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THE AMERICAN NATURALIST [Vol. XL VIII 



reason of the conflicting interests, the varying conditions 

 necessary for existence, and the varying methods of re- 

 sponse to these conditions, forms a self-contained and 

 self-regulating system of activities. 



HI. EELATIVE INFLUENCE OF DIFFERENT ORGANISMS WITHIN 

 THE ASSOCIATION— DOMINANCE 



The plant ecologist determines which plants in an asso- 

 ciation are of greatest importance (dominant) by ob- 

 serving which species tend to increase at the expense of 

 others, which are most abundant, most frequent, largest, 

 etc. Competition among plants in a grassland associa- 

 tion is mainly for space, and the dominant species are 

 usually determined with considerable accuracy after some 

 study. With the animals the consideration of dominance 

 involves greater complexity. The important relations 

 between conflicting animal species are those in which 

 they obtain food, are removed from competition, or 

 escape enemies. These relations are in each case most 

 directly concerned with food. The plant-eaters of the 

 association thus form a dominant group within the asso- 

 ciation, since predaceous and parasitic animals, and 

 scavengers in large part, depend upon them for existence. 

 Individual species within the various food-groups, how- 

 ever, present such striking differences in importance, 

 that we can not speak of all plant-eaters as dominant 

 forms, or that all animals of other food habits are un- 

 important. It is merely probable that the phytophagous 

 group will contain a larger proportion of dominant spe- 

 cies. This appears to be the condition in the bunch-grass 

 association. 



A. Factors of Dominance Among Animals 

 The success of an animal species within an association 

 is due to the resultant effect of a large number of factors. 

 Among these may be mentioned number of individuals, 

 size, activity, voracity, concentration of food, rapidity of 

 growth, rapidity of reproduction, and wideness of dis- 

 tribution in space and in time. Dominance signifies more 



