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Jannett, F.J., Jr. 1998. Small mammal community dynamics in Cook County, Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 21 pp.


This research included the fifteenth year of consecutive annual monitoring of the small mammal populations in Cook County, Minnesota. Nineteen sites were included and censuses were attempted at two additional sites. The sites include those where rock voles (Microtus chrotorrhinus), heather voles (Phenacomys intermedius), and smoky shrews (Sorex fumeus) were previously secured. Numbers of small mammals (1984-1997) were analyzed with respect to data on pine marten (Martes americana) (1985-1996). All specimens of the three small mammal species mentioned above which have been secured 1992-1997 are tallied by year and site.

At the 19 monitoring sites, the average number of rock voles declined in 1997 from 1996. The average number of red-backed voles (Clethrionomys gapperi) increased from its 15-year nadir in 1996. Smoky shrews were more common than in any previous year except 1995. There was a total of 15 smoky shrews secured in 1997. No heather voles were secured in 1997. At the two sites with attempted censuses for which only Museum Special traps were used, there was no second peak of specimens of rock voles as had occurred previously with a two-stage-two-trap-type enumeration.

Analysis of pine marten-small mammal interaction included two measures of marten populations: the estimated size of the population and the ratio of juveniles to adult females. Numbers of small mammals analyzed with respect to the marten were those of rock voles, red-backed voles, deer mice (Peromyscus maniculatus), all species of shrews combined, and all species of small mammals combined. Analyses of correlations without time lags indicated that there were large and significant correlations between the numbers of prey (red-backed voles, shrews, all small mammal species combined) and the ratio of juvenile to adult female pine marten, thus supporting the hypothesis that prey availability correlates with success of breeding and/or raising young. There was a large and significant negative correlation between red-backed vole numbers and the population estimate of marten, thus suggesting a depressive effect of marten on red-backed voles. There was no significant correlation when prey indices were lagged one year nor when marten indices were lagged one year.

Numbers of specimens of small mammal species of greatest interest secured in 1992-1997 included four heather voles at two sites (two consecutive years), 117 smoky shrews at 19 sites, and 1,236 rock voles at all 21 sites. Smoky shrews and rock voles were secured each year. Heather vole occurrence is sporadic, despite uniformity of collecting effort and personnel. Smoky shrews seem to be increasing in numbers. Rock voles are relatively stable and common, despite a "crash" in their numbers in 1997, only the second such event in 15 years.

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