V38 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



landscape, more or less sharply distinguished from its surroundings. 

 A lower limit is set for the size of biotopes in their relation to 

 geography; and this limit is not necessarily valid for the biocoenosis 

 or for habitat niches of ecology. 



A biocoenosis forms a unit whose members are mutually dependent. 

 It is accordingly not practical to separate the study of plant and 

 animal elements. But it is none the less a necessity, for the sake of 

 the division of labor, to carry out such a division at least to the extent 

 of placing the consideration of the animals in the foreground. The 

 dependence of the animal element on the plants is especially evident. 

 The plants are the producers, the accumulators of food, storing the 

 energy of sunlight in complex organic compounds, by means of their 

 chlorophyll, which are required by the animals, which are the con- 

 sumers. In their metabolism these higher compounds are broken down 

 and carbon dioxide and nitrogenous salts are formed, which are used 

 in turn as foodstuffs by plants. 



If these mutual relations were present in every community, and if 

 the amounts of foodstuffs supplied were equivalent in both directions, 

 the biocoenoses would form closed systems, would be "autarchic"; 4 

 but it is very rare that the biocoenosis does not require food supplies 

 from without, and usually food is contributed to other biocoenoses. 

 The nearest approaches to such closed systems are perhaps furnished 

 by lakes without outlets or by oases in deserts. Some biocoenoses are 

 wholly dependent on the outer world for their food supply, and may 

 then consist entirely of animals, as in caves, or in the communities of 

 the lightless depths of the sea. 



Within the biocoenosis, the plants are most directly dependent upon 

 the biotope. Land and shore plants are confined to the earth, whose 

 chemical constitution regulates their food supply. They are unable to 

 withdraw from the periodic climatic influences. The plant cover of the 

 given biotope, however, to a large extent determines the nature of the 

 animal population. The vegetation affords the animals not only food 

 but also shelter; it determines their mode of motion, and sometimes 

 hinders orientation. In the water the plant metabolism supplies an 

 abundance of oxygen. On account of the role of plants as the source 

 of animal food, the carnivores are dependent on them also, though 

 secondarily. The protective devices of plants against browsing may 

 exclude certain animals completely from a biocoenosis; the presence of 

 the unpalatable ferns must have an important influence on the com- 

 position of a biocoenosis in which they predominate. Conversely, ani- 

 mals by grazing hold the vegetation at a pre-climax stage or by over- 

 grazing may cause a retrograde replacement of the vegetation. 



