The Evolution of Communities 231 



parasites, and 20 or 30 species of trees comprising a forest may be 

 preyed upon by many hundred species of insects. The number of 

 species involved is a function of the evohitionary rate of increase 

 in number of species and of species survival in the individual 

 taxonomic groups. 



Dominants and Subdominants 



After communities reached some critical stage of complexity, the 

 members of the community became segregated into two classes: 

 dominants and subdominants. In biotic communities some species 

 or group of species produce special ecological conditions which, 

 superimposed over the general climatic and edaphic conditions of 

 the area, have a profound limiting effect on the occurrence of 

 other species in the community. These influential species are known 

 as dominants, and they often are the species at the base of many 

 food chains of the community. The non-dominant species are called 

 subdominants. 



Examples of such dominants are common. In the temperate de- 

 ciduous forests of eastern North America and other parts of the 

 world, various aggregations of tall deciduous trees constitute the 

 dominants of their respective communities. The dense summer 

 shade of these trees superimposes a curious set of conditions on 

 denizens of the community, increasing the humidity, decreasing 

 temperature extremes, and reducing the sunlight materially for all 

 organisms occurring within the canopy. In winter and spring, 

 when the trees are without leaves, the sunlight within the tree 

 cover is much greater. Many subdominant shrub and herb species 

 of plants grow only under these conditions, for example members 

 of the genera Aesculus, Smilax, Trillium, Collinsia, Laportea, and 

 the woodland species of Viola, Dentaria, and Cornus. In their 

 evolutionary history these plants have evidently become adapted 

 to the understory conditions of the deciduous forests where they 

 form a stratum of distinctive and competitive organisms and a base 

 for extensive food chains. The situation holds for birds and other 

 animals which may utilize trees for nesting sites or shelter. 



In the humid coniferous forests such as those of northwestern 

 North America, various species of evergreen conifers are the domi- 

 nants. These forests modify the humidity, temperature, and sun- 

 light conditions during the summer and continue to modify humid- 

 ity and sunlight conditions during the winter. As a result of the 

 year-round dense and continuous shade cast within the tree canopy, 

 only a sparse understory subdominant biota exists except in small 



