40 J. E. Hobbie 



and the large belowground storage of energy and nutrients, will help resist 

 the effects of grazing. In fact, when lemmings are excluded from 

 experimental plots, there is a buildup of standing dead vegetation, a 

 reduction in the depth of seasonal thaw, an increase in the thickness of the 

 moss layer, a decrease in vascular plant productivity, and a change in 

 species composition. 



No single factor has been found to control the lemming population 

 cycles. One contributing factor may well be the high year-to-year variation 

 in the nutrient content of plants; at the low levels found in Barrow plants, 

 nutrients have been found to affect lemming reproduction. Another factor 

 is the thickness and condition of the snow cover. Temperatures as low as 

 — 25°C, frequently found at the ground surface during winter, severely 

 stress the animals and may prevent winter reproduction. Vertebrate 

 carnivores may kill a significant percentage of the lemming population 

 during the winter beneath the snow (weasels) or during the summer 

 (jaegers, owls). 



Carnivores 



The lemming cycles also produce a cycle in abundance of their 

 predators (Pitelka et al. 1955). In a high year, pomarine jaegers 

 {Stercorarius pomarinus), snowy owls {Nyctea scandiaca), short-eared 

 owls {Asio flammius), least weasels (Mustela nivalis), ermine (A/. 

 erminea), and arctic fox {Alopex lagopus) may all be abundant. The avian 

 predators are migratory and arrive between late April and early June. 

 Because these large birds eat four to seven lemmings a day, they breed 

 only when lemmings are abundant. Weasels immigrate into the area when 

 lemmings begin to be abundant; they reproduce during the winter and their 

 number may be high (150 km~^) at snowmelt. Foxes breed inland from 

 Barrow but appear at Barrow in late fall and will prey actively on 

 lemmings when they are abundant. 



The common smaller birds (seven species of shorebirds and two 

 buntings) arrive in the first days of the spring melt and begin to breed at 

 once (80-100 pairs km "^). They rely at first on Diptera larvae (especially 

 craneflies) and on fat reserves. Later, when adult insects become available, 

 the birds take these. At the end of the summer, the birds turn again to 

 insect larvae, this time to the chironomids (Diptera). By the time 

 emergence is complete, the smaller birds have cropped about 30% of the 

 adult insects from the tundra surface; they have little effect on the larval 

 insects (taking less than 1%). 



Feeding by insectivorous birds serves to bring energy and material 

 from belowground pools into aboveground circulation. These birds are 

 also an alternate food source for the larger avian predators. The owls and 

 jaegers usually nest and roost on elevated sites, such as mounds and 



