190 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 



FlG. 88. Dead chestnut, killed by blight, in a forest stand of which they 

 once were important members. Cherokee National Forest, Tenn.— U. S. For- 

 est Service. 



for, unless the land is cultivated, the honeysuckle cannot be elim- 

 inated without considerable trouble. 



Parasites.— A parasite is completely dependent upon its host for 

 its existence and thereby becomes a factor in the environment of a 

 community. When conditions are favorable for the host, a certain 

 amount of parasitism can be tolerated with little apparent effect. 

 Parisitic fungi and bacteria are almost constantly present but cause 

 no serious disturbance of a community unless conditions become 

 unusually favorable for their increase. Then they may cause death 

 of enough hosts to produce a change in dominance or to destroy 

 the community. Such occurrences are usually local and may be 

 followed by gradual recovery of the original community. How- 

 ever, when a parasite is introduced from afar, it may be so effec- 

 tive in its new environment that disaster results. 166 Chestnut blight 

 has practically eliminated chestnut in the eastern United States, 

 and oak is now dominant where oak-chestnut occurred before. 

 Dutch elm disease 62 is gradually spreading from New England, 

 where it first appeared, although its spread has been somewhat re- 

 tarded by the drastic procedures used to check it. 



Parasitic seed plants are not usually of much ecological signifi- 



