466 ESSENTIALS OF ZOOLOGY 



total destruction of the biotic communities of a region. When 

 denudation occurs, a region does not long remain unpopulated, but 

 the slow process of redevelopment of a community characteristic 

 of the existing climate begins as soon as the destructive force has 

 ceased its action. This process of development is called succession 

 because it is characterized by the appearance, first, of pioneers, 

 which give way to other groups of organisms which, in turn, alter 

 conditions in such a way that still others find conditions favorable 

 for their existence. The final result, after a long time, is the de- 

 velopment of a community which is in equilibrium with its en- 

 vironment and will not change unless the environment clianges. 

 Such a community is called a climax community. An analogy might 

 be sought, again, in human experience. The human pioneers who 

 first penetrated into the broad plains of the ''old west" conquered 

 the wilderness to such an extent that some who required more of 

 the comforts of civilization were able to find suitable homes. Many 

 of the original pioneers, however, moved on to new frontiers wlien 

 the land became too thickly settled. The "climax human commu- 

 nity" has not yet been attained, as man is forever seeking more 

 perfect adjustment to his environment. 



It is often possible to study the stages in the development of a 

 community without waiting for the entire process to take place in 

 one locality. For example, the broad flood plains of many of onr 

 rivers are subject to frequent inundations. Such inundations may 

 remove all living things from their paths, but since the flood ph-iins 

 are so broad and the course of the river is never the same from year 

 to year, the same tracts are not covered each spring, and areas mny 

 be found which have been undisturbed for a few weeks, a few 

 months, a year, two years, five years, fifty years, etc. In tlie flood 

 plain of the Canadian River (in Oklahoma) mud flats from Avliich 

 the water has receded recently are soon occupied by an assemblage 

 of organisms including blue-green algae, two small beetles, and a 

 fly. Tiger beetle larvae come a little later, and then seedlings of 

 willow and cottonwood. During the course of a year or so blowing 

 sand is deposited so that the level of the ground is raised several 

 feet, willows and cottonwoods increase in size, and other plants 

 are added. Soil and sand continue to be deposited, the trees in- 

 crease in size, and the ground becomes more thickly covered by 

 vegetation. Finally, after a hundred years or so, a still "higher level 

 is attained and the willows and cottonwoods give way to oaks and 



