. The Canadian field-naturalist. . Figure 1. Mapof Cook County, Minnesota, showing localities where Rock Voles were captured during 1980, 1981, and 1982 (closed circles); open circles indicate Rock Vole localities of Buech et al. (1977) and Timm (1974). Numbers identify Cook County and USDA Forest Service roads. other authors (Kirkland and Jannett 1982; Timm et al. 1977). Martin (1971) reported a relatively constant ratio of 3 C gapperi per M. chrotorrhinus; Kirkland (1977) and Kirkland and Knipe( 1979) observed ratios of about and , respectively, in West Virginia and New York. In the p
. The Canadian field-naturalist. . Figure 1. Mapof Cook County, Minnesota, showing localities where Rock Voles were captured during 1980, 1981, and 1982 (closed circles); open circles indicate Rock Vole localities of Buech et al. (1977) and Timm (1974). Numbers identify Cook County and USDA Forest Service roads. other authors (Kirkland and Jannett 1982; Timm et al. 1977). Martin (1971) reported a relatively constant ratio of 3 C gapperi per M. chrotorrhinus; Kirkland (1977) and Kirkland and Knipe( 1979) observed ratios of about and , respectively, in West Virginia and New York. In the present study, the overall ratio of Red-backed Vole: Rock Vole numbers was (137/98), and Red-backed Vole numbers exceeded those of Rock Voles at only 19 of the 59 sites. Thus, the relative abundance of Rock Voles observed in this study appears to have been higher than previously reported. Peromyscus maniculatus, Blarina brevicauda, and Sorex cinereus were commonly observed at Rock Vole sites (29%, 20%, and 17% of the localities, respec- tively). Other small mammals captured at Rock Vole localities included Sorex palustris (1 site), Eutamias minimus (3), Napeozapus insignis (I), Synaptomys cooperi (2), and Microtus pennsylvanicus (2). The extent to which habitat distribution of Rock Voles is influenced by interactions with Southern Bog Lem- mings {S. cooperi) or Meadow Voles {M. pennsylva- nicus) is not known (see Timm et al. 1977). In that context, it is significant that these other two microtine rodents appear to coexist with M. chrotorrhinus at relatively few sites in northeastern Minnesota. As described above, potential sampling sites were selected on the basis of presence of exposed rocks. This common feature of Rock Vole localities reported in this study was thus determined a priori. Nonethe- less, presence of boulders was highly predictive of presence of M. chrotorrhinus: 59 of 69 (86%) sites characterized by boulders produced Rock Voles, regardless of vegetative comp
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