276 Origin of Biomes and Succession 



plains species or are known from even farther west (Riegel, 1942). 

 Only one of the abundant leafhoppers (Polyamia rossi) has not 

 been found to date in any other area (DeLong, 1948). Thus if 

 these Illinois sand areas and their distinctive species disappeared 

 and new sand areas later came into existence, it is probable that 

 the present subclimax communities would be almost fully reconsti- 

 tuted. Forms such as Lithospermiim and many insects would colonize 

 the new sand areas from western prairie communities, and forms 

 like Aristida tuberculosa might come in from eastern communities. 

 The few species now absolutely obligate to the area (the leaf- 

 hopper Polyamia rossi might be one) would have become extinct, 

 and the new community would conceivably differ from the old 

 only in lacking these few species. 



This example of the sand areas points to the possibility that a 

 community which is highly distinctive because of its particular 

 combination of species, may completely disappear, then later be 

 almost completely reconstituted upon the recurrence of specific 

 ecological conditions. 



The sphagnum bogs in central North America are likewise prod- 

 ucts of glaciation. They occur in basins gouged in the earth by ice 

 action. When they filled with water these basins formed ponds or 

 lakes with only a slight current or none at all. In the central part 

 of the continent these bogs are gradually disappearing because 

 of plant succession and drainage. Presumably no additional ones 

 will be formed until another ice sheet again extends southward. 

 Whereas this community is distinctive as a unique combination of 

 organisms in central North America, the individual species all have 

 a wide range in contiguous non-bog communities to the north 

 (Forest Service of Canada, 1949). Thus when all the central bog 

 communities disappear, the species will not vanish. If the pot-hole 

 lakes and ponds again form in the area, the same species could 

 colonize them and reconstitute the sphagnum-tamarack community. 



Undoubtedly geologic events have made many other unusual but 

 ephemeral topographic features which, during their relatively short 

 lives, have supported communities composed of an unusual combi- 

 nation of species, each species occurring elsewhere in some other 

 community. In an evolutionary sense the chief interest of these 

 unusual types is to illustrate the fortuitous nature and plasticity of 

 communities, to highlight the fact that many species do occur in 

 more than one distinctive community, and to emphasize the short 

 geologic life of some types of communities, such as the tamarack 



