344 G. O. Batzli et al. 



correlated with the vegetation types given in Chapter 1 (Table 1-4): 



1. Ridges: well-drained raised areas with Salix heath vegetation (I, 

 II). 



2. High-centered polygons: polygons with well-drained centers and 

 with wet troughs and ponds between them (I, III, V, VI). 



3. Low-centered polygons: polygons with mesic centers and with wet 

 troughs between them (II, III, IV, V, VI). 



4. Graminoid flats: wet meadow with mixed graminoid vegetation 

 (V, VI). 



5. Carex flats: wet meadow dominated by Carex (VI). 



Although densities varied, the pattern of habitat use was fairly con- 

 sistent from year to year. In early summer, well-drained areas dominated 

 by high-centered polygons were the most heavily used, and twice as many 

 lemmings were captured there as in any other habitat. By late summer the 

 use of such areas decreased markedly, while use of areas dominated by 

 low-centered polygons and of low-lying meadows increased. This pattern 

 reflects the fact that low habitats are flooded in early summer but be- 

 come drier as the summer progresses. The extremes of the moisture gra- 

 dient, ridge and Carex meadow, received substantial use only when pop- 

 ulation densities were high. Collared lemmings, when present, usually 

 were found in ridge or high-centered polygon habitats (Figure 10-6). 



Density estimates and signs of activity on four 2.25-ha live-trapping 

 grids in the immediate vicinity of the Biome research area gave a slightly 

 different picture of habitat use. During winter nest densities were greater 

 in high-centered polygons than other habitats, but population densities 

 at snowmelt and winter clipping of vegetation were greater in low- 

 centered polygons (Table 10-1). Clipping rates indicate the amount of 

 winter foraging, while nests may reflect the intensity of reproduction. All 

 indicators of activity support the notion that the polygon and pond habi- 

 tats, which had relatively uniform vegetation heavily dominated by 

 Carex, were used least by lemmings. Summer population densities and 

 the rates of clipping indicate that the low-centered polygons and mea- 

 dows were used most heavily, particularly in the summer of 1972 and the 

 winter of 1973. At lower densities, in the summer of 1973 and the winter 

 and summer of 1974, use of high-centered and low-centered polygons 

 was more similar. 



Use of the meadow depended upon moisture conditions. The sum- 

 mer of 1972 was relatively dry, so lemmings moved into the lower, wetter 

 portions of the grid as standing water receded, and the overwintering 

 population was similar to that on low-centered polygons (Table 10-1). 

 The summer of 1973 had late rains, however, and much of the meadow 

 was flooded at freeze-up. As a result, the overwintering population dur- 

 ing 1974 was only a third of that on low-centered polygons. 



