SYSTEMS OF COACTIONS 



109 



(Fig. 21), and similar consequences may follow infestations by other 

 epidemic or endemic insects, notably twig borers and bark beetles. 



Fig. 21. — Aspens killed after three successive years of defoliation by the larvae 

 of a noctuid moth; Pikes Peak, Colorado. (Photo by Edith Clements.) 



SYSTEM OF COACTIONS 



Since processes take the leading part in a dynamic system, the 

 primary division is made upon this basis, as follows: (1) shelter and 

 housing, (2) food, (3) materials, (4) reproduction, (5) social group- 

 ing, and (6) attachment. The logical subdivision of these is first with 

 respect to -the active agent (coactor) and second with reference to the 

 passive organism (coactee), but the difference between land and water 

 is such as to warrant a preliminary grouping into land and water 

 communities, those of the latter being treated incidentally in the 

 chapters on aquatic climaxes. Among coactors, animals are first con- 

 sidered because of their vastly larger number and importance in this 



