Origin of Biomes and Succession 



285 



Fig. 125. Period during which adult ovipositing flies were present on fresh 

 cattle droppings. The upper four species were members of the first succes- 

 sional stage; the lower three species were members of the second. (From 

 Mohr. ) 



ORIGIN OF NEW TERRESTRIAL BIOMES 



Since the appearance of the first biome many additional ones have 

 come into existence. Three principal processes have led to the 

 origin of the new biomes: 



(1) Colonization of areas in which no life previously existed. 



(2) Substitution of dominants which change the physical aspect 

 of the biome. 



(3) Changes of climate resulting in new combinations of domi- 

 nants, each combination having different physical character- 

 istics. 



These processes are an expression of the dynamic nature of both 

 the species making up communities and the environment. 



1. Colonization of Abiotic Areas 



Few large areas in the world today lack life. Summit areas in the 

 highest mountain systems, limited polar areas, and a few desert 

 areas do lack life, but less extreme ecological settings have at least 

 a scattering of living organisms. When the tropical rain forest or 

 its earliest equivalent was the only terrestrial biome, the opposite 



