26 THE STUDY OF PLANT COMMUNITIES * Chapter II 



standing of the community as a whole is possible only after infor- 

 mation is complete on the individual strata. 



Dependence.— Within any community some species, although 

 a part of the community, are at the same time dependent upon the 

 whole for their survival. To a great extent, these are inconspicuous 

 organisms, which, at first glance, might well be overlooked or 

 ignored. Most of the bryophytes and thallophytes, as well as a few 

 vascular plants, require the special conditions provided by larger 

 seed plants; shade and moisture are usually of greatest importance 

 to their survival. Such dependent organisms would soon disappear 

 if the dominant vegetation were removed. 



Epiphytes grow on the trunks, the branches, and even on the 

 leaves of the larger plants. In subtropical and tropical forests they 

 may be conspicuous because of both size and abundance. In for- 

 ests of temperate zones they may be easily overlooked, for they 

 are usually mosses, liverworts, or lichens. These may be restricted 

 to certain communities, and sometimes individual species will grow 

 only on specific trees. Fungi, including bacteria, make up an im- 

 portant part of many communities, especially forests. Here they 

 may be parasitic and cause diseases that may at times become so 

 serious as to destroy a stand or even to eliminate a community. 

 Other saprophytic fungi, living in the soil or litter of the forest 

 floor, although dependent upon the community, likewise contrib- 

 ute to its perpetuation through their activities in decomposition of 

 organic matter. Still others, again often host specific, live in an as- 

 sociation with the roots of vascular plants in a relationship termed 

 mycorhiza (see Fig. 91). 



Finally, animals, largely as dependents but also as influents, are 

 likewise a part of the biotic community. Large species such as deer, 

 which move about freely, are not necessarily associated with a 

 single community. However, many smaller, less widely ranging 

 species are definitely restricted to single communities, and even 

 some birds and flying insects may be constantly associated with 

 certain types of vegetation. Many beetles, borers, moths, etc. are 

 extremely destructive parasites, while other similar small animals 

 live on the remains of dead plants. The animals are apt to be re- 

 lated to the community through food requirements and, if present 

 in large numbers, may have extremely destructive effects. 



