The Herbivore-Based Trophic System 367 



75% of the food items in pellets of the parasitic jaeger. Data were avail- 

 able for the long-tailed jaeger only for the 1955-58 period when this spe- 

 cies took fewer birds and many more insects than did the other jaegers. 

 Long-tailed jaegers are most common in July, when adult craneflies 

 (Tipulidae) are abundant on the tundra surface. 



Comparable data on prey selection by snowy owls in relation to lem- 

 ming density are lacking. Pitelka et al. (1955) noted that Lemmus makes 

 up the bulk of the prey, but they also found owl pellets containing the re- 

 mains of a variety of birds, ranging in size from the Lapland longspur to 

 the old-squaw duck, as well as the remains of a least weasel. Examination 

 of pellets over many years shows that the fraction of the diet consisting 

 of lemmings is as large for snowy owls as for pomarine jaegers and may 

 be even larger at low to moderate lemming densities. 



Arctic foxes depend not only on lemmings but also on carrion dur- 

 ing the winter and on birds and their eggs during summer. Using the lem- 

 ming population dynamics model, a lemming mortality equivalent to 

 13.5 kg ha"' was estimated during the winter (September-May) of a lem- 

 ming high. These carcasses could constitute an important food source. 

 Mullen and Pitelka (1972) investigated the disappearance of lemming 

 carcasses by placing dead lemmings on the tundra in autumn. They 

 found that virtually all carcasses disappeared by the following spring. In 

 some cases other lemmings were implicated, but weasels were few, and 

 winter observations indicated that foxes were primarily responsible for 

 the removal of carcasses. 



No data are available on the summer diet of foxes in the Barrow re- 

 gion, but in the Prudhoe Bay region Underwood (1975) found remains of 

 Lemmus and Dicrostonyx in 86% of 50 fresh scats in an inland area and 

 75% of 24 scats in a coastal area, despite generally low densities of mi- 

 crotines during the 1971 summer. Birds were found in 50% of the scats 

 from inland and 63% of the scats from the coast. Underwood (pers. 

 comm.) reports finding as many as 30 lemming carcasses in a single fox 

 den at other sites on the Arctic Slope. It is likely that the functional re- 

 sponse of foxes to lemming density is much greater than their numerical 

 response. 



Data on the winter diet of both species of weasel derive from obser- 

 vations of remains and scats at lemming nests (MacLean et al. 1974), and 

 thus may be biased in favor of lemmings. However, in the absence of al- 

 ternative prey during winter, it seems safe to say that virtually the entire 

 diet consists of lemmings. During summer weasels also take birds and 

 their eggs, and a functional response to lemming density is probable. 



Selective predation upon sex or age classes within the lemming pop- 

 ulation could influence the population dynamics of the lemmings. Since 

 both jaegers and owls regurgitate pellets containing the bones of their 

 prey, it is possible to estimate the frequency of capture of various classes 



