EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



697 



stable unity and the evolution of the 

 ecosystem. 



In the following pages, we attempt to 

 analyze the evolutionary processes inte- 

 grating different species, and we shall then 

 review aspects of the more complex assem- 

 blages found in the biocoenoses and com- 

 munities. 



ACTION 



Community evolution involves the ac- 

 tion or effect of the physicochemical habi- 

 tat upon organisms. Adaptations to the 

 physical environment result from a long 

 process of sorting through natural selection 

 (p. 640). The community often contains 

 species in different taxonomic categories 

 with similar adjustments to similar eco- 

 logical factors (Convergent Evolution, p. 

 666; see also Gleason, 1939). 



For example, the existence of sand 

 cherry {Primus pumila) and marram grass 

 {Ammophila arenaria) side by side in an 

 Indiana foredune association is probably 

 not the result of any important interaction 

 between these species, but is rather the 

 consequence of similar adaptations of both 

 species to climatic, light, soil, and moisture 

 conditions. Likewise, the presence of an 

 ant lion larva (Myrmeleon immaculatus) , 

 a tiger beetle larva {Cicindela lepida), a 

 seaside locust (Trimerotropis maritima), 

 and a sand spider (Geolycosa pikei) in a 

 poplar dune association does not indicate 

 adaptation to each other, but rather struc- 

 tural, physiological, and behavioristic ad- 

 justment to rather extreme conditions of 

 substrate, temperature, and moisture, char- 

 acteristic of this stage in the sand dune 

 succession (Wheeler, 1930). 



Cole (1946) has analyzed the cryptozoa 

 (terrestrial fauna in the dark habitat under 

 stones, logs, and bark of trees) and finds 

 that this fauna (as studied under boards), 

 although similar in various geographic re- 

 gions, lacks interspecies integration and is 

 largely the result of common habitat re- 

 quirements and tolerances. Such an assem- 

 blage does not exhibit the interspecies 

 adaptations of a biocoenose, but may have 

 many organisms that avoid the stiffer com- 

 petition of the more integrated associations. 



Temporal changes in physical conditions 

 may give rise to a succession of species 

 adapted to each stage in the development 



of the community. Obviously, adaptive evo- 

 lution is a slower process than the change 

 in the factors during ecological succession 

 in the development of the dunes associa- 

 tions. Only succession taking hundreds of 

 thousands of years might enable a certain 

 number of organisms to become adapted 

 to the changing conditions. Possibly the 

 organisms could keep up with the environ- 

 mental modifications if competition with 

 better adjusted species were reduced or 

 absent. 



The physicochemical environment may 

 affect the composition of an assemblage of 

 organisms that shows no evolved integra- 

 tion and represents only a statistical entity. 

 Such a grouping is at the bottom of an 

 ascending series of interspecies systems. 



REACTION 



The reaction of the organisms upon the 

 environment often changes the physical, 

 and indirectly the biotic factors, to which 

 many species are adjusted (Jennings, 

 1944). The gradual accumulation of hu- 

 mus, through the incorporation of decaying 

 vegetation and animal matter into the soil, 

 alters the moisture-holding capacity and the 

 rate of evaporation from the surface. This 

 factor is responsible for much of the succes- 

 sional sequence found in the Indiana sand 

 dunes from the drier foredunes to the mesic 

 woodlands (pp. 566-569). Many organisms 

 living in the climax forest on the dunes 

 are thus dependent upon the community 

 sequence, but nevertheless have not neces- 

 sarily become adapted to each other, but 

 rather to the physical conditions resulting 

 from the reaction of organisms on their 

 environment. 



A good example is the succession of ant 

 species in these dunes (Talbot, 1934). The 

 ants in this series are generalized scaven- 

 gers or predators. The foredune has a 

 mean average daily evaporation about 

 three times that of the beech-maple climax, 

 and a decrease in evaporation rate may be 

 demonstrated through the community se- 

 quence on sand from the foredunes to the 

 climax forest. Pheidole bicarinata is found 

 nesting in soil from the foredunes through 

 the oak stage. Paratrechina parvula oh- 

 scuriventris nests in the soil of the pine and 

 oak stages. Monomorium minimum is found 

 from the pine dunes through the oak 



