The Herbivore-Based Trophic System 381 



cific nutrients are known to be required by rodents (National Academy 

 of Sciences 1972), but the exact requirements of lemmings are unknown. 

 Some nutritional work has been done on lemmings and their forage, 

 which forms the basis for our tentative conclusions regarding the role of 

 forage quality in population dynamics. 



Calculations of the energy requirements of lemmings during popula- 

 tion buildup to a peak of 150 ha"' showed that in a normal high year suit- 

 able forage would be completely utilized before snowmelt (Batzli 1975a). 

 Some high population levels seem to reach 225 ha"' or more before de- 

 clining, so insufficient available energy appears to be contributing to 

 population decline in late spring of some years. Death may occur directly 

 by starvation since the average level of body fat in carcasses we collected 

 before and during snowmelt was about 2%, the level at which starving 

 lemmings die in the laboratory. The continued decline of populations 

 through the summer can not be related to lack of available food, but 

 there may be continuing effects of earlier undernutrition. 



Reproducing females require considerably more energy than non- 

 reproducing females. So insufficient forage relative to energy require- 

 ments may also explain why there are fewer pregnancies and smaller lit- 

 ters during the winter breeding season. The level of available graminoids 

 then is one-tenth that of midsummer. Lemmings may not be able to 

 maintain the necessary rate of forage intake on such a dispersed re- 

 source. Recent experiments show that rate of forage intake increases line- 

 arly with forage availability (Batzli et al., in press). 



Although there is considerable variation from year to year and site 

 to site, tundra graminoids are often low in calcium and phosphorus 

 (Table 10-6). Batzli (unpubl. obs.) found that the temperate microtine 

 Microtus californicus does not reproduce well when fed a diet with levels 

 of calcium similar to the highest amounts found in tundra graminoids. 

 Lemmings, however, perform well on such forage; and metabolic experi- 

 ments have shown that nonreproductive subadults are in slight positive 

 balance for all minerals except sodium. But when lemmings are repro- 

 ducing and when the nutrients in forage are at their lowest levels, this 

 may not be true. Laboratory animals fed natural forage ate more ad libi- 

 tum than was required to meet their energy needs, and fat levels rose to 

 15 to 20*^0 of body weight. In the field, where energy requirements are 

 greater, fat averages only 3.5°7o of body weight. Apparently the ability to 

 process large amounts of vegetation, which is related to low digestibility 

 for energy, allows lemmings to do well on a diet that would not support 

 temperate microtines. The simulation model of lemming nutrition (see 

 Nutrition and Energetics) led to the same conclusion and indicated that 

 reproductive success might be curtailed in years of poor forage quality. 



Schultz (1969) conducted experiments in which the nutrient status of 

 tundra vegetation was changed. By fertilizing heavily he increased the 



