INTRODUCTION 



437 



lake are two examples in an almost infinite 

 series of communities that owe their exist- 

 ence to an almost infinite variation in the 

 earth's total environment. The species popu- 

 lations that compose the community are 

 never isolated units, unrelated to each otlier. 

 Their existence is possible only by the con- 

 tinued existence of other species popula- 

 tions of the community, since the fife of 

 each organic member of a species depends 

 upon the fulfillment of two broadly inter- 

 preted necessities, nourishment and pro- 

 tection. 



The inevitable chain of consequences 

 may be summarized thus: Anabofic cellular 

 demands require abnost continuous satis- 

 faction. In most plants these vital require- 

 ments are inorganic salts in the substratum 

 or surrounding medium, carbon dioxide, 

 water, and a portion of the radiant energy 

 of the sun. The photosynthetic input is 

 in turn utiHzed directly by herbivorous 

 animals, and hence indirectly by carni- 

 vorous animals or less commonly (Darwin, 

 1875; Wray and Brimley, 1943) by car- 

 nivorous plants, and still more indirectly by 

 saprophytic and saprophagous organisms. 

 Thus the demand for nourishment must be 

 fulfilled by the environment, and food is a 

 prime ecological influence. The late WilHam 

 Bayfiss (1924, p. 548) sums up this gen 

 eral idea by stating that "the whole exist- 

 ence of Uving organisms on the earth de- 

 pends on the receipt of radiant energy from 

 the sun . . ." 



It follows that continuous activity would 

 eventuate in excessive demands followed by 

 exhaustion and death. Periodic recuperation 

 is usually accompanied by relative in- 

 activity, and in this condition the animal 

 seldom responds as rapidly or completely to 

 external stimuli, and hence is exposed to 

 natural enemies during periods of physio- 

 logical recuperation. Rest and sleep, or their 

 physiological equivalents, are consequently 

 generally consummated within a more or less 

 sheltered place. This is the habitat niche 

 or home. Physiological recuperation, there- 

 fore, is consumated within the environment, 

 and sheltering is an ecological influence. 



These two general requirements of food 

 and shelter are selfish in that their satis- 

 faction prolongs the life of the individual. 

 A third basic drive, reproduction, is more 

 concerned with the future of the species, 

 although its immediate fulfillment in bi- 



sexual species, where more or less area 

 must be quartered by an individual of one 

 sex in search of one of the opposite sex, 

 requires environmental adjustment. 



These three organismal drives, with their 

 various ecological adjustments, are saUent 

 features of the organism, and were in- 

 cluded by Wheeler (1911) in his formal 

 definition. Having gone this far in defining 

 an organism, we should realize that if this 

 definition is even approximately sound, then 

 organisms would tend to form natural 

 groups of foods and feeders— in other words, 

 would form communities. Since each kind 

 of organism inherits a more or less specific 

 arrangement of genes, the resulting proto- 

 plasmic demands are similarly more or less 

 restrictive. It follows that communities are 

 composed, not of a random assortment of 

 species, but of ecologically compatible 

 species populations whose collective eco- 

 logical requiiements of food, shelter, and 

 reproduction are satisfied, in the last 

 analysis, by a certain range of environ- 

 ments. Therefore, communities with broadly 

 similar requirements have a broadly similar 

 range of environments, and their collective 

 adjustments produce a broadly similar com- 

 munity pattern. Upon this basis, a com- 

 munity may be said to have a characteristic 

 anatomy, an equally characteristic physi- 

 ology, and a characteristic heredity. 



The formation of the community may be 

 considered as a resultant of ecological se- 

 lection, in which the building blocks, or or- 

 ganisms, unable to exist alone, fall into 

 place to produce a self-sustaining whole of 

 remarkable complexity. Organization of 

 such an accumulation is obligatory and the 

 universality of the community is the proof 

 of this general proposition. 



The functional integrity of the commu- 

 nity is a logical extension of the facts exam- 

 ined, since it becomes apparent that the 

 community must be the natural unit of or- 

 ganization in ecology, and hence is the 

 smallest such unit that is or can be self- 

 sustaining, or is continuously sustained by 

 inflow of food materials. It is composed of 

 a variable number of species populations, 

 which occupy continuous or discontinuous 

 portions of the physico-biological environ- 

 ment, the habitat niches. Thus a bracket 

 fungus contains certain kinds of myceto- 

 colous animals (Weiss, 1920, 1920a, 

 1920b; Park, 1931a). These saprophytes 



