366 G. O. Batzli et al. 



Ermine likewise show a numerical response to lemming density, al- 

 though the response is less than that of the least weasel (MacLean et al. 

 1974). Ermine usually occur on the Barrow peninsula when lemming 

 populations increase, but the density of ermine is probably less than 10% 

 of the density of the least weasel. 



Weasels differ from avian predators in their response to declining 

 lemming populations. Low lemming density leads to reproductive failure 

 and reduction of the population of adult jaegers and owls. By early fall, 

 regardless of nesting success, all avian predators have departed. Weasels 

 are less mobile; they remain through the summer into the post-high win- 

 ter, and exert considerable predation pressure upon the declining lem- 

 ming population. Eventually, by death or emigration, weasel density 

 falls to undetectable numbers that characterize the low phase of the lem- 

 ming cycle. 



Nutrition and Energetics of Predators — Functional Response 



Predators characteristic of the coastal tundra at Barrow are categor- 

 ized as obligate or facultative lemming predators (Table 10-6). Obligate 

 predators are those whose presence depends upon an adequate popula- 

 tion of lemmings. Such predators show the largest numerical response 

 and the absence of a strong functional response. Facultative predators 

 are able to maintain a population at times of low lemming density by use 

 of alternative food sources. Their numerical response to lemming density 

 is small or even inverse, but they show a marked functional response. 

 This section will consider two aspects of the functional response of 

 predators: 1) changes in prey selection associated with changes in the 

 density of primary and ahernate prey, and 2) bioenergetic factors which 

 determine rate of prey capture and consumption. 



Clear differences in the food habits of jaegers are evident between 

 years and between species (Maher 1974). Remains of lemmings were 

 found in 98% of the pellets of pomarine jaegers taken in the high lem- 

 ming years of 1956 and 1960, indicating the strong dependence of breed- 

 ing pomarine jaegers upon lemmings. Lemmings were found in 10097o of 

 75 pellets collected in 1959, when a few pairs of pomarine jaegers at- 

 tempted breeding during a period of much lower lemming density. Even 

 when lemming populations were low, lemmings were found in 41 % of 56 

 stomachs of transient, nonbreeding pomarines collected in 1957 and 

 1958. This slight functional response was of little importance because the 

 birds were rarely seen after mid-June 



Parasitic jaegers preyed more on birds and their eggs than did the 

 other jaegers, particularly when lemming densities were low (Maher 

 1974). However, during the 1956 lemming high, lemmings accounted for 



