EcotoGy 753 



335. Types of Interactions between Species 



The members of two different species may affect each other in any 

 one of several different ways. If neither population is affected by the 

 presence of the other, so that there is no interaction, the situation is 

 termed neutralism. If each population is adversely affected by the other 

 in its search for food, space, shelter, or some other fundamental require- 

 ment for life, the interaction is one of competition. If each population is 

 benefited by the presence of the other, but can survive in its absence, 

 the relationship is termed protocooperation. But if each population is 

 benefited in some way by the other, and cannot survive in nature without 

 it, the relationship is termed mutualism. Commensalism refers to the 

 relationship in which one species is benefited and the second is not 

 affected by existing together, and amensalism to the relationship where 

 one species is inhibited by the second but the second is unaffected by 

 the first. Where one species affects the second adversely but cannot live 

 without it, the relationship is one of parasitism or predation; parasitism 

 if one species lives in or on the body of the second and predation if 

 the first species catches, kills and feeds upon the second. The older term 

 symbiosis, "living together," is used by some authors as a synonym of 

 mutualism and by others in a wider sense as a term including mutualism, 

 commensalism and even parasitism. 



336. Competition 



Two species may compete for the same space, food, light, or in 

 escaping from predators or disease; these may be summarized as com- 

 petition for the same ecologic niche. Competition results in one species 

 dying off, or being forced to move to a different space or use a different 

 food. Careful ecologic studies usually reveal that there is only one species 

 in an ecologic niche (Cause's rule). One of the clearest examples of 

 competition was provided by the classic experiments of Cause with pop- 

 ulations of paramecia. When either of two closely related species, Para- 

 meciutn caudatuin or Paramecium aurelia, was cultured separately on a 

 fixed amount of bacteria as food, it multiplied and finally reached a con- 

 stant level (Fig. 37.4). But when both species were placed in the same cul- 

 ture vessel with a limited amount of food, only Paramecium aurelia was 

 left at the end of sixteen days (Fig. 37.4). The Paramecium aurelia had 

 not attacked the other species, or secreted any harmful substance; it 

 simply had been more successful in competing for the limited food sup- 

 ply. Studies in the field generally corroborate Cause's rule. Two fish-eat- 

 ing, cliff-nesting birds, the cormorant and the shag, which seemed at first 

 glance to have survived despite occupying the same ecologic niche, were 

 found upon analysis to have slightly different niches. The cormorant 

 feeds on bottom-dwelling fish and shrimps whereas the shag hunts fish 

 and eels in the upper levels of the sea. Further study showed that 

 these birds typically have slightly different nesting sites on the cliffs 

 as well. 



