COMMENSALISM 



Commensalism defines the coaction in 

 which two or more species are mutually associated in 

 activities centering on food and one species, at least, 

 derives benefit from the association while the other 

 associates are neither benefited nor harmed. It is 

 often difficult to establish definitely the nature of the 

 relations between species ; and phenomena considered 

 at one time to be commensalism have been later found 

 to be parasitism or mutualism. The concept of com- 

 mensalism has been broadened, in recent years, to 

 apply to coactions other than those centering on food ; 

 cover, support, protection, and locomotion are now 

 frequently included (Baer 1951). 



The remora fish are remarkable for having the 

 spinous dorsal fin modified to form a sucking disk on 

 top of the head by means of which they become at- 

 tached to the body of the shark, swordfish, tunny, 

 barracuda, or sea turtle. They are of small size and 

 are not burdensome to the host. The host benefits 

 the remora, however, for when the host feeds, the 

 scraps of food floating back are swept up by the rem- 

 ora. Many small animals become attached to the out- 

 side of larger ones, such as the protozoans Tricho- 

 dina and Kerona on Hydra, vorticellids on various 

 other aquatic organisms, branchiobdellid annelids on 

 crayfish, and so on. Commensals may also be in- 

 ternal ; consider, for instance, the harmless protozoans 

 that occur in the intestinal tract of mammals, includ- 

 ing man. 



The pitcher of the pitcher plant found in bogs 

 furnishes a breeding site or home for certain species 

 of midge flies, mosquitoes, and tree toads. Many 

 kinds of micro-organisms, both plant and animal, live 

 in the canal system of sponges. 



The nest of one species often furnishes shelter and 

 protection for other species as well. Ant nests may 

 contain guest species of various other insects. Large 

 hawk nests sometimes have nests of smaller species 

 tucked in their sides (Durango 1949) ; some birds 

 place their nests close to wasps, bees, or ants for the 

 protection offered by these insects (Hindwood 1955). 

 Woodchuck burrows are used also by rabbits, skunks, 

 and raccoons, especially in the winter. During dry 

 periods the water in crayfish burrows, a meter below 

 the ground surface, often teem with entomostraca 

 (Creaser 1931). 



stratification, niches, and food chains. The com- 

 munity, too, is a thing born, and it exhibits the same 

 characteristics of growth and old age as do individ- 

 uals. There is succession of stages to the climax com- 

 munity like the series of instars in the life cycle of 

 an insect. If the community is injured, it heals the 

 wounds in its structure through secondary succession. 

 The community is self-sustaining in that it absorbs 

 energy from the sun and metabolizes it at various 

 trophic levels in order to do work. There is division 

 of labor, analogous to the functions of the various 

 organs in the body of a single individual ; plant spe- 

 cies manufacture the food that animals need, and 

 dominant species create environment conditions 

 within the community suitable for the existence of 

 other species. There is transmission of stimuli, in- 

 tercommunication, between individuals and species by 

 voice, odor, sight, and contact. There is control over 

 the numbers of individuals of each species in the bal- 

 ance of nature. The result is that the biotic commun- 

 ity is a highly integrated recognizable unit in which 

 species exhibit various degrees of interdependency. 

 The existence of each component depends to a certain 

 extent on coooperation between them all, so that the 

 community responds and behaves as an organic en- 

 tity. That such complicated interrelationships have 

 come about through evolution indicates that they have 

 survival value for the component species involved. 



PARASITISM 



Parasitism is the relation between two in- 

 dividuals wherein the parasite receives benefit at the 

 expense of the host; parasitism is therefore a form 

 of disoperation. Parasitism is mainly a food coac- 

 tion, but the parasite derives shelter and protection 

 from the host, as well. A parasite does not ordinarily 

 kill its host, at least not until the parasite has com- 

 pleted its reproductive cycle. Were the parasite to kill 

 its host immediately on infecting it, the parasite 

 would be unable to reproduce and would quickly be- 

 come extinct. The balance between parasite and host 

 is upset if the host produces antibodies or other sub- 

 stances which hamper normal development of the 

 parasite. In general the parasite derives benefit from 

 the relation while the host suffers harm, but tolerable 

 harm. 



COMMUNITY ORGANIZATION 



Classification 



The final stage in the evolution of coopera- 

 tion is the biotic community. Analogous to a multi- 

 cellular individual, the community is composed of or- 

 ganic units, in this case organisms and species rather 

 than cells and tissues. It has a definite anatomy in its 



Parasites are commonly classified as ectopara- 

 sites, those which live on the outside of the host, and 

 endoparasites, those which live in the alimentary 

 tract, body cavities, various organs, or blood or other 

 tissues of the host (Baer 1951). Ectoparasites may 



178 Ecological processes and dynamics 



