Origin of Biomes and Succession 297 



cessional communities which follow denudation and of biomes, the 

 largest existing ecological units. A much more complex series of 

 successional stages has evolved in terrestrial biomes than seems to 

 be the case in aquatic ones. 



The evolution of these interlocking communities forming biomes 

 and their successional stages has been accompanied by a remark- 

 able increase in complexity within the community. In spite of this, 

 the addition or extinction of a single critical species may cause 

 drastic changes in the ecological structure of the entire biome. In 

 several biomes, however, the taxonomic composition of the domi- 

 nants has changed radically many times, but the general physical 

 and ecological nature of the biome has remained unchanged. Be- 

 cause of this situation, the dominants and subdominants of the 

 communities comprising these biomes have evolved independently. 



