The Herbivore-Based Trophic System 339 



Mean densities for the entire tundra at Barrow are mainly useful for 

 considering annual trends (Figure 10-2). The densities were calculated by 

 calibrating results from extensive snap-trapping done at seven sites in 

 five habitat types (Pitelka 1973). Local densities may depart markedly 

 from overall densities, but the general trends from year to year were simi- 

 lar in all habitats. 



A description of the sequence of events during a standard cycle can 

 begin with the development of a high population. During the pre-high 

 winter, lemmings reproduce in nests constructed out of dead grass and 

 sedges and placed at the base of the snowpack. The population grows 

 rapidly and reaches a peak in late spring. Breeding ceases during May, so 

 few young are still in the nest during snowmelt, but juveniles continue to 

 be recruited into the trappable population until early June. Before snow- 

 melt there may be signs of stress. Many lemmings burrow to the surface 

 and wander about, sometimes dying (Rausch 1950, Thompson 1955b). 

 During snowmelt massive cHpping of graminoids and disruption of moss 

 and lichen carpets are revealed, and lemmings scurry everywhere. Large 

 numbers of predators attack the exposed lemmings. Particularly promi- 

 nent are pomarine jaegers {Stercorarius pomarinus), snowy owls {Nyctea 

 scandiaca) and least weasels {Mustela nivalis). During the summer lem- 

 ming survival declines, and the population crashes to a low level, where it 

 remains for one to three years. 



While this may be the general scenario, careful analysis of trapline 

 data indicates that each cycle has peculiarities of its own (Pitelka 1973). 

 In 1956, 1963 and 1969 populations increased under the snow, but de- 

 clined before it was possible to measure maximum densities. In 1956, 

 considered a peak year, an early snowmelt began in May, exposing the 

 lemmings to avian predators. In 1963 and 1969 predation under the snow 

 by weasels was unusually heavy (Pitelka 1973, MacLean et al. 1974), and 

 normal peak densities of more than 100 ha'' were never reached. The 

 highest recorded density occurred during the 1960 peak, which lasted 

 through the summer despite heavy predation and widespread destruction 

 of habitat. In contrast, during 1965 the population declined to less than 

 1 % of its initial density during the course of the summer. The decline fol- 

 lowing the population peak in 1971 was not as great, and, unlike all other 

 post-high summers, in the summer of 1972 lemmings were present in 

 moderate numbers. The population did not reach its usual low density of 

 less than 0.5 ha"' until 1974. During the pre-high summer of 1964 densi- 

 ties reached unusually high levels, but the population merely doubled 

 during winter to produce the 1965 high. The pre-high summer of 1970 

 represents the other extreme: densities remained low, and the population 

 increased by a factor of 250 during the ensuing winter. The combination 

 of events since 1965, which has been especially peculiar compared with 

 previous cycles, has been described in detail by Pitelka (1973). 



