Populations of the Water Vole (Microtus richardsoni) occur along headwater streamlets at timberline in the Rocky Mountain front ranges of Alberta. At the 2000-m contour riparian distances between potential habitat patches ranged from 749 to 24,651 m, whereas overland distances ranged from 157 to 5104 m. Reproduction is restricted to a snow-free (May–August) period of 60 to 112 d in which females typically produce 1 to 2 litters of 2 to 10 young. Based on home range overlaps, the mating system appears to be polygynous. Over 4 habitat patches and 9 patch-seasons, reproduction was dominated by overwintered adults and began with effective populations (Ne) of 9.2 (4.1 males and 6.8 females) occupying 3- to 4-ha patches of mesic vegetation. Territorial behavior opposes recruitment into these breeding populations. Few males have an opportunity to breed in their natal colony in their 1st summer, but 5% of 1st-litter female young (50% known to be philopatric) may do so. Young of subsequent litters, including any born to 1st-litter females, are unlikely to breed in the season of their birth. Episodes of predation may facilitate settlement and breeding of philopatric young of the year, or “floaters” of unknown origin. In a typical colony, breeding generated a mean surplus above replacement (potential emigrants) of 8 males and 6 females. Disappearance rates of overwintered adults (the number of voles present in 1 trapping session and not in the next divided by the number of intervening days) remained low until August. Disappearance of the 1st cohort of young averaged 1.8%/d for females and 3.7 %/d for males. Young persisting into September in their natal colonies were the most likely to be present in the following spring. The socially determined window for recruitment appears limited to late summer and early fall. Winter trapping indicated that low temperatures and heavy snow cover inhibit movement between habitat patches during the winter months. The limited genetic analysis available indicates that variation is maintained in local gene pools despite restriction to small isolated habitat patches, small effective population sizes, and territorial behavior. Conservation concerns may arise due to the habitat specificity of M. richardsoni and the constraints on population dynamics and gene flow imposed by the small size and isolation of habitat patches and the harshness of the subalpine environment.
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