216 The Evolution of Communities 



competition was an important element which set in motion selec- 

 tion pressures leading either to specializations toward indirect com- 

 petition or to the origin of food chains. 



INDIRECT COMPETITION 



A simple type of competitive interference leading to indirect com- 

 petition is illustrated by two mites (Acarina) studied by Lienck 

 and Chapman ( 1951 ) . In New York the closely related two-spotted 

 mite Tetramjchus himaculatus and the European red mite Parate- 

 tramjchus pilosus feed together on unsprayed apple trees. Para- 

 tetramjchus pilosus normally reaches its maximum abundance about 

 August 1 and Tetramjchus himaculatus about September 5. In 

 1950 an acaricide applied very early in the season to a series of 

 orchards almost eliminated the Paratetramjchiis but had no discern- 

 ible effect on the Tetramjchus. Under these conditions the Tetra- 

 mjchus reached a greater abundance, and its time of peak abun- 

 dance was appreciably earlier (Fig. 92). In unsprayed orchards 

 the lower population and later peak of abundance of Tetramjchus 

 himaculatus would therefore appear to be due to the competitive 

 interference of Paratetramjchus pilosus. Under prolonged competi- 

 tion the greater survival of the late season Tetramjchus populations 

 would presumably initiate selection pressures tending to stabilize 

 genetically the later appearance of the species. 



Exploitation 



As Elton has stressed (1927), the organizational backbone of a 

 community is its food chains, showing which species feed on which 

 (referred to as tropliic levels). It might seem that very different 

 species must have had to evolve before one species could feed on 

 another, but this is not a necessary postulate. Many unicellular or- 

 ganisms feed on other species of unicellular organisms, so that it 

 is not necessary for the aggressor species to be a monster in order 

 to prey on another. It is probable that the first such predator was 

 a unicellular species which started feeding to a limited extent on 

 the partially grown stages of another somewhat similar fellow spe- 

 cies in the community. The predatory indi\'iduals presumably ma- 

 tured faster or reproduced better than others, and from this situa- 

 tion natural selection led to a wholly predatory species. Inferential 

 support for the evolution of prcdatism between closely related 

 species comes from many observations. 



In the Triholium populations studied for competitive effects, the 



