North American deciduous forest biociation is to 

 be contrasted, for instance, with the pond-marsh 

 biocies. 



When two plant or animal communities merge, 

 either by interminghng of species in the same habitat 

 or by juxtaposition of different communities in the 

 same region, the resultant transitional state is called 

 an ecotone. Ecotones occur between consecutive com- 

 munities in serai development on an area as well as 

 between adjacent existing local or geographic com- 

 munities. 



SUMMARY 



A community is an aggregation of organ- 

 isms in a distinctive combination of species. The 

 community and the habitat in which it occurs con- 

 stitute an ecosystem. Inherent within the community 

 are forces of dominance which control the species 

 composition, and of influence which affects the abun- 

 dance, health, and activities of organisms. Dominance 

 is exerted primarily through reactions of organisms 

 on the habitat, influence primarily by coactions of 

 organisms on one another. The relative importance 

 of each of the various species within the community 

 is evaluated on the bases of exclusiveness, abundance, 

 time of activity, secondary groupings, and influence. 

 Reproduction and growth brings a production of or- 

 ganic matter ; the rate at which formation of it takes 

 place is called productivity. 



Communities are constantly changing, the result 

 of reactions and coactions of the organisms, and cli- 

 matic, physiographic, and evolutionary processes. 

 This change is one of succession, an orderly replace- 

 ment of one community by another until a climax, 

 especially evident in bioseres, is reached. 



The community may be considered as a highly in- 

 tegrated self-contained organic unit or as merely an 

 aggregation of independent species whose preferanda 

 coincide in the same habitat. These are extreme 

 points-of-view ; an intermediate one is adopted in this 

 book. 



The gross structure of the community is the pri- 

 mary basis for distinguishing and recognizing it. On 

 land, this structure is characterized by type of vege- 

 tation ; in water, by the life-form of the predominant 

 organisms, which are usually animals. Communities 

 are then subdivided according to their taxonomic 

 composition. An aggregation of species is given com- 

 munity status if at least 50 per cent of the predomi- 

 nant species are exclusive to or characteristic of it. 

 Animal communities are named for the type of vege- 

 tation, life-form of predominating species, or habitat, 

 depending on which is the most conspicuous feature ; 

 and secondarily for the predominant two or three ex- 

 clusive or characteristic species that it contains or 

 for the geographic area in which it occurs. Biomes 

 are major geographic community units. Biociations 

 are secondary climax communities distinguished by 

 the distinctiveness of their predominant animal spe- 

 cies. Biocies are the serai equivalents of biociations. 



30 Background 



