ECOLOGY AND ZOOGEOGRAPHY 



637 



1 12-yea 



lbs 



4^ yearlings 



Figure 446. An ecologic pyramid, to show graphically the quantitative relationships in a 

 simplified alfalfa-calf-boy food chain. An enormous mass of plant material is necessary to produce 

 a relatively small amount of beef, and a large quantity of beef is required to grow a 12-year-old 

 boy. Data from Odum. 



Commensalism 



Commensal ism is a term that indicates an 

 association in which one partner is benefited 

 and the other is neither injured nor bene- 

 fited; the one benefited is called a commen- 

 sal. The term really means eating at the 

 same table. This type of association prob- 

 ably occurs in all large groups of animals, 

 sometimes between species belonging to the 

 same phylum, class, or order, and sometimes 

 between those of different phyla. For ex- 



ample, peculiar tropical fish called remoras 

 are modified for life as commensals. The 

 first dorsal fin forms a sucker by means of 

 which the fish attaches itself to sharks, tur- 

 tles, whales, other large aquatic animals, and 

 floating objects such as boats. They are able 

 to swim, but are more often carried about by 

 other animals. Their food consists of other 

 fish and probably scraps left from the meal 

 of the shark or other animal to which the 

 individual is attached. Many animals find 

 the bodies of sponges excellent places in 



