420 The Community 



studies of the dynamic ecological relationships within the community. 

 The extensive series of investigations on Wisconsin lakes will serve as 

 an example (Juday, 1943). 



The foregoing discussion has shown that the biocenose is composed 

 of species characteristic of the area, but these species may not be 

 limited to the area nor is the biocenose limited to a fixed set of species. 

 This concept of the flexibility of the biocenose is well illustrated by 

 studies made by Bird ( 1930 ) of the interrelationships of members of 

 communities occurring in the parkland region of Manitoba. The 

 chief food relations of certain of the species within and between com- 

 munities are indicated in Fig. 11.7. A great many other, smaller 

 species are present in each community represented in the diagram, 

 and many other dependencies exist besides the food relationships. 

 This oversimplified picture nevertheless shows that certain kinds of 

 plants and animals form a characteristic nucleus in each community 

 but many of the individual species occur in more than one biocenose. 

 The principal members of each biocenose have most of their require- 

 ments fulfilled within their own biotope but interrelations between the 

 biocenoses are brought about by animals that move from one biotope 

 to another for foraging or other purposes. 



STRATIFICATION OF THE COMMUNITY 



Many communities exhibit a structure, or recognizable pattern, in 

 the spatial arrangement of their members. A community may be 

 divisible horizontally into "subcommunities"— that is, units of homo- 

 geneous life-form and ecological relation. The zonations described 

 in earlier pages of this chapter are horizontal structural units of this 

 sort. Of more general occurrence is the aspect of structure that in- 

 volves vertical changes, or strotificotion, within the community. In 

 some communities a complex stratification is present, but in others the 

 vertical dimension is so much compressed that the entire biocenose 

 consists essentially of only one stratum. Lichens pioneering on a 

 rock ledge represent an extreme example of a one-layered community, 

 but, at a later stage when higher-growing mosses and herbs have be- 

 come established and bacteria are present in a layer of humus beneath 

 the holophytic plants, the beginnings of stratification are apparent, 

 albeit on a very small scale. 



In a grassland community subterranean, floor, and herbaceous sub- 

 divisions can be recognized. The subterranean stratum contains the 

 roots of the principal vegetation and forms the permanent residence 

 of the soil bacteria, fungi, and protozoans, as well as a host of insects, 



