232 The Evolution of Communities 



glades where a large tree has fallen and a shaft of sunlight reaches 

 the ground. In other types of communities this highly modifying 

 action of the dominant species is either less evident or less under- 

 stood. This is especially so in grassland and aquatic communities. 

 In grassland communities the dominant species are grasses and 

 other herbs which bring about ecological influences concerned 

 chiefly with the soil, involving extreme competition for water and 

 the kind of soil cover available for other organisms. Here can grow 

 only those plants which are hardy to sunlight throughout the year 

 and which are able to compete in some way for space and water 

 with the dominant species. Here also tree-nesting birds and mam- 

 mals would be excluded. 



In marine communities the system of dominant and subdominant 

 species may be of an entirely difi^erent nature. All the communities 

 have a common dependence on the currents and their load of 

 plankton and other nutrient material. At least along the shore 

 and on the bottom of relatively shallow seas, distinctive com- 

 munities are indicated by assemblages of larger organisms. The 

 dominant species in these assemblages or communities are those 

 which first colonize the area and make it possible for other organ- 

 isms to live there. For example, tube-making worms bind or over- 

 lay sandy or muddy substrates and form a firm anchoring area and 

 in this way make possible the existence of other organisms which 

 could not exist on the bare substrate. Above these bottom com- 

 munities are free-swimming or floating aggregations of species 

 which presumably have a community structure based chiefly on 

 food chains. In these nektonic communities, the plankton could 

 well be considered dominants. The situation in fresh water com- 

 munities seems comparable. 



The role of the dominant organisms in the community may be 

 threefold: (1) they may be important in the food chains; (2) they 

 may prevent the establishment of other types of dominants; and 

 (3) they usually superimpose on the community special ecological 

 conditions in addition to or modifying those of the general climate 

 or substrate. It is possible that this third role has some expression 

 in all communities, but more information is needed before reach- 

 ing conclusions. 



The dominant species themselves are dominant simply because 

 they are the species best adapted to living in the area under the 

 ecological conditions prevailing in the area, including not only 

 conditions of the climate and substrate, but also of competition, 

 predatism, and other biotic features of the community. However, 



