364 Relations between Species 



neutral, attempts to classify many actual associations in these sub- 

 divisions may not be profitable. The scheme is chiefly useful in clari- 

 fying the types of relationship to be kept in mind while interspecific 

 reactions are being examined. 



Species A Species B Relation 



SYMBIOSIS 



When members of two species are living together in a symbiotic 

 relationship, the benefit received by one or both of them most fre- 

 quently involves the provision of food, but it may also involve shelter, 

 substratum, or transport. The association may be continuous or 

 transitory, obligate or facultative. The two symbionts may be in close 

 contact, with their tissues actually intermingled, or one partner may 

 live within a cavity of the other or attached to its surface. In some 

 instances contact between the individuals is transitory, and in some 

 the two species may influence each other without actual contact. 

 Associations in which both species derive benefit are termed mutual- 

 ism; those in which only one species is benefited and neither is harmed 

 are termed commensalism. 



Mutualism 



Mutualism with Continuous Contact. The most intimate type of 

 mutualism is seen in those associations in which contact between the 

 symbionts is close and is often permanent as well as obligatory. A 

 classic example of this sort of relationship is furnished by the lichens, 

 which are composed of a matrix formed by a fungus within which 

 cells of an alga are embedded. The fungus holds moisture and makes 

 minerals available for both partners, in return for which the chloro- 

 phyll-bearing alga manufactures carbohydrates for itself and also for 

 the colorless fungus. The fungus of a lichen can never grow in nature 

 without the associated algae, and the algae, although generally similar 

 to independent species, are probably dependent upon the fungus 

 under natural conditions. The dual personality of the lichen may be 



