Origin of Bionics and Succession 271 



munity as the one erased. For example, in an Illinois stream reduced 

 by drought to a few stagnant pools, almost the full original comple- 

 ment of fish and aquatic invertebrates recolonized the stream 

 within a few months after normal stream flow was resumed (Lari- 

 more, Childers, and Heckrotte, 1959). If successional communities 

 occur, they would appear to be ephemeral ones composed of uni- 

 cellular organisms and those of the type studied by Yount ( 1956 ) 

 (Fig. 118). In these areas, the single subclimax period lasted from 

 forty-six to two hundred days, in sharp contrast to the fifty to 

 one hundred years often required for the development of even the 

 most short-lived series of subclimax communities leading to a ter- 

 restrial climax community. 



Following denudation, a complete aquatic community may be 

 attained by the step-by-step addition of units of food chains. In 

 north temperate streams a full year might be required for reconsti- 

 tution of bottom communities because many of the species have 

 only one generation a year, and dispersal is accomplished only by 

 one stage of the life history. In marine habitats, bottom commu- 

 nities may be reconstituted through the successive colonization of 

 species forming necessary means of attachment for others. 



Geologic Age of Subclimax Species 



If succession were a one-way process, terrestrial climax communities 

 would gradually cover all the areas of the earth, and no areas would 

 remain for the persistence of species able to exist only in subclimax 

 communities, but this is obviously not the case. Subclimax plants 

 and animals contain many species groups and sometimes genera, 

 such as the grasses Digifaria and Danthonia, which must be of 

 considerable geologic age, dating back certainly to the Oligocene 

 and perhaps to the Cretaceous, and which probably have occurred 

 in the same subclimax relationship for most if not all of this time. 

 It is reasonable to conclude therefore, that if individual, ephemeral 

 subclimax communities disappear a certain time after becoming 

 established in each locality, a constant succession of freshly denuded 

 or inundated areas must have been available to these subclimax 

 organisms for many millions of years, and these fresh areas must 

 have been within dispersal distance of the older areas. In other 

 words, some processes must operate constantly to reverse the nor- 

 mal direction of succession. 



