The Herbivore-Based Trophic System 337 



Some mammalian herbivores also occur sporadically on the tundra. 

 Ground squirrels {Spennophilus parryii) may reach impressive densities 

 along river banks and beach ridges, where substrate suitable for con- 

 structing their winter hibernacula can be found. Hares {Lepus othus, = 

 timidus, and L. arcticus) rarely reach significant numbers on tundra, 

 though summer herds of 100-150 occasionally appear (Batzli 1975a). The 

 two remaining groups of mammalian herbivores, the microtine rodents 

 (Lemmus, Dicrostonyx and Microtus) and the ungulates {Rangifer and 

 Ovibos), frequently reach high densities over wide areas of tundra, and 

 represent most of the biomass in the first link of the herbivore chain in 

 tundras. 



Differences in body size and mobility of the microtine rodents and 

 ungulates lead to different tactics for dealing with the severe arctic cli- 

 mate (Batzli et al. 1981). The large ungulates have more insulation and 

 can withstand lower temperatures (Scholander et al. 1950). When winter 

 snow conditions or temperatures become intolerable, they can travel long 

 distances to more favorable habitats. In contrast, the small microtines 

 have poor insulation and must rely upon increased metabolism to main- 

 tain body temperature. Because they lack the ability to move long dis- 

 tances, they must select or create favorable microhabitats in order to sur- 

 vive severe winter conditions. Large body size gives ungulates the oppor- 

 tunity for a long life span and iterative reproduction. However, two 

 other characteristics associated with large body size — a longer develop- 

 mental period and an increased parental investment in each offspring 

 (Pianka 1970) — reduce the ability of ungulates to respond rapidly to fa- 

 vorable conditions by reproducing, and populations remain relatively 

 stable. Small microtines, by comparison, suffer greater mortality in se- 

 vere environments, but their shorter developmental time and greater fe- 

 cundity allow them to respond rapidly to favorable environmental condi- 

 tions. The following exposition describes and compares the most impor- 

 tant components of the herbivore-based trophic systems at Barrow, dom- 

 inated by microtine rodents, and at Prudhoe Bay, dominated by 

 ungulates. 



HERBIVORY AT BARROW— LEMMINGS 



Introduction 



In the coastal tundra at Barrow the brown lemming {Lemmus siber- 

 icus, =trimucronatus) is the dominant herbivore. The density of trap- 

 pable animals (post-weanlings) may reach 225 ha"'. Collared lemmings 

 {Dicrostonyx torquatus, = groenlandicus) are usually scarce, about 0.1 

 per hectare, though densities on elevated ground may be higher. They 



