. Numerous <br />white-tailed prairie dog (Cynomys leucurus) were anticipated to be distributed <br />throughout proper habitats within [his region, but none were found. In fact, <br />there was no evidence that prairie dogs have been active near the Craig Sta- <br />tion site (Colorado-Ute Power Plant) within the past decade. Sagebrush voles <br />(Lagurus curt at us) were also expected in sagebrush communities, but none were <br />trapped. This observation may be explained by the fact that over time, the <br />species experiences violent fluctuations in abundance. At the low point of <br />it's population flux, this vole is apparently distributed unevenly throughout <br />its suitable range. <br />• At all sites and during most seasons, the deer mouse (Peromyscus maniculatus) <br />was by far [he most abundant of trappable rodents. On the Williams Fork Moun- <br />tains, where species richness and total abundance of small mammals were <br />greater than in all other trapping localities, population trends of the deer <br />mouse were clear: deer mice showed a substantial increase from [he low popula- <br />tion in winter (February) to higher numbers in June following recruitment from <br />spring births. Finally, rising [o a population peak in the fall near the <br />close of [he reproductive period. A[ the "Summit Site" on the Williams Fork <br />Mountains, red-backed voles (Clethrionomys gapperi) and long-tailed voles <br />(Microtus longicaudus) generally displayed the same demographic patterns. This <br />seasonal fluctuation is typical of small mammal species which have a tremen- <br />dous potential for rapid population increases during favorable periods of the <br />year. <br />Proportion of deer mouse juveniles and sub-adults relative to the number of <br />adults at different periods of Che year (Figure E-1) demonstrates [hat [he <br />• November population of deer :nice is overwhelmingly adult in composition. <br />E-21 <br />