400 

 300 

 200 

 100 



FIG. 1 01 2 Seasonal growth and decline of a nest colony of 

 common hornets, Kespo Cfoio, in France (Bodenheimer in B/o/. 

 Rev. 1937, after Janet). 



animal community. This animal community, the de- 

 ciduous forest biociation, begins its ecesis with the 

 establishment of the first trees, and with each ad- 

 vanced stage in the plant sere more and more animal 

 species invade. The establishment of the complete 

 animal community is not attained until the late or final 

 stages of the plant sere are reached. In the sand and 

 pond seres there are three or four distinct animal 

 communities recognizable that definitely succeed one 

 another, but the ecesis of each animal community is 

 not accomplished until there has been a succession of 

 several plant communities. One must not expect, 

 therefore, the complete ecesis of a distinct animal com- 

 munity to correspond with each distinct plant commu- 

 nity but only when there is a change and full develop- 

 ment of a distinct type of vegetation. 



SUMMARY 



Dispersal of animals into new areas may be 

 a range e.xpansion of a species if individuals find un- 

 occupied niches, are able to overcome competition, or 

 are able to acclimatize and adapt to the conditions. 

 Dispersal is chiefly by the immature stages that are 

 broadcast randomly in all directions, conveyed pas- 

 sively by wind, current, or other animals, respond to 

 environmental factors by directed movements or 

 taxes, or find their way by trial and error. Dispersal 

 continues at measurable rates until a physiographic, 

 climatic, or biotic barrier is reached. Factors that in- 

 duce dispersal include population pressure, failure of 

 food supplies or loss of favorable homesites, opening 

 up of new areas elsewhere, and pre-adaptation for 

 new and different niches. 



The origin of taxonomic groups of organisms may 

 be traced to various centers, usually distinguished by 

 the occurrence of the greatest amount of differentia- 

 tion within the group and the convergence of lines of 

 dispersal. According to Matthew, the major verte- 

 brate groups first evolved in temperate Asia ; accord- 

 ing to Darlington, that event took place in the Oriental 

 tropics. The present distribution of related forms 

 over the face of the world has been accounted for by the 

 continental drift theory and the likelier idea of dis- 

 persal over land bridges that have periodically come 

 and gone during geological time. 



The fauna of continental islands is derived mainly 

 from the adjacent mainland over such bridges in the 

 past, but the fauna of the more distant oceanic islands 

 are often unique and unbalanced, and dependent on 

 the accidental dispersal of miscellaneous species. The 

 fauna of North America is derived principally from 

 Eurasia, South America, and by autochthonous de- 

 velopment. 



Migration, like dispersal, involves movements and 

 invasion of new areas, but differs from dispersal in 

 that the movements are periodic back-and-forth move- 

 ments between two areas. Migrations may be annual 

 or daily, or may be in the form of changes of habitat 

 at different stages in a life cycle. Annual migrations 

 may be latitudinal, altitudinal, or local. Annual migra- 

 tion is best developed in birds but also occurs in mam- 

 mals, fish, and insects. 



Ecesis is the establishment of organisms in an area 

 into which animals have come by dispersal or migra- 

 tion. This involves the establishment of individuals, 

 the growth of populations, the invasion of more and 

 more species, and finally the development of mature 

 communities. The growth of species populations and 

 of complex communities commonly follows a sigmoid 

 or logistic curve that may be defined in mathematical 



FIG. 10-13 Ecesis of nesting bird species in central Illinois dur- 

 ing the prevernal and vernal aspects (compiled from Smith 

 1930). 



62 Ecological processes and dynamics 



