Relations between Species 363 



Interrelations between species may be beneficial to both parties, 

 harmful to both parties, or beneficial or harmful to one and neutral 

 in respect to the other. Every gradation may be found between these 

 conditions. The beneficial effect of the presence of another species 

 is sometimes a vital necessity; but in other instances in which only a 

 trivial advantage is provided, decision is often difficult as to whether 

 the relationship is actually beneficial or merely neutral. Positively 

 harmful relations grade off in similar fashion to those that produce 

 only a minor inconvenience or are essentially neutral. The nature 

 of the relationship may change during the life cycle of one or both 

 of the species concerned. Furthermore, as with intraspecific relations, 

 the classification of an interaction between two species as beneficial 

 or harmful depends upon whether consideration is given to the imme- 

 diate effect on the individual or to the long-range effect on the species 

 as a whole. In our present discussion we shall consider first the na- 

 ture of interspecific relations as they act within the Iffe span of the 

 individual, and subsequently their more remote consequences to the 

 success of the population and to the evolution of the species. 



With the foregoing qualifications in mind, we may divide the in- 

 terrelations between species into two main categories: (1) symbiosis, 

 in which one or both species are benefited and neither species is 

 harmed; and (2) antagonism, in which at least one of the species is 

 harmed. Some authors extend the meaning of symbiosis to embrace 

 all types of interrelations including harmful ones; other authors, taking 

 the other extreme, limit the term to relations between a plant and an 

 animal, or to relations that are mutually beneficial. The present defi- 

 nition of symbiosis, which literally means "living together," seems 

 more logical and more in keeping with established usage. Symbiotic 

 associations are divided into those of mutualism (both species bene- 

 fited) and commensalism (only one species benefited). Relation- 

 ships of antagonism between species embrace antibiosis, exploitation 

 (including parasitism and predation), and competition. 



Various ecological aspects of the foreging subdivisions will be dis- 

 cussed in the following sections. E. F. Haskell's more elaborate clas- 

 sification of "coactions" between species is discussed and illustrated 

 by Burkholder (1952). A simphfied arrangement of these inter- 

 specific relations may be represented as tabulated on page 364. 



In this scheme ( -j- ) indicates an increase in a beneficial life process 

 as the result of the association, ( — ) indicates decrease or harm, and 

 (0) indicates no significant effect. Since these relationships grade 

 into one another and sometimes change, and since it is often difficult 

 to determine whether the effect on one of the species is essentially 



