tive," in which the parasites are restricted by the host, although 

 seedhngs may be destroyed; "less primitive," when the parasites 

 are less restricted and cause rapid and widespread destruction of 

 the host tissues; and "specialized," in which some degree of tem- 

 porary mutualism occurs and disorganization of host tissue is re- 

 duced or delayed. ^° 



Mutualistic relations may be of great advantage to plants, for 

 without the special absorbing capacity of the fungi, or the nutri- 

 tive features of associated bacteria, they might not be able to 

 utilize the resources of the environment adequately. If both 

 associates are present, otherwise uninhabitable sites may be occu- 

 pied, but there may be delay in invasion because two different 

 organisms must migrate into the new area and come together. 

 However, the advantages of mutualistic relations seem to out- 

 weigh the disadvantages, as indicated by their widespread 

 occurrence. 



ECOLOGICAL SUCCESS OF A SPECIES 



The ecological success of a species or a population depends 

 upon its capacity to cope with its physical environment and with 

 the associated plants and animals in the relationships outlined 

 in the preceding principles. This capacity has been attained over 

 a long period of evolution during which the fitness of adaptations 

 to the physical environment and to interspecific association be- 

 came increasingly close. The adaptiveness is generally so efficient 

 that new migrants or hybrids and mutants can find opportunity 

 to grow only in disturbed areas where the original vegetation has 

 been destroyed or impaired. It appears that every species has its 

 ecological niche and that most niches are fully occupied until 

 disturbed. 



The degree of success of an individual organism depends upon 

 how well its processes are integrated with the environment and 

 with other organisms and how well it functions as a member of 

 the species population. Similarly, the success of a population de- 

 pends upon the integration of population processes and relation- 

 ships with other species populations and with the environment. 



54 • Species azmd Popimlatioims 



