The Herbivore-Based Trophic System 343 



age, sex and reproductive class for a standard cycle based upon data 

 from 1961-65 (Figure 10-5). In the model, survival rates for October 

 through April were not varied with time and should be viewed as average 

 values over the winter. 



One of the most striking results of these simulations was the small 

 increase in survival of lemmings required to build a peak population 

 (Figure 10-5). Sucklings accounted for about half of the population dur- 

 ing the pre-high summer and one-quarter of the peak population, During 

 this time, reproductive rates were no better than the previous year, but 

 suckling and female survival improved slightly. Even with high reproduc- 

 tive rates survival rates of suckhngs and juveniles during the pre-high 

 summer needed to be relatively high for the population to increase as ob- 

 served in the field. During the population crash the survival rates of 

 young animals remained very low so that at times the population became 

 almost completely adult. Survival rates for sucklings are based only on 

 litters whose mother also survives; when a nursing female dies her litter 

 also dies. Thus, improvement of female survival has a double effect on 

 population growth. 



Habitat Use 



Lemmings do not make equal use of various habitat types, so any 

 analysis of herbivory must consider spatial patterns of habitat utiliza- 

 tion. We use two scales in our analysis, mesotopographic units and mi- 

 crotopographic units (Figure 10-6). 



Snap-trap lines run by Pitelka for 19 years in the Barrow area pro- 

 vided information on habitat utilization. These lines were divided into 

 five habitat types, which can be arranged along a moisture gradient and 



in Ez.v n.iD Ez.s yi 



Vegetation Type 



i2,y,2i 211 y,3n in ni.iz 



Small, low-centered polygon 

 (Polygon grid) 



Meadow 

 (Meodow grid) 



Meodow 



Large, low-centered 

 polygons ond ponds 

 (Pond grid) 



Higti-centered 

 polygons 

 (Upland grid) 



FIGURE 10-6. The relative utilization by lemmings of habitat types on 

 the Barrow research area. The numerals above the topographic profile 

 indicate vegetation types (Table 1-4). 



