180 



ANALYSIS OF THE ENVIRONMENT 



in mountains stands much higher in the 

 tropics, at 16,000 to 18,000 feet, descending 

 gradually to sea level in higher latitudes. 

 The snow level in mountains is also affected 

 by such other conditions as moisture con- 

 tent of the air, exposure, and the general 

 wind pattern. 



In any serious attempt to evaluate the 

 ecological significance of snow as such, it 

 is important to keep its ecological relations 

 as distinct as possible from those of low 

 temperatures, storms, and glaciers, with all 

 of which it has obvious relationships. Snow- 

 fall is usually heavier in forested country 

 than in the more open grasslands. It is not 

 so great as might be expected in polar 

 regions, since these tend toward dryness, 

 but enough snow is usually present in the 

 tundras in winter so that these may be 

 characterized as regions of snow dunes. 



The quahty of snow varies from the wet, 

 heavy snow of warmer weather to the gran- 

 ular or even powdery snow characteristic of 

 more intense cold. The surface ranges from 

 the extreme softness found in forests to the 

 hard sleet or the icy slickness of frozen rain, 

 and the depth may reach several feet of 

 even undrifted snow. Thawing and freezing 

 produce a hard crust over the surface that 

 is frequently strong enough to bear the 

 weight of large animals, including man. The 

 crust hinders, or even prevents, entrance or 

 escape from the snow by the animals that 

 retire under its protecting cover. Snow is 

 a poor conductor of heat, partly because of 

 the air held within its mass, and thus pro- 

 vides protection for many small forms that 

 penetrate or are covered by it. Snow is fre- 

 quently an aid to hibernating animals and 

 acts as a protective mulch for many plants. 

 In deciduous forest, at latitude 45 degrees 

 north, for example, the ground beneath the 

 snow may remain entirely unfrozen through 

 a severe winter. A good snow cover pre- 

 vents frost heaving of the soil and so re- 

 tards spring erosion, and practically or 

 completely prevents wind erosion. Pre- 

 cipitation falling as snow on unfrozen 

 ground frequently melts slowly enough to 

 avoid a direct run-off and permits much of 

 the moisture to penetrate the soil. 



Large drifts in mountains or open coun- 

 try may have decided local effects on 

 vegetation and associated animal life, since 

 they persist as snow cover long after the 

 snow has disappeared elsewhere. Thus in 

 the Steens Mountains, in Oregon, areas of 



twisted and dwarfed poplar are sharply set 

 off from the surrounding sage brush; these 

 areas are confined to the lee of mountain 

 crests, and are obviously conditioned by 

 long-persistent snowdrifts. 



Snow covers much of the available food 

 for many animals and is a factor causing 

 migration, both of birds and of mammals, 

 toward areas that are somewhat or entirely 

 free from its direct influence. The autumnal 

 migrations move away from the snow-cov- 

 ered regions near the polar zones, especially 

 in the north; or there may be much shorter 

 migrations down from the higher country 

 in mountains. 



An assemblage of small mammals, nota- 

 bly small rodents, tunnel through the snow 

 among or near the underlying mosses or 

 grasses and so get access to buried food 

 and obtain protection from winter cold. 

 Snow not only covers much food, it also 

 serves somewhat as a natural ladder for 

 those animals capable of using it, enabhng 

 gnawers, such as snowshoe rabbits, to 

 browse in winter on otherwise unavailable 

 twigs and woody stems. Snow can also be 

 used as a cold-weather substitute for drink- 

 ing water, although its low mineral content 

 presents a special condition. In the tundra, 

 the nonhibernating collared lemmings (Di- 

 crostonyx) breed during winter beneath the 

 snow. 



Snow impedes or prevents the locomotion 

 of mammals and of running birds; even 

 mammals with long legs, such as deer or 

 moose, can not travel through overdeep 

 snow. A common adaptation is the evolution 

 of relatively large feet that act as natural 

 snowshoes. Thus ptarmigan and grouse 

 have feathered feet or develop elastic ex- 

 tensions (Fig. 36); the Siberian ptarmigan, 

 Lagopus lagopus, has a body weight of 

 about 15 gm. per square centimeter of foot 

 surface, in contrast with Perdix perdix of 

 the steppes, in which this ratio is about 40 

 gm. per square centimeter (Formosov, 

 1946). The snowshoe rabbit of North Amer- 

 ica (Lepus americanus) , like Lepus timidus 

 of northern Eurasia, has widened feet that 

 enable it to nm over the surface of the 

 snow. The feet of this snowshoe hare are 

 more than double the size of those of a 

 Kansas jack rabbit, which weighs over twice 

 as much (Seton, 1909). The corresponding 

 relation appears between the size of the 

 feet of the Canada lynx and bobcat (cf. p. 

 163). A factor of some ecological impor- 



