The Herbivore-Based Trophic System 373 



the snowpack has developed can produce ice layers in the snowpack, and 

 these also inhibit the formation of depth hoar. 



The subnivean atmosphere has been suggested as another important 

 feature of the winter environment of lemmings. The dense, wind-packed 

 snow offers high resistance to gaseous diffusion. Kelley et al. (1968) and 

 Coyne and Kelley (1974) demonstrated a buildup of subnivean carbon di- 

 oxide in fall, and again in spring. On two occasions, concentrations rose 

 rapidly to 700 and 1500 ppm. While well above normal levels of 320 

 ppm, these values appear to be too low to influence the physiology of 

 lemmings. Alveolar air in mammalian lungs is normally 5 x 10* ppm CO2, 

 and that level could easily be maintained by a slight change in respiratory 

 rate (Johnson 1963). 



Snow chimneys, tunnels dug by lemmings to the snow's surface, are 

 frequent when lemming populations are high. Melchior (pers. comm.) 

 found that they are used most during fall and spring, when release of gas 

 under the snow is to be expected. Lemming tracks are often seen in fresh 

 snow around the chimneys. Sometimes the tracks may be traced to the 

 same or another chimney, indicating that the lemming returned to the 

 subnivean environment. In other cases the tracks lead to a dead lemming 

 or to signs of predation by foxes or owls. There are obviously risks asso- 

 ciated with ventures above the snow. The advantages are unknown but 

 may include escape from the toxicity of the subnivean atmosphere or dis- 

 persal to new habitat by an easier route when the depth hoar layer is 

 poorly developed. 



In summary, reproduction regularly subsides at snowmelt and dur- 

 ing freeze-up before the snowpack develops, indicating that these are pe- 

 riods of stress for lemmings. Mortality rates, particularly from exposure 

 to predators, are high at snowmelt. Circumstantial evidence suggests that 

 a shallow snowpack and low winter temperatures inhibit winter repro- 

 duction, but we found no significant correlation between snow depth and 

 spring population densities at Barrow over the past 25 years (r = +0.18; 

 p > 0.25). If the depth hoar layer of the snowpack is poorly developed, 

 winter populations decline, apparently because insufficient forage is ac- 

 cessible to them. Adequate snow depth and a well developed hoar layer 

 are necessary but not sufficient to allow a population increase during 

 winter. Unfavorable abiotic factors may contribute to the decline of high 

 populations, but they do not appear to be necessary for the decline. Poor 

 winter conditions can reduce populations to extremely low levels and pre- 

 vent recovery of the population, thereby altering the timing of peaks. In 

 summer, flooding may restrict the habitat available to lemmings, but 

 reproduction and survival do not appear to be directly affected. 



