308 ECOLOGY 



expect lack of accord within and between plant and animal communities 

 under such conditions. In these cases, however, conditions are only 

 temporarily out of adjustment, due to rapid physiographic changes, and 

 we note from the data presented that plant and animal communities 

 are usually in agreement. The exceptions are often apparent only and 

 due to the emphasis of species instead of mores and growth form. From 

 this viewpoint and with such exceptions as are noted, plant and animal 

 communities are probably in agreement the world over. 



IV. RELATIONS OF COMMUNITIES 



I. SUCCESSION CAUSES 



Succession is no doubt one of the most important and widespread 

 of the phenomena discovered by the ecologists up to the present time 

 (120, 197). Simply stated, it means that on a given fixed area organisms 

 succeed one another, because of changes in conditions. These changes 

 make impossible the continued existence of the forms present at any 

 given time; with the death or migration of such forms, others adapted to 

 the changed conditions occupy the area, whenever such adapted forms 

 are available. The changes referred to result from physical or bio- 

 logical causes, or combinations of the two. It is probable that the causes 

 of the changes are frequently complex combinations of various factors. 



We have among the physical causes changes in climate and changes 

 in topography. All degradation of land is a cause of succession. Such 

 geological processes are well understood and treated in textbooks on 

 geology and physiography. 



The biological causes of succession lie chiefly in the fact that organ- 

 isms frequently so affect their environments that neither they themselves 

 nor their offspring can continue to live at the point where they are now 

 living. Every organism adds certain poisonous substances to its sur- 

 roundings, and takes away certain substances needed by itself. It 

 frequently thus so changes conditions that its offspring cannot live and 

 grow to maturity in the same locality as the parents. However, by 

 these same processes it prepares the way for other organisms which can 

 live and grow in the conditions thus produced. 



Obviously, those organisms whose decaying bodies and excretory 

 materials are not removed or distributed by their wanderings will 

 modify their environments most. Organisms which remain in one 

 place do nothing which tends to remove the results of their own existence, 

 and frequently modify their environments in manners detrimental to 



