Antagonism 381 



may have evolved in some instances into mutualism and in others into 

 parasitism. The possibility of evolutionary transformation in either 

 direction may be imagined from such flexible associations as those of 

 the various mycorrhizae. 



The interspecific relationships described in this section are flexible, 

 and their nature may change even within the lifetime of the individ- 

 uals concerned. At one moment the tissue of the giant clam is pro- 

 viding a favorable environment for the growth of zooxanthellae; at 

 another moment it is digesting these unicellular organisms. Fungi 

 associated with the roots of the tomato plant are ordinarily harmless, 

 or even beneficial; but under certain conditions these fungi become 

 definitely parasitic on the same plant. Another turn-about relation- 

 ship, which will be more fully described in Chapter 12, occurs in 

 stands of "old field" pine. The grass in abandoned fields in central 

 New England forms a favorable bed for the development of seeds 

 that blow in from white pine trees. When the pines have grown into 

 a dense grove, however, the resulting deep shade and thick carpet of 

 fallen needles will kill the grass beneath. These examples will suf- 

 fice to indicate how easily the line between symbiosis and antagonism 

 may be crossed. 



ANTAGONISM 



Relations between members of different species in which one or 

 both are harmed during the life span of the individuals concerned 

 are included under the general heading of antagonism. These antag- 

 onistic relations are not necessarily harmful for the population or for 

 the species as a whole. From the broader viewpoint interspecific 

 antagonism may have beneficial effects through controlling abun- 

 dance or influencing the course of evolution. Exploitation and com- 

 petition are necessary for the very existence of the community, and, 

 when kept in suitable balance, make possible its perpetuation. Our 

 scrutiny of the relations between individuals of different species there- 

 fore will lead to a consideration of the relations between interacting 

 populations. The latter are involved in functional adjustments of the 

 community as a whole which will be discussed in subsequent chap- 

 ters. As with other ecological factors, when the interaction between 

 species is extended from individuals to populations and the entire 

 community, additional relations with new significance emerge. 



Antagonistic relations of one species toward another involve harm 

 that may be inflicted in one or more of several ways: (1) Species 

 A may produce a poisonous substance or a change in environmental 

 condition that is inimical to species B without species A deriving any 



