The Evolution of Communities 213 



short supply. Common examples include seasonal succession of in- 

 sect species having the same host, diurnal rhythms of hunting ani- 

 mals, or insect species each feeding simultaneously only on roots, 

 leaves, stems, or flowers of the same plant host. This situation is 

 commonly said to avoid competition, but it does not. The competi- 

 tion is simply less direct, hence called indirect, in comparison with 

 a situation in which competitors utilize the commodity in short 

 supply simultaneously, side by side, and in the same fashion. In- 

 stances of indirect competition would seem in general to be spe- 

 cialized conditions which evolved from conditions of direct com- 

 petition. 



A surprisingly large number of species are in direct competition. 

 Especially among the insects, many species may feed on the same 

 host at the same time without any discernible differences in food 

 utilization among the competitors. It seems reasonable to suppose 

 that this kind of mixture of competing species is the simple one 

 from which specialized forms of coexistence arose and also that it 

 is the kind most like the first primeval mixtures of species. 



Several investigators, following the generalization called Cause's 

 Law by Lack (1947) that no two species can occupy the same 

 niche at the same time, have theorized that such mixtures could 

 not exist. This generalization is highly unrealistic because it is 

 doubtful if natural conditions are ever as idealized or static as 

 the generalization requires (which was explained in Chapter 6). 



Competitive mixtures of this sort are found today, and several 

 sets of pertinent experiments based on laboratory "communities" of 

 two or three species indicate that such mixtures could have oc- 

 curred in primeval communities. The multi-species composition of 

 competitive mixtures could have been maintained by either un- 

 usual competitive balance or oscillations of ecological factors. 



UNUSUAL COMPETITIVE BALANCE 



In tests for competition between two species of Tribolitim flour 

 beetles. Park (1954, 1955) found that under some combinations of 

 temperature and humidity all the replicate tests did not have the 

 same results (Fig. 90). In some of these experiments the "loser" 

 in 90 per cent of the tests won in the other 10 per cent, and in 

 another set of experiments the "loser" in about 66 per cent of the 

 tests won in the other 33 per cent. 



Comparing two species of Drosophila, Sokoloff (1955) found an 

 even more delicately balanced competitive equilibrium between 

 Drosophila pseudoobscura and D. persimilis. Although D. persimilis 



