428 



Succession and Fluctuation 



dispersal over a period of time. Even non-motile animals and plants 

 are able to extend their ranges with surprising rapidity when condi- 

 tions are suitable. 



Barriers. Against this insistant pressure for dispersal, barriers exist 

 that retard or prevent the movement of certain species. Ecological 



SUMMER 1921 

 WINTER 1919-21 



SUMMER 1926 

 WINTER 1924-26 



SUMMER 1932 

 WINTER 1930-32 



SUMMER 1941 

 WINTER 1939-41 



Fig. 12.1. Westward dispersal of the starling as shown by advancing limit of 

 breeding range ( solid line ) and winter records ( black dots ) outside the breeding 

 range. Star indicates unusually advanced breeding record in 1934. ( Kessel, 



1953.) 



barriers may be either physical or biological. The action of many 

 physical barriers is easily understood since they may be too wide to 

 cross all at once and unsuitable as a way station because of local con- 

 ditions. A great expanse of salt water or of dry land acts as an effec- 

 tive barrier against the dispersal of fresh-water forms. A rugged 



