SECTION IV. THE COMMUNITY 



25. INTRODUCTION 



The community concept of modern ecology 

 is one of the fruitful ideas contributed by 

 biological science to modern civilization. 

 Its importance is threefold. Through its 

 numerous direct and indirect applications it 

 is of value to such practical fields as agri- 

 culture, animal husbandry, wild life con- 

 servation, and medicine. Natural order- 

 ability, made clear by comprehension of the 

 concept, is important to philosophical 

 thought. Lastly, it is of especial importance 

 to the professional ecologist. Certain phases 

 of this concept have been developed in pre- 

 vious chapters, and the ground has been 

 prepared for the study of the community 

 in our discussion of interspecific and intra- 

 specific populations. Other aspects to be 

 presented shortly will, we hope, demon- 

 strate further the productiveness of the 

 community principle as developed within 

 the last fifty years. 



In large, the major community may be 

 defined as a natural assemblage of organ- 

 isms which, together with its habitat, has 

 reached a survival level such that it is rela- 

 tively independent of adjacent assemblages 

 of equal rank; to this extent, given radiant 

 energy, it is self-sustaining. This general 

 definition will be extended and modified, 

 without appreciably altering its pertinence; 

 we shall later recognize various levels of 

 interdependence within this larger com- 

 munity. Some exceptions will be noted; for 

 example, certain cave communities require 

 a periodic input of energy (these will be 

 discussed later). The teim "community" has 

 been used in other senses, but for present 

 purposes the concept of the major com- 

 munity as just defined is exactly expressed 

 by the well-known black oak community 

 on establi.shed dunes at the southern end 

 of Lake Michigan fCowles, 1901, p. 62; 

 Shelford, 1913,' p. 229; Fuller, 1914, 1925; 



Park, 1930, 1931, 1931a; Talbot, 1934; 

 Strohecker, 1938). 



Many years ago Mobius (1880, p. 721) 

 recognized that a natural assemblage of 

 organisms constitutes a community, and 

 stated: "Every oyster bed is ... a com- 

 munity of living beings, a collection of 

 species, and a massing of individuals, which 

 find everything necessary for their growth 

 and continuance . . . ." Obviously Mobius 

 did not mean that the oysters alone formed 

 the community, but that the collection of 

 species, which were mutually interdepend- 

 ent, and hence self-sustaining, formed the 

 community. Thus, the important copper ion 

 concentration necessary for the setting of 

 the oyster spat (Prytherch, 1934), and the 

 location of the oyster bed in marine httoral 

 areas where this ion could be available from 

 river systems, the plankton upon which the 

 oysters feed, the oyster-sponges (Clionidae) 

 and oyster-drill (Urosalpinx) and starfish 

 (Asterias), which prey upon the oyster, 

 the collective ectoparasites and entopara- 

 sites— all these and many other elements 

 combine to form the oyster community of 

 Mobius and, in an expanded sense, a part 

 of the whole major marine community. 



This commvmity principle rests upon two 

 diverse considerations: the universality of 

 the concept, and the functional integrity of 

 the community. The first is so obvious 

 that a few words will suffice. Wherever 

 observations are made, it is found that 

 plants and animals— rarely animals or plants 

 alone— are not segregated into ecologically 

 disparate entities, but rather form natural 

 groups. Such groups are communities, and 

 the realitv of an oak forest or a lake is so 

 evident that we are apt to take such a 

 natural, self-sustaining assemblage for 

 granted; consequently its underlying signif- 

 icance may escape us. The forest and the 



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