The Herbivore-Based Trophic System 371 



relative to standing crop of lemmings peaked at lemming densities of 25 

 ha"' and dropped off rapidly at lower or higher densities. 



Osborn did not simulate predation for summers when lemming pop- 

 ulations were low, but there is some evidence suggesting that predation 

 relative to standing crop is quite high. Pomarine jaegers do not defend 

 territories when lemming populations in early June are less than 10 ha"'. 

 At these densities lemmings are distributed in small patches, upon which 

 wandering predators prey. The percentage of lemmings taken may be 

 greater than during years when lemmings are protected from other pred- 

 ators by pomarine jaegers. 



During 1972 one of the live-trapping grids was located within an 

 area that was defended by a nesting pair of pomarine jaegers and we ob- 

 served minimum survival rates for adult lemmings of 40 to 70% per 28 

 days. On two other grids not defended by jaegers lemming survival rates 

 were less than 20% per 28 days. During 1973 and 1974 no jaegers nested 

 in the study area, and adult survival rates were less than 30% per 28 days 

 on all grids. Wandering jaegers and owls were relatively common in all 

 three years. Weasels were present in 1973, and in 1974 an arctic fox re- 

 sided in the study area. Animals on trapping grids not defended by terri- 

 torial jaegers appeared to be healthy, and all females were reproducing, 

 so predation by other predators appeared to be the most likely cause of 

 the high mortality. 



FACTORS INFLUENCING LEMMING POPULATIONS 



Abiotic Factors 



One of the most dramatic features of tundra is the rapid transition 

 between the mild summer and the severe winter. Because they neither mi- 

 grate nor hibernate, lemmings must function throughout even the most 

 extreme conditions. During the summer, temperatures near the ground 

 are above freezing, but they are nearly always below the lower limit of 

 thermoneutrality (17 °C) of lemmings (Figure 10-7). Hence, even summer 

 temperatures cause metabolic rate and food demand to be elevated above 

 the minimum value, but they pose little direct threat to survival. 



Habitat flooding at the time of snowmelt and again, in some years, 

 following July and August rains may be a greater danger to lemmings. 

 Much of the low-lying habitat, including meadows, polygon troughs and 

 basins of low-centered polygons, becomes inundated. Since water des- 

 troys the insulation value of lemming fur, and the high specific heat of 

 water makes it an effective heat sink, lemmings must keep dry most of 

 the time. Slight flooding (2 to 3 cm deep) can make a habitat unsuitable 

 except for occasional foraging. At snowmelt the subnivean habitat col- 



