358 



COMMUNITY ECOLOGY: 



relations because distance alone is an effective bar- 

 rier to possible activities between communities, and 

 stability is important because only a stable com- 

 munity (climax) can complete with another habitat 

 and still maintain its identity. When two or more 

 climaxes compete with one another, each retains its 

 identity; however, when a climax competes with an 

 unstable, or serai, community, the climax replaces 

 the serai stage. 



Community interrelations exist because of natural 

 movements of materials between communities. For 

 example, wind or water can move soil, organic debris, 

 or organisms, and organisms can travel between com- 

 munities. These movements are not uncommon; 

 daily and seasonal movements of animals have al- 

 ready been mentioned. Therefore, some of the re- 

 sources of one community can actually be used by 

 another. 



ENVIRONMENTAL INFLUENCE 



Communities tend to have some control over the 

 climate of their immediate area. In creating shade, 

 plants reduce temperature and decrease moisture 

 loss; in slowing wind, reduce drying; and in provid- 

 ing litter, prevent soil moisture and temperature 

 extremes. Animals by reducing vegetation can re- 

 verse these plant effects. Plants have many other 

 actions upon their environment. They can deplete 

 the soil of its moisture and minerals; in turn, they are 

 important agents of soil formation. Plants can change 

 atmospheric composition by releasing more oxygen 

 into the air during the day and more carbon dioxide 

 at night; however, plants generally use more carbon 

 dioxide during the day than they release during a 

 twenty-four hour period. Vegetation also retards 

 water runoff, and thus retards erosion of the soil in 

 the community. 



Animal influences normally are less striking, or at 

 least usually less beneficial. Animals do aid in soil 

 formation when burrowing species mix the soil; how- 

 ever, their most pronounced actions are destructive. 

 For example, overgrazing can increase erosion, 

 cause environmental change, and ultimately create 

 drastic alteration in community composition. 



STRATIFICATION 



Stratification involves the influence of layering 

 within a habitat. For example, a community may 



have tree, shrub, herbaceous, and moss layers. Each 

 of these layers, like a rock layer, is called a stratum; 

 collectively, the layers are strata. The phenomenon of 

 vertical distribution is called stratification. Stratifica- 

 tion occurs because a stand as a rule has vertical dis- 

 tribution of both its ecological factors and organisms, 

 but only the layering of life is implied by the concept 

 of stratification. Both plants and animals are strati- 

 fied, both in water and on land. However, further 

 discussion here is limited to land plants and the 

 animals associated with this plant layering. 



Classical segregation of layers is based on whether 

 or not the plants contain chlorophyll (i.e., are or are 

 not green) and on whether or not the plants have free 

 support. The classification follows (Figure 19.5). 



Green Plants: Mechanically Independent. This 

 category includes trees, shrubs, herbs, and mosses. 



Green Plants: Mechanically Dependent. Climbers are 

 subdivided according to their attachment mech- 

 anisms. Twiners wrap around other plants because 



tree 



shrub 



soprophyl 



Figure 19.5 The strata or layers of plants in a community. 



