The Herbivore-Based Trophic System 365 



1.6 fledged young, well above the maximum of 1.1 fledged young per 

 jaeger nest (2 eggs x 55% success rate) reported by Maher (1970) for 1960. 



Mammalian predators are difficult to observe, and major popula- 

 tion changes occur in winter, so less is known about their population 

 fluctuations. The role of the arctic fox is modified by the proximity of 

 the ocean and by human activities. Although foxes hunt lemmings, they 

 also have access to an abundant supply of carrion, primarily carcasses of 

 marine mammals and eiders that are crippled and lost by hunters. Foxes 

 are trapped commercially by man during the winter. As a result, fox den- 

 sity shows little correlation with lemming density (Figure 10-11), 

 although a strong numerical response is reported elsewhere (MacPherson 

 1969). An adult female fox regularly hunted and caught lemmings on the 

 Biome research area throughout the summer of 1974, but foxes are usu- 

 ally more abundant in winter than in summer. The main denning areas 

 are inland, possibly because of the low density of breeding waterfowl in 

 the coastal tundra. In some areas foxes are known to take large numbers 

 of waterfowl eggs (Underwood 1975). 



Of all predators the least weasel appears most closely tied to the lem- 

 ming. Least weasels may be absent from the coastal tundra at Barrow 

 during years when the lemming population is low, but they appear during 

 lemming peaks. Presumably, recolonization occurs from areas to the 

 south where the lemming cycles are of lower amplitude and where local 

 asynchrony of fluctuations of coexisting microtine species may allow 

 maintenance of a more stable predator population (Pitelka 1957b). Nev- 

 ertheless, the strong numerical response that occurs during most high 

 winters results from reproduction. The reproductive cycle of Mustela 

 nivalis differs from that of other small mustelids in that delayed implant- 

 ation does not occur. This allows the least weasel to make a rapid repro- 

 ductive response to increasing or high lemming populations. Juveniles 

 collected at Barrow on 18 May and 6 June 1963 attest to the occurrence 

 of winter breeding. Four pregnant females collected during summers 

 with high lemming densities contained 15, 12, 12 and 3 (x = 10.5) em- 

 bryos, compared with an average of 4.8 for temperate zone females (B. 

 Fitzgerald, pers. comm.). Thus, a dramatic reproductive response to 

 lemming density appears to be present. 



By immigration and reproduction least weasels can increase from 

 nearly zero to maximum densities over the course of a pre-high summer 

 and high winter (MacLean et al. 1974). Thompson (1955b) estimated the 

 peak density at 25 km"^ during the 1953 lemming high, a value that 

 Maher (1970) considered conservative. Maximum densities since 1969 

 have been far less than this. MacLean et al. (1974) reported 59 least wea- 

 sels collected during summers following winters when lemming densities 

 increased (1953, 1956, 1960, 1963, 1965, 1969), but only seven specimens 

 in all other summers. 



