Ecology of Animals 653 



the northern ice cap to the borders of the coniferous forests. The com- 

 paratively monotonous rolhng landscape is not relieved by any tall vege- 

 tation or large hills. The many depressions are filled with water, and it 

 is but a few inches down to the permanently frozen soil known as perma- 

 frost. Sphagnum and lichens such as reindeer moss are the characteristic 

 vegetation, with many shrubby and herbaceous plants in the drier areas. 

 During the brief summer, the area abounds with flowers and ground- 

 nesting birds. To the discomfiture of human beings, dipterans are the 

 characteristic insects. The mosquitoes and biting flies emerge in hordes 

 from the boggy ground to attack any mammal. Among the mammals, 

 the musk ox, reindeer, caribou, arctic fox, and arctic wolf are charac- 

 teristic forms. Large numbers of rodents including the lemming live in 

 the low-growing vegetation. 



The main stress faced by the animals dwelling in this biome is the 

 extreme cold of winter and the accompanying depletion of food. Most 

 animals migrate to more favorable areas, for hibernation is impossible in 

 the permanently frozen ground. Many of the rodents survive this ex- 

 treme cold by burrowing under the snow and feeding on dry vegetation. 



Northern Coniferous Forest. — Like the tundra, the coniferous 

 forest is circumpolar and very extensive. Its vegetation consists of 

 massive conifers with much understory herbaceous and woody growth. 

 Occasional patches of deciduous trees such as aspens and poplar are 

 found. Due to the great abundance of food, there is a rich and varied 

 fauna. This fauna includes such large forms as the moose and deer, the 

 smaller lynx, foxes, wolves, martens, and many minks, weasels, and 

 ermine. The rodents, including the porcupine, pine mouse, deer mouse, 

 and others are abundant. Hosts of birds, including warblers and 

 thrushes nest in the conifers. These colorful migrants spend only their 

 brief nesting period in this area, returning with the approach of winter 

 to more southern regions. Again the chief stress is cold and the reduced 

 food supply of winter. 



Temperate Deciduous Forest. — The temperate deciduous for- 

 est is much more difficult to define and characterize inasmuch as it is 

 more varied. All biomes may be divided into small units known 

 as associations. The composition of this forest is so varied that its as- 

 sociations are easily recognized. In the eastern portion of the United 

 States, the beech-maple forest, the oak-hickory, the oak-chestnut, and 

 the mixed mesophytic are all distinct types. All are characterized by 



