374 G. O. Batzli et al. 



Predation 



The general characteristics and feeding patterns of predator popula- 

 tions have been discussed above. In this section the impact of predation 

 on the population dynamics of lemmings will be evaluated. Osborn's 

 (1975) simulation indicated that in some years avian predators could ac- 

 count for 88% of the early summer mortality of lemmings. The major 

 impact came at snowmelt when lemming densities were about 25 ha"', 

 and percent predation dropped off rapidly at higher densities. Maher 

 (1970) calculated the total impact of all predators on lemming popula- 

 tions during a high year. The calculations were based on the assumption 

 that no young lemmings were weaned until mid-July and that mean litter 

 size was six. Both assumptions are conservative (BatzH et al. 1974). He 

 concluded that predation could not prevent population growth during 

 the summer if lemming densities were greater than 65 ha"' at snowmelt. 

 Since lemming densities during high years are usually greater than 100 

 ha"' at snowmelt, something in addition to predation must account for 

 declines during the following summer. Predatory impact at high lemming 

 densities is reduced by the protection afforded lemmings by territorial 

 pomarine jaegers. During summers with lower population levels, when 

 jaegers are not territorial, predation rates may be high enough to prevent 

 population increases. Thus, predation contributes substantially to the 

 rapid decline of lemming densities during some summers, and may even 

 prevent population growth in others, but it is not sufficient to cause the 

 decline of lemming densities from peaks. 



During winter weasels are the most important lemming predators, 

 and there is some evidence that winter predation rates may be sufficient 

 to restrict the growth of lemming populations. During 1963 and 1969, 

 lemming populations reproduced under the snow, but by snowmelt the 

 populations had been reduced, and there was evidence of intense preda- 

 tion by weasels (Mullen 1968, MacLean et al. 1974). Maher (1967) re- 

 ported a similar circumstance on Banks Island. 



Pearson (1966) argued that the most significant effect of predators 

 on cycling populations of microtines is the reduction of populations to 

 extremely low levels, which delays their recovery. Maher (1970) and 

 MacLean et al. (1974) supported this view as it applied to lemming popu- 

 lations. The increased mortality that adult lemmings experience during 

 summers when populations are low lends further credence to such a role 

 for predation. However, high populations of lemmings usually develop 

 under the snow and not during summer, so the critical period is winter. If 

 winter predation regulates lemming populations, intensity of predation 

 should be negatively correlated with the change in lemming density. Our 

 data for the winters of 1969 through 1974 do not show this trend. During 



