698 



ECOLOGY AND EVOLUTION 



stages. Formica truncicola obscuriventris 

 nests in the soil of the oak associes. Myr- 

 mica scabrinodis nests in the soil from the 

 pine dunes through the beech-maple cli- 

 max. Formica fiisca var. subaenescens nests 

 in both the soil and logs of the oak-maple 

 and beech-maple climax. 



This sequence illustrates an indirect eflFect 

 of the reaction of organisms on their en- 

 vironment without correlation with adap- 

 tive coaction. The interspecies relations of 

 these ants are rather general and not 

 limited to one or a few species of other or- 

 ganisms. 



In contrast, a succession of gall insects 

 in the same series is doubtless produced 

 by the coaction between the species of in- 

 sects and the host plants to which they are 

 limited (p. 699). 



The most dramatic eflFects of animals on 

 the community are found in the activities 

 of forms that control their own environ- 

 ment by building nests or other structures, 

 such as those of the social insects (p. 

 425), beavers, and man. Before the dawn 

 of history, the beavers had a greater phys- 

 iographic influence than man. One-tenth 

 of the flat meadow land of parts of the 

 Rocky Mountain region is the result of their 

 work' (Hill, 1943; p. 672). 



It is a truism that organisms are 

 adapted to many factors in the environ- 

 ment (p. 634). DiflFerent combinations of 

 factors may result in entirely different com- 

 munity composition, as was pointed out 

 in the preceding pages (Chap. 26). Like- 

 wise, it is apparent that many organisms 

 are adjusted to more than one distinct com- 

 munity as well as to different parts of the 

 same community. As we have just seen (p. 

 696), the remarkable caenogenetic evolu- 

 tion of the larval stages of numerous ani- 

 mals allows for double or multiple com- 

 munity relations, particularly among the 

 insects with complete metamorphosis. Se- 

 lection pressures unquestionably are pro- 

 duced by these environmental modifica- 

 tions, and in time the germinal patterns 

 of the evolving organisms in turn cause a 

 degree of organic evolution of the physical 

 as well as of the biotic environment (p. 

 672). 



COACTION 



Many intraspecies populations evolve as 

 units, and the broad concept of individ- 

 uality must include various levels of popu- 



lation integration (p. 683). Such intra- 

 species populations have been referred to as 

 supraorganisms, superorganisms, or epior- 

 ganisms (Emerson, 1939; Gerard, 1942). 

 The problem that now confronts us is the 

 possibility that interspecies associations 

 evolve as supraorganismic units. This ques- 

 tion is properly an important aspect of the 

 evolution of biocoenoses and communities, 

 and the reality of ecosystems is sub- 

 stantiated if the answer is in the affirma- 

 tive. The evolution of the mechanisms of 

 community integration is largely concerned 

 with the origin of coaction between organ- 

 isms. 



Clements and Shelford (1939) say that 

 coactions constitute the chief bonds in the 

 community (p. 348). The elementary unit 

 with which we are dealing in this division 

 is the coactive species pair. Relations be- 

 tween two species {coaction) may be 

 roughly classified under disoperation (both 

 harmed through a deleterious effect or 

 mutual elimination), exploitation (one or- 

 ganism benefits, while the other is harmed), 

 toleration (neither harmed), and coopera- 

 tive mutualism (both benefit). 



There may be every gradation between 

 these categories, and contrasting aspects of 

 the relation of two organisms may occur at 

 different stages in their development, or 

 even at the same time. For example, the 

 larva of a histerid beetle (Fig. 258) living 

 with termites in British Guiana was ob- 

 served eating young termites at the same 

 moment that worker termites were ficldng 

 exudate organs on its abdomen. Thus this 

 insect, while deriving many advantages 

 from the termite hosts, was both harming 

 and benefiting the society to which it had 

 become conspicuously adapted. 



A basic "proto-cooperation" (p. 395) 

 between organisms may set a stage for 

 later evolution, but may not have evolved 

 through selection. Preadaptation (p. 642) 

 may involve cooperative relations on oc- 

 casion. Proto-cooperative relationships may 

 also be conceived of as the fitness of the 

 biotic environment— to paraphrase Hender- 

 son's concept (p. 73). 



Interspecies integration, when viewed in 

 evolutionary terms, is the result of the 

 operation of natural selection on pairs of 

 species or upon larger associations in re- 

 lation to the reciprocal interactions. 



