The Herbivore-Based Trophic System 409 



BatzH (1978) reviewed evidence for similar effects of herbivores in other 

 ecosystems. Although the effects may not be as dramatic, herbivory in 

 grasslands and forests may also increase the rate of nutrient cycling and 

 change the distribution of nutrients. 



SUMMARY 



A single species, the brown lemming {Lemmus sibericus), dominates 

 the herbivore community at Barrow. The number of trappable animals 

 per hectare increases to a peak of 150 to 250 every three to six years and 

 may drop to less than 1 in the years between. 



A simulation model shows that a dramatic increase in the popula- 

 tion can be produced by a slight improvement in the survival rate of adult 

 females and their young. The population increases occur during those 

 winters when the structure of the snowpack allows access to food, when 

 forage quality is high, and when predatory mammals are scarce. The 

 high reproductive potential of lemmings then allows the population to in- 

 crease greatly before the snow melts. 



Only catastrophic mortality can explain the radical declines. Current 

 evidence suggests that the mortality, at least early in the summer, is 

 caused by overgrazing accompanied by a rise in the number of predatory 

 birds. However, the effects of increased social interactions on the disper- 

 sal and genetics of high populations have not been studied sufficiently. 



Examination of the nutrient-recovery hypothesis as an explanation 

 for cyclic fluctuations in lemming density leads to the conclusion that it 

 requires modification. Although changes in the nutritional quality of the 

 vegetation may affect lemming populations, lemming activity does not 

 appear to produce the short-term effects required to alter nutrient con- 

 centrations in soil and plants as proposed. 



The herbivore community in the Prudhoe Bay region is more diverse 

 and more stable than that at Barrow. Caribou (Rangifer tarandus) pro- 

 vide the greatest herbivore biomass. The brown lemming and the collared 

 lemming {Dicrostonyx torquatus) exist in about equal numbers, but their 

 density is an order of magnitude less than at Barrow. Even so, lemmings 

 may consume three to six times as much vegetation as caribou because 

 their metabolic rates are higher and their forage is less digestible. Ground 

 squirrels {Spermophilus parryii) are also important herbivores in more 

 restricted habitats. 



Comparison of ungulate and microtine grazers reveals two very dif- 

 ferent, but equally successful, suites of adaptation to herbivory on tun- 

 dra. The short development times and large litters of the microtines give 

 them a high population growth rate. In order to fuel its high metabolic 

 rate a microtine must fill its gut often. But because it eats faster and has a 



