10 



Relations 

 between Species 



In natural situations the presence of other organisms of different 

 species is an unavoidable and also a necessary part of the environment. 

 The existence of other species may be crucially important in the pro- 

 vision of food, shelter, or some other necessity. Contrarywise, vari- 

 ous kinds of animals and plants are undesirable neighbors; but the 

 presence of these species must nevertheless be dealt with as an in- 

 fluence received from the surroundings. Certain interactions among 

 species in an area are prominent and clearly discerned, but others 

 are of a subtle nature not easily studied. Some of the relationships 

 form an integral part of the operation of the ecological complex as a 

 whole, whereas others may be of only minor consequence. In this 

 chapter we shall continue our analytical approach to ecological prob- 

 lems by surveying the various types of relationships between species 

 and considering the operation of each separately. In subsequent 

 chapters the combined operation of these interactions will be con- 

 sidered in relation to the composition and adjustment of the commun- 

 ity and to the functioning of the ecosystem as a whole. 



In attempting to delineate the various types of interrelations be- 

 tween species one realizes that a great complexity exists. Animals 

 have relations with other animals, plants with other plants, many 

 animals are dependent upon plants in their environment, and some 

 plants are dependent upon animals. All gradations exist from rela- 

 tionships that are vital and lifelong to those that are casual and tempo- 

 rary. Interdependency may exist between species of widely different 

 kinds and sizes— as between mighty redwood trees and microscopic 

 bacteria, or in the animal kingdom between elephants and fleas. In 

 some instances one species has an exclusive relation with another— 

 sometimes with one short life stage of the other species; but in other 

 instances species are quite flexible in their dependencies upon their 

 neighbors. 



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