Origin of Biomes and Succession 283 



may have an entirely different evolutionary history than climax 

 species. 



EVOLUTION OF SUCCESSION 



The most plausible explanation for the evolution of community 

 succession is that it arose as a competitive relationship such as gave 

 rise to seasonal succession. Let us consider a simple case in which 

 two species A and B evolved in isolation and then occupied the 

 same area. Let us assume first that A germinated better on bare 

 ground and B germinated better where A was already growing, 

 and second that B eventually outcompeted A. B would gradually 

 supplant A over the range of A in which B could live. Because it 

 is highly improbable that two species would have identical ranges, 

 A would continue to exist along one periphery of the range of B, 

 and for a short time in any new denuded areas occurring within 

 the range of B. This latter arrangement is the subclimax-climax 

 relationship. The competitive restriction of A to a subclimax posi- 

 tion and B to a climax position would change the selection pressures 

 acting on the two species and theoretically lead to adaptations 

 establishing each species more firmly in its respective position. A 

 would presumably evolve toward faster maturation and greater 

 seed or spore dispersal, B toward longer life and possibly larger 

 stature. 



If subsequent events divided the range of this two-species sys- 

 tem so that A divided into lA and 2A, both ecologically similar to 

 A, and B into IB and 2B, and these divided units again mixed, two 

 communities would have come into existence, the subclimax com- 

 posed of lA and 2A, and the climax composed of IB and 2B. 



It is also possible that the original mixture contained many spe- 

 cies, and that, through the same mechanism as the one described, 

 the subclimax and climax components came into existence as mix- 

 tures of species rather than as single species. 



Support for this thesis is found in the successional microcom- 

 munities occurring in certain microhabitats. Three overlapping but 

 distinct microcommunities of this type occur in cattle droppings 

 (Mohr, 1943). Mohr found that the first stage is dominated by a 

 group of five or six species of extremely rapidly growing fly maggots 

 and lasts about five days; the second is dominated by a few slower 

 growing species of maggots and ends about fifteen days later; and 

 the third is dominated by even more slowly growing beetle larvae 

 and extends until the dung is reduced to little more than soil (Fig. 



