ECOLOGY 755 



presumably as camouflage. The coelenterates benefit from the associa- 

 tion by getting bits of food when the crab captures and eats an animal. 

 Neither crab nor coelenterate is absolutely dependent upon the other. 



339. Mutualism 



When both species gain from an association and are unable to sur- 

 vive separately, the association is termed mutualism. It is probable that 

 associations begin as commensalism and then evolve through a stage of 

 protocooperation to one of mutualism. A striking example of mutualism 

 is provided by the relationship of termites and their intestinal flagel- 

 lates. Termites have no enzymes for the digestion of wood, yet that is 

 their staple diet. Certain flagellate protozoa that live only in their in- 

 testines do have the enzymes to digest the cellulose of wood to sugars. 

 Although the flagellates require some of this sugar for their own metab- 

 olism, there is enough left over for the termite. Termites are unable to 

 survive without their intestinal inhabitants; newly hatched termites in- 

 stinctively lick the anus of another termite to get a supply of flagellates. 

 Since a termite loses all of its flagellates along with most of its gut 

 lining at each molt, termites must live in colonies so that a newly 

 molted individual will be able to get flagellates from a neighbor. The 

 flagellates are provided with plenty of food in a well protected, rela- 

 tively constant environment; they can, in fact, survive only in the in- 

 testines of termites. 



340. Amensalism 



Commensalism, protocooperation and mutualism are types of posi- 

 tive interactions, ones in which one or both members of the associated 

 pair derive benefit from the association yet neither is harmed by it. 

 Negative interactions between species— amensalism, parasitism and pre- 

 dation— are those in which one species is harmed by the association. If the 

 second species is unaff^ected, the relationship between the two is termed 

 amensalism. Organisms that produce antibiotics and the species in- 

 hibited by the antibiotic are examples of amensalism. The mold Pe}ii- 

 cillium produces the antibiotic penicillin which inhibits the growth of 

 a variety of bacteria, but the mold is unaffected by the bacteria. The 

 clinical use of these bacteria-inhibiting agents has had the unexpected 

 effect of increasing the incidence of fungus-induced diseases in man 

 which are normally kept in check by the presence of the bacteria. When 

 the bacteria are killed off by the antibiotics, the pathogenic fungi have a 

 golden opportunity. 



341. Parasitism and Predation 



It is incorrect to assume that the host-parasite and predator-prey 

 relationships are invariably harmful to the host or prey as a species. 

 This is usually true when such relationships are first established, but 

 the forces of natural selection tend, in time, to decrease the detrimental 



