384 G. O. Batzli et al. 



be scarce. Only when all three of these conditions prevail can a high pop- 

 ulation be attained. A high population may begin to decline because of 

 inadequate availability of forage, but only high mortality, resulting from 

 predation or some other factor, and reduced recruitment can make the 

 decline continue through the summer. Therefore, it does not seem that 

 any single extrinsic or intrinsic factor can explain the population dynam- 

 ics of lemmings. Rather, lemming populations respond to a number of 

 factors that act and interact concurrently to determine the timing and 

 amplitude of fluctuations (Figure 10-15). 



Dramatic fluctuations in the lemming population occur because the 

 high reproductive output of lemmings during a single favorable winter 

 allows them to increase their density by 100-fold or more, and these high 

 levels cannot be sustained. The population usually peaks every three to 

 six years, which implies that winters meeting all the necessary conditions 

 occur every three to six years. Why favorable winter conditions are 

 spaced at that interval is not clear. The underlying causes of the cyclic 

 pattern are not likely to be random. But considering the variability in 

 cycles during the past 25 years (Figure 10-2), and the relationships be- 

 tween factors that influence population density (Figure 10-15), it seems 

 clear that random factors, particularly weather, strongly influence the 

 population dynamics of lemmings. 



HERBIVORY AT PRUDHOE BAY— CARIBOU 



Introduction 



The herbivore community in the Prudhoe Bay region is more varied 

 than that at Barrow. There are caribou, willow ptarmigan and ground 

 squirrels in addition to two kinds of lemmings. Each of the three small 

 mammals requires a different habitat (Figure 10-16). Ground squirrels 

 prefer stream bluffs, stabilized sand dunes near rivers, and pingos, where 

 soil conditions allow construction of deep burrows. Within their home 

 range the density and biomass of ground squirrels is high, but overall 

 density is low (Table 10-3) because of their patchy distribution (Figure 

 10-16). Collared lemmings live around pingos, on stream banks and on 

 polygonal terrain (Feist 1975, Batzli, unpubl. obs.). Brown lemmings 

 prefer wetter habitats, as they do at Barrow. They are usually found in 

 polygonal terrain with high-centered polygons and well-developed 

 troughs where the vegetation is dominated by a variety of graminoids. 

 The range of density and biomass of lemmings in the Prudhoe Bay region 

 is relatively low (0.01 to 10 animals ha"' or 0.2 to 150 g dry wt ha"') com- 

 pared with Barrow (Table 10-3). Much of the Prudhoe Bay vegetation is 

 dominated by Carex and Salix spp., which grow on low, flat areas, often 



