374 Relations between Species 



significant role in the establishment of trees in new areas. Many 

 other illustrations of mutualism, both of an intimate and of a casual 

 nature, will occur to the reader. 



Commensalism 



When members of different species are associated in such a way 

 that only one of the organisms is benefited but neither is harmed, the 

 relationship constitutes commensalism. Such associations no doubt 

 began by the mere toleration of "guests" near the usually larger host 

 species, or on or in its body. If the guest derived some benefit with- 

 out interfering with the host, the relationship would tend to persist. 

 Casual association may have led to a partial or a complete dependency 

 on the part of the guest. Obligate commensalism established in this 

 way may have evolved further in some instances to give rise to mutual- 

 ism, on the one hand, or parasitism, on the other. If the host species 

 became adapted to take some advantage of the close proximity of its 

 guest, a mutualistic symbiosis would result. However, if during the 

 course of evolution the guest species imposed more and more upon 

 its host, finally overstepping the bounds of hospitality and inflicting 

 harm upon the host, the relationship would change to exploitation and 

 perhaps to parasitism. 



The advantage derived by the commensal involves the provision 

 of substratum, shelter, or transport, and very frequently of food. 

 Commensalism means "eating off the same table" as guest messmates. 

 "The messmate does not live at the expense of his host; all that he 

 desires is a home or his friend's superfluities" (Pearse, 1939). As 

 would be expected, the circumstances of commensalism are more 

 variable than those of mutualism. Although in some instances the 

 commensal is in continuous contact with its host-attached to a sur- 

 face or retained within a cavity-more frequently the guest is free to 

 come and go at irregular intervals. Sometimes the commensal species 

 can associate with only a single host species, but often considerable 

 species flexibility is observed. 



Commensalism with Continuous Contact. Commensals in more or 

 less permanent contact with their hosts are represented by a great 

 variety of epiphiftes and epizoans. Many tropical orchids, brome- 

 liads, and other "air plants" grow perched on horizontal branches or 

 in forks of trees or hanging in streaming festoons (Fig. 10.7). Fa- 

 miliar in the northern coniferous forest are the hanging "mosses" 

 Usnea and Alectoria that gave rise to Longfellow's well-known lines: 



