(liictioii on the lower slojies l)efore they do on the 

 higher slopes. Uirds appear to construct more com- 

 pact nests, as insulation against cold, at high than 

 at low altitudes, and they huild these nests on the 

 exposed south sides of thickets or trees where they 

 henelit from the heat of the sun (Heilfurth 1936). 



The low o.xygen pressure at high altitudes ap- 

 pears to he more critical for the warm-hlooded mam- 

 mals than it is for birds, which are adapted to fly at 

 liigh elevations anyway, or for invertebrates and 

 plants that have much lower rates of metabolism and 

 oxygen requirements (Hall 1937, Kalabuchov 1937). 

 Mammals moving up to high altitudes may become 

 temporarily acclimated through increases in rate of 

 respiration, in rate of heart heat, in number of red 

 blood cells, and in hemoglobin, but these adjustments 

 are seldom as elTective as in those species which are 

 permanent residents at high altitudes. Species accli- 

 matized to low oxygen pressures are affected in a 

 reverse manner when they move to low altitudes. 

 Some mammals that have a wide altitudinal range, 

 particularly the pocket gopher Thomomys. are, like 

 plants, smaller in size at high than at low elevations 

 w-ith a continuous gradation between the extremes 

 (Davis 1938). 



Human relations 



There are few humans living permanently in 

 the high mountainous regions of North America, but 

 white man goes up there, taking sheep and goats to 

 summer pasture, and for recreational or sight-seeing 

 purposes. Man does occupy the Tibetan plateau of 

 Asia. There, he depends on the yak to plough his 

 fields and to furnish meat, butter, and milk. Although 



lUK'li ot the ])l;iteai 

 iiilk't, corn, and wl 



^llMMAKV 



oo rugged and cold for crops, 

 ire grown in sheltered valleys. 



The tundra biome extends beyond the tree 

 line in the far North and on high mountains. It has 

 low ])recipitation, low temperatures, a short growing 

 season, and, in the arctic extreme seasonal changes 

 in length of day and night and a permafrost in the 

 ground. Vegetation is bush or mat tundra, grass 

 tundra, or lichen-moss tundra. Perpetual snow and 

 ice occur in extreme areas. The tundra flora is prob- 

 ably derived from serai stages in the Arcto-tertiary 

 flora which became segregated as the continent cooled 

 in the Pliocene and Pleistocene. 



The arctic tundra biociation is fairly uniform 

 faunistically in North America and Eurasia, although 

 two faciations are distinguishable. Mammals, birds, 

 mosquitoes, and flies are the most conspicuous ani- 

 mals with springtails and mites predominant in the 

 soil. White color is common among mammals and 

 birds, especially during the winter. Acclimatization 

 to cold is highly developed in many resident forms. 

 Some small mammals remain active under the in- 

 sulating cover of snow during the winter months. 

 Most birds migrate. Cycles of abundance are pro- 

 nounced in several species. The Eskimo and Lapp 

 mainly hunt and fish for a living. 



The alpine tundra biociation is best developed in 

 the Tibetan plateau in central Asia. Only a few spe- 

 cies of mammals and birds are peculiar to the bioci- 

 ation. Mammals are physiologically adjusted to the 

 low oxygen pressure. Man finds habitation in this 

 area difficult except during the summer months, when 

 he brings his sheep and goats for pasture or comes 

 for recreation. 



Tundra biome 323 



