Origin of Biomes and Succession 265 



reef assemblages. By Cretaceous time, specialized massive pele- 

 cypods or Rudistids formed reefs of wide extent in the low lati- 

 tudes. In the Cenozoic, development of the present "coral reef" 

 environment took place. This included the specialization of the 

 symbiotic algae-hexacoral association and the encrusting Lithotha- 

 mian calcareous algae which together provide the physical frame- 

 work for existence of many other invertebrate groups such as the 

 regular echinoids, branching bryozoans, and certain gastropods 

 and pelecypods. 



These reef communities have thus formed a true biome in which, 

 as in the tropical rain forest, newly evolved dominants presumably 

 produced the same physical and ecological environment as the 

 species they replaced. Thus taxonomic change in the dominants 

 probably led to no change in the selection pressure acting on the 

 subdominants, with the result that evolution of the subdominants 

 would have been generally independent of evolution in the dom- 

 inants. 



Other communities existing over a shorter span undoubtedly 

 exemplify the same principles. The temperate deciduous hardwood 

 forests (Chaney, 1940) have been in existence probably since earli- 

 est Cenozoic time and the more xeric-adapted subtropical sclero- 

 phyllous tree and shrub communities probably since Eocene time 

 (Axelrod, 1958). Many of the present-day dominants in these 

 communities are remarkably similar to the fossil forms first as- 

 sociated with the community. It seems highly likely that in these 

 communities, as postulated for the tropical rain forest and coral 

 reef communities, the subdominant species evolved independently 

 of the dominants. 



The biome may therefore be considered as a series of overlapping 

 communities moving through time, occasionally dividing and re- 

 uniting with a consequent increase both in constituent species and 

 ecological complexity. In this moving column species additions 

 and deletions occur irregularly and in large measure independ- 

 ently, and the evolution of the subdominant species is independent 

 of that of the dominants. As long as the dominant species, regard- 

 less of taxonomic composition, produce the same environmental 

 effects, the whole preserves its identity as a relatively stable eco- 

 logical unit. 



