The concept of the biotic community is basic to 

 an understanding of ecology. We will here be con- 

 cerned only with laying a foundation of general prin- 

 ciples. Details will come in later chapters, but for 

 proper orientation we must know something about 

 how the community is organized, how it functions, 

 and how it may be recognized. 



INTERNAL STRUCTURE 

 AND PROCESSES 



Communitv and ecosystem 



Background: 



The Biotic Community — 



Structure 



and Dynamics 



A community, or biocenose, is an aggregate of 

 organisms which form a distinct ecological unit. Such 

 a unit may be defined in terms of flora, of fauna, or 

 both. Community units may be very large, like the 

 continent-wide coniferous forest, or very small, like 

 the community of invertebrates and fungi in a de- 

 caying log. The extent of a community is limited 

 only by the requirement of a more or less uniform 

 species composition. 



A different community occurs in each diflferent 

 habitat and environmental unit of larger size, and in 

 fact the composition and character of the community 

 is an excellent indicator of the type of environment 

 that is present. Since plant communities and animal 

 communities occur together in the same habitat and 

 have many interrelations, the one can scarcely be con- 

 sidered independently of the other. Together they 

 make up the biotic community, and the biotic com- 

 munity along with its habitat is termed an ecosystem 

 (Tansley 1935). The ecosystem is the best unit for 

 the study of the circulation of matter and flow of 

 energy between organisms and their environment. 



Comnumities may be distinguished as major or 

 minor. Major cotnmunities are those which, together 

 with their habitats, form more or less complete and 

 self-sustaining units or ecosystems, except for the 

 indispensable input of solar energy. Minor communi- 

 ties, often called societies, are secondary aggregations 

 within a major community and are not, therefore, 

 completely independent units as far as circulation of 

 energy is concerned. When in this book communi- 

 ties are spoken of the reference is to major communi- 

 ties unless otherwise indicated. 



Dominance 



When a number of species come together to 

 form communities, each fits into a different niche and 

 plays a different role in the internal dynamics of the 

 community. Dominance is the relative control ex- 

 erted by organisms over the species composition of 



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