No. 571] 



TERRESTRIAL ASSOCIATIONS 



443 



social and antagonistic, the latter relations being with 

 food-species, competitors and enemies. Correlations of 

 the various kinds of characters with relations involving 

 food, competition and enemies, are given. According to 

 its ecological constitution, each organism finds a status 

 in the association, the whole being a self-contained and 

 self-regulating system of activities. 



Dependencies within the association are concerned 

 mainly with sources and interchange of material and 

 energy. Dominant plants (the most influential species) 

 are those most intimately correlated with physical en- 

 vironment, as indicated by aggressiveness, abundance, 

 frequence, size, etc. Dominant animals are most numer- 

 ous among phytophagous forms. Dominance in an ani- 

 mal species includes dependence of other animals npoD it 

 (for food) plus the ability to thrive in spite of the drain 

 upon its numbers. The degree of specialization of be- 

 havior is a convenient index of the relative influence of 

 animals in the association. The dominant animals are 

 moderately specialized, and carry on the ordinary work 

 of the association. The highly specialized animals make 

 use of space otherwise unoccupied, and food material not 

 available to other species, or not taken by other forms. 

 Least highly specialized animals act as a check upon 

 undue departure from biotic equilibrium. 



The association may be divided into minor groups of 

 organisms, both in space and in time. Space-division is 

 vertical, resulting in strata, and horizontal, resulting in 

 sub-habitats of greater or less magnitude. The strata 

 and sub-habitats present a larger or smaller degree of 

 discontinuity and of internal variability. Time-distribu- 

 tion is diurnal, seasonal and annual. There are also 

 time-variations produced by variability of weather condi- 

 tions and by oscillatory disturbances. 



The relations between plant and animal assemblages 

 have long been known, in a general way, to be intimate. 

 Plants and animals agree in similar response to common 

 environmental influence, and in types of geographic dis- 



