No. 571] TERRESTRIAL ASSOCIATIONS 



425 



than mere ability of a species to thrive in its surround- 

 ings : the species of greatest influence are those on which 

 the greatest number of other animals depend ; thus domi- 

 nant species arc successful, but successful species are not 

 always dominant. Species which are relatively free from 

 competition or which have comparatively few enemies 

 may be successful, but are not dominant, and are usually 

 not numerous. Species which are successful and at the 

 same time extremely abundant, usually form the food of 

 a large number of other animals, as it appears to be the 

 rule that no considerable source of food within the asso- 

 ciation is left unused. Dominance in a species, then, 

 would seem to include the dependence of other animals 

 upon it, plus the ability to thrive in spite of the drain 

 upon its numbers. 



B. Ceitekia of Dominance Among Animals 

 The factors mentioned as contributing to the success 



of a species, and the numbers of animals dependent upon 

 the species, are all indications of the degree of its domi- 

 nance. It appears that another criterion is available, 

 which perhaps expresses the summation of many factors 

 which contribute toward dominance. This is the degree 

 of specialization exhibited by the species in its adjust- 

 ment to a particular place in the association. Dominant 

 animals appear to be those of moderately specialized 

 habits rather than those of highly specialized, or rela 

 tively unspecialized, habits. 



C. Specialized and Unspecialized Animals 

 Each species may be referred to a position in the scale 



of specialization in habit. The degree of specialization 

 of the species is well seen in the food-habits, though all 

 the habits are to be considered. The most abundant food 

 in the sand prairie is plant material bunch-grasses. The 

 majority of the plant-feeders are adapted to eat herbage 

 of nearly any kind : they are not restricted to particular 

 species or particular parts of plants. They are non- 

 selective feeders. Grasshoppers, cutworms and certain 



