have had a share in bringing them into existence. We notice in any region 

 chiefly the plants, both because they stay put and because they are usually 

 present in masses of individuals of the same kind. The animals manage for 

 the most part to remain out of sight. The many species of plants and animals 

 of any region, however, make up a coherent whole. 



The different species depend upon one another not alone in the food chains 

 — or, rather, food cycles. They depend upon each other also for "shelter". 

 Thus birds and mammals hide in the trees, or smaller plants live in the shade 

 of larger ones. And they depend upon each other for "services" — as in the 

 case of insects transporting pollen or of mammals transporting seeds. We may 

 regard some of the activities as in the nature of "protection" — as when ants 

 keep the plant lice in check on a shrub. Such a grouping of many different 

 species that depend upon each other in these different ways is sometimes 

 called a "natural community". 



What Determines the Composition of Natural Communities? 



Life on the March In every plant and animal species population con- 

 stantly presses in all directions. From wherever there is an established popu- 

 lation, to wherever it can find a place to take hold, life is on the move. What 

 enables species to move forward? What obstructs this movement.f^ 



The climate, the contours of the earth, large bodies of water, may restrict 

 some plants and animals pretty closely. On the other hand, winds carry 

 seeds and spores over all kinds of obstacles, and the rivers distribute living 

 forms. Ocean forms become widely distributed, being restricted chiefly by 

 climatic conditions. The birds often fly over obstacles that block other species, 

 and they often carry seeds and spores far from their place of origin. 



On the relatively low mountains in the eastern United States, for example, 

 one finds species of plants and animals that are typical of the Canadian zone of 

 life. Plants on the barren tops of these mountains are typical of the arctic 

 region and of the higher Alps, the so-called Arctic Alpine life zone. It is quite 

 a thrill to recognize one of these Alpine species after an exhilarating climb to 

 the summit. Lichens and mosses grow on barren rocks and in protected cre\ - 

 ices above the timber line, under climatic conditions which no higher forms of 

 life can long endure. 



There is much evidence to show that the distribution of plants and animals 

 today is in some ways unlike that of the distant past. Thus we find in the 

 arctic coal deposits which must have been produced ages ago, although the 

 conditions there today are impossible for plants that can form coal. How did 

 coal deposits get into the arctic regions? Was the coal formed farther south 

 and then somehow shifted into the arctic? Or were conditions in these parts 

 of the world favorable to plant life in past ages? 



563 



