370 



G. O. Batzli et al. 



FIGURE 10-12. The impact of avian predators on lem- 

 ming populations in 4 years as indicated by the percent- 

 age of mortality accounted for by predators. Lemming 

 densities are given in Figure 10-2. (After Osborn 1975.) 



found in winter nests. High predation rates — 20% of nests on Banks 

 Island (Maher 1967) and 35*^0 of nests at Barrow (MacLean et al. 1974) — 

 have been reported. The actual percentage of the population consumed 

 was probably much higher since weasels usually consume more than one 

 lemming per nest. But exact predation rates could not be calculated be- 

 cause the number of lemmings using each nest and the number of nests 

 built by each lemming are unknown. 



In order to assess the impact of avian predators on lemming popula- 

 tions, Osborn (1975) modeled the numerical and functional responses of 

 snowy owls and pomarine jaegers. His model contains the information 

 discussed above, plus growth curves of Watson (1957) for the snowy owl 

 and Maher (1970) for the pomarine jaeger. He calculated the mortality 

 rates for lemming populations when they were high enough to ensure 

 breeding by the two avian predators, and expressed predation as the 

 percentage of mortality (Figure 10-12). 



In general, the model indicated that avian predation accounted for 

 the greatest amount of mortality at snowmelt in early June when non- 

 breeding pomarine jaegers were still present. In 1956 and 1965, when the 

 lemming population declined rapidly from peak levels, predation became 

 an important source of mortality again in late summer when lemming 

 populations were low. In 1964 and 1973, when lemming populations in- 

 creased in late summer, percent predation declined. The simulation for 

 the early summer of 1972 showed the maximum impact of predation: 

 88% of total mortality. In 1960, when lemming densities were at their 

 highest recorded levels, absolute predation rates were also highest (79 g 

 ha"' day"'), but avian predation accounted for only 39% of the mortal- 

 ity. Finally, Osborn found that in early June the amount of predation 



