Communilij Concept 



407 



The step-like occunence of the vegetation within a community, or 

 between commmiities, may react on the environment to impose a 

 zonation on some of the physical features of the habitat, and this in 

 turn may cause a zonation in the minor constituents of the community. 

 Even without this effect, however, the dependence of certain plants 

 and of many animals upon specific kinds of vegetation for food, 

 shelter, or other needs produces abrupt changes in the distribution 



Fig. 11.3. Zonation of vegetation in a semicircle around a shallow pool, and 

 also between the pool and a sand beach at Woods Hole, Mass. ( 1 ) Bulrushes 

 {Scirpus americunus), (2) marshmallows {Althaea ofpcimilis) , (3) poison ivy 

 {Rhus Toxicodendron), (4) bayberry {Myrica Penn.si/Ivanica) , (5) beachgrass 

 {Spartina uherniflora), and (6) wild roses {Rosa nigosa). 



of the minor inhabitants corresponding to that of the dominating 

 species. Continental and altitudinal tree lines have already been 

 mentioned as illustrations of relatively abrupt limits to communities 

 without correspondingly sharp changes in the physical environment. 

 Many other examples could be cited of the action of the vegetation in 

 complicating the environment and sharpening the transition between 

 biotopes. Thus for a variety of reasons the boundaries of certain 

 biocenoses are clearly distinct, but in many other instances one 

 biocenose gives place gradually and irregularly to another. 



