628 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



standpoint of the association the behavior of these active pioneering 

 animals corresponds to the trial activities in the behavior of the indi- 

 vidual animal. These activities are not different in kind from those 

 which are involved in normal maintenance. They are those which 

 form the initial stages in the establishment and extension of the asso- 

 ciation in a new locality or the reestablishment in an old one, and 

 thus lead to a sequence or succession of associations. Ecological suc- 

 cession thus consists in an orderly sequence or series of associations 

 which occur successively and form a genetic series. 



9. ASSOCIATIONAL SUCCESSION. 



A succession of associations takes place either through a trans- 

 formation of older ones, or through the origin of a new one on a 

 surface which has been newly formed and has had no population. A 

 favorable habitat without a population of animals is comparable in 

 some respects to a vacuum ; it exists as a condition of unstable equi- 

 librium which tends to change toward a more stable state. The 

 active life of animals tends to lead them into all possible habitats, 

 and where they find the conditions favorable for existence they tend 

 to survive and thus bring about the establishment of an association. 

 Each association, like the individual animal, has a certain amount of 

 iniity and tends to maintain or perpetuate itself. But the stability of 

 associations is only relative, and some are much more stable than 

 others. Naturally the unstable ones are those which show succession 

 most readily. Thus if we destroy a few trees in a hardwood forest 

 and produce a glade, a large number of the characteristic animals 

 of the dense forest will disappear and be replaced by animals which 

 normally frequent open places; then in a few years sprout-growth 

 and young and suppressed trees will change the conditions so much 

 that the kind of forest animals which were eliminated for a time 

 will begin to return; and when the new gTowth is replaced by the 

 mature forest the animals of the mature forest will return and a new 

 equilibrium will be formed. In such a forested region the glade is 

 to be looked upon as an unstable condition, which through a succes- 

 sion of associations will later arrive at a relatively stable condition, 

 which is able to perpetuate itself indefinitely under existing condi- 

 tions. Such an association is considered a cllmnx^ or the culmination 

 of a series of successions under existing conditions. The succession of 

 associations leading to a climax represents the process of adjustment 

 to the conditions of stress, and the climax represents a condition of 

 relative equilibrium. Climax associations are large units, and are 

 the resultants of certain climatic, geological, physiographic, and bio- 

 logical conditions. 



