The Herbivore-Based Trophic System 345 



TABLE 10-1 Summary of Indicators of Brown Lemming 

 Activity in Four Habitats at Barrow 



Note: Densities are seasonal extremes. Habitat types are illustrated in Figure 10-6. 



These observations support several generaHzations. First, brown 

 lemming activity, both summer and winter, tends to be concentrated in 

 polygonal terrain, which has a mixture of relatively dry and wet habitat. 

 Second, low, wet areas with vegetation heavily dominated by Carex and 

 dry ridges with Salix heath are the areas least utilized by brown lem- 

 mings. Third, the use of low-lying meadows with mixed graminoids 

 varies depending on seasonal and annual moisture conditions. 



Lemming activity is neither randomly nor uniformly dispersed with- 

 in the larger topographic units discussed above, and the use of local mi- 

 crotopographic units must also be considered. Habitats with well- 

 developed high-centered and low-centered polygons have the greatest mi- 

 crotopographic relief and support the greatest winter nesting. Ninety- 

 three percent of 139 nests examined in 1974 were located in polygon 

 troughs. Winter clipping of vegetation is also concentrated in troughs, 

 where the density of shoots is higher, but patches of clipping occur in all 

 microtopographic units. In 1973 we found clipping percentages of 25 to 

 50% in troughs, where graminoid shoot densities were about 3000 m'^ 

 15% on rims with 2000 shoots m'\ and 5% in basins with 1000 to 1500 

 shoots m'^ 



Summer activity patterns have been more intensively studied than 

 winter patterns. Using techniques of radio tracking. Banks et al. (1975) 

 showed that individual lemmings may move over 1 km day"'; however, 

 most lemmings live in home ranges of highly variable size. Females tended 



