360 G. O. Batzli et al. 



amounts of moss in lemming diets, up to 40% in winter, even though the 

 digestive efficiency for energy of mosses is low. Lemmings fed only 

 mosses reduce their intake and quickly starve. ResuUs from the model 

 suggest that mosses serve as nutrient supplements because they are 10 to 

 20% higher than graminoids in phosphorus concentration and 200 to 

 300% higher in calcium. The low digestive efficiency for energy of 

 mosses requires a larger food intake, and hence assures a larger intake of 

 nutrients. But mosses cannot be used exclusively as forage because of low 

 digestible energy or other nutritional deficiencies. 



Finally, reproduction during winter, when energy demand is already 

 high, becomes more understandable with the hypothesis that growth and 

 reproduction are limited by nutrient availability rather than energy 

 availability. In fact, the high energy demand of thermoregulation may 

 assure an adequate intake of nutrients, and conditions for reproduction 

 may be nearly as favorable in winter as they are in late summer, if suffi- 

 cient forage is available to meet energy demands. 



PREDATION ON LEMMINGS 

 Introduction 



Predators are conspicuous in the tundra at Barrow during the sum- 

 mer of a lemming high, and their populations have received considerable 

 attention (Pitelka et al. 1955, Maher 1970, 1974, MacLean et al. 1974). A 

 separation of avian and mammalian predators also distinguishes migra- 

 tory predators from those remaining through the winter (Table 10-6). 

 Their relative abundance throughout a standard lemming cycle is shown 

 in Figure 10-11. 



During the high winter, when the major increase to a high lemming 

 population occurs, the only predators are the arctic fox and two species 

 of weasels. Occasional snowy owls are seen in a pre-high winter, but they 

 are so scarce as to be negligible. The immigration of snowy owls begins in 

 late winter (April) of a high year and nesting commences in mid-May, 

 well before snowmelt. At snowmelt they are joined by pomarine jaegers, 

 glaucous gulls and, in some years, short-eared owls (Pitelka et al. 1955). 

 The total intensity of predation rises dramatically and reaches its max- 

 imum at snowmelt, when lemmings are both most numerous and most 

 exposed. A period of intense territorial activity follows, both within and 

 between species. Nonbreeders are forced to marginal habitat, if they re- 

 main at all. The breeding population, with some attrition of unsuccessful 

 birds, remains through the rest of the season. Migration of jaegers occurs 

 in August, and snowy owls follow in September and early October. 

 Following the departure of avian predators, foxes and weasels are again 



