362 



G. O. Batzli et al. 



Trappable Lemmings 



(0 



c 

 m 

 a 



0.10 

 0.05 



-t-t- 



-t— t- 



\HX- 



I . Snowy Owl 



2. Pomarine Jaeger 



3. Other Joegers 



4. Glaucous Gull 



5. Short-eared Owl 



^ 



Arctic Fox 



2. Leost Weasel 



3. Ermine 



Prehigh 

 Summer 



High Winter 



fc^=£ 



a 



High Posthigh Winter Posthigh Prehigh Winter 



Summer Summer 



FIGURE 10-11. The estimated densities of predators during the course 

 of a standard lemming cycle for the coastal tundra at Barrow. Periods of 

 snowmelt and freeze-up are indicated between summer and winter. 

 (After Pitelka et al. J 955, and unpublished observations of authors.) 



the only predators. Presumably, these remain until the lemming popula- 

 tion has declined to the point that it will no longer sustain predators. 

 Foxes then switch to alternative food, especially dead waterfowl and sea 

 mammals, but weasels may disappear from the Barrow peninsula alto- 

 gether between lemming highs. Avian predators are usually scarce in the 

 post-high summer. 



Abundance of Predators — Numerical Response 



The rate at which predators take lemmings is the product of the nu- 

 merical response (number of predators per unit area) and the functional 

 response (number of prey taken per predator) to prey density (Holling 

 1959). If lemming density is insufficient to support breeding when jae- 

 gers and owls arrive at Barrow, they quickly move on, and only nomadic 

 individuals are seen throughout the summer. If the density of the lem- 

 mings is intermediate to high, both snowy owls and pomarine jaegers es- 

 tablish breeding territories, and in both species the density of the terri- 

 tories depends upon the abundance of lemmings. Maher (1970) con- 

 cluded that pomarine jaegers did not establish territories if the density of 



