224 



The Evolution of Communities 



the season, or bored deeper into a host, and in this way avoided 

 a parasite, or predator, or an inimical chmatic condition, then 

 mutants bearing these changes would persist in the species and 

 would bring about a specialization relating to food. From their 

 nature alone it might be difficult to determine whether some of 

 these changes had occurred because of non-competitive or com- 

 petitive selection pressures. 



The indeterminate nature of these observations is illustrated by 

 studies of Broadhead and Thornton ( 1955 ) on a group of closely 

 related psocid insects (Corrodentia) in an English forest. Three 

 closely related species of Elipsocus inhabit the branches of several 

 tree species; E. westwoodi and E. hyaJimis feed only on algae, 

 thus being in direct potential competition, whereas E. rnclachlani 

 may feed on algae but usually feeds on lichens, thus having only 

 slight competitive relations with the other two. Although Broad- 

 head and Thornton (see also Broadhead, 1958) found evidence 

 that the populations were below a level which would bring about 

 active competition, there is definite evidence of two points of spe- 

 cialization: 



(1) mclachlani, the lichen feeder, has evolved a behavior pattern 

 which actively chooses feeding sites where lichen is present, 

 and 



(2) hijalimis and westwoodi, the alga feeders, occur in markedly 

 different densities on several tree species although presum- 

 ably eating the same food (Fig. 96). 



Home 

 tree 



6 

 5 -< 



••- t: -go o D 

 so k; ij- = oo 





^ J> w w w w 



E. hyolinus 



— E, westwoodi 



E. mclachlani 



Fig. 96. Relative frequency of three species of Elip.wciis on trees in York- 

 shire. (Based on data from Broadhead and Thornton, 1955.) 



