442 



THE COMMUNITY 



plan is to be anticipated when it is realized 

 that a community is, in large part, the ob- 

 ligatory gathering of many organisms for 

 survival. In tliis sense the community is a 

 supraorganism. Just as the common struc- 

 tural plan of moUusks pervades many thou- 

 sands of species, belonging to hundreds of 

 genera and dozens of families, general com- 

 munity structure is discernible through a 

 wide variety of types. Similarly, as each 

 family of moUusks presents a particular 

 taxonomic habitus, so each community 

 type (the forest community, for example) 

 shows a particular modification of general 

 organization. 



One of the outstanding general principles 

 of community organization is that of 

 stratification. For present purposes, stratifi- 

 cation is apphed in the broadest meaning 

 of the term and embraces all objectively 

 delimitable vertical or horizontal layers of 

 organisms, their by-products, or the results 

 of their activities upon the environment. 

 Consequently, when this principle is ap- 

 pUed to communities, and the total volume 

 occupied is examined for evidence of strati- 

 fication, nearly all communities share a 

 well-defined lamination into either (1) a 

 column of strata upon a vertical organismal 

 gradient ("layers" of many botanists, strata 

 in the limited meaning of some authors; 

 Lippmaa, 1939); or (2) a series of strata 

 on a horizontal organismal gradient (zones 

 of many hydrobiologists; belts or girdles); 

 or (3) more commonly the community is 

 at least partially separable into both verti- 

 cal and horizontal series of strata. 



Complete stratification is uncommon, al- 

 though most communities show this pattern 

 in parts of their organization. In other in- 

 stances one gradient will be obvious, while 

 another will be much less apparent, as in 

 the marine littoral on sand or mud where 

 the horizontal stratification is well devel- 

 oped and obvious to the eye, while the 

 vertical gradient is not so apparent until 

 samples from different depths are analyzed. 



The reasons for stratifications are fairly 

 obvious. The process is effected (1) by 

 initial colonization of a stratified environ- 

 ment by what may be thought of as pri- 

 mary residents. These invading plants 

 and animals belong to species, the popula- 

 tions of which adjust to the stratified envi- 

 ronment and hence are directly stratified 

 as a result of their specific tolerations and 



adaptations. (2) As soon as this initial 

 stratification of organisms is consummated, 

 there is a tendency for the process to be 

 reenforced as a consequence of the very 

 bulk of the organisms, their excretions and 

 by-products, and lastly through the chemi- 

 cal reactions taking place between these 

 by-products and the physical stratification. 

 (3) Finally other organisms take up tem- 

 porary or permanent residence as a direct 

 response to the presence of initial residents, 

 rather than to the initial environmental 

 stratification. These may be considered as 

 secondary residents in general, although 

 they may be divisible into secondary, ter- 

 tiary, and so on, depending upon their 

 orientation with respect to the first comers. 



The primary and secondary species may 

 or may not be irreplaceable in the com- 

 munity. At the primary level the role of 

 numerous species may be essential, but 

 transferable within an ecologically equiva- 

 lent group, so that we have the principle 

 of community stratification affected by the 

 principle of ecological replaceabifity. There 

 are undoubtedly fewer species of primary 

 residential value than there are of second- 

 ary residential value, which brings to mind 

 the further suggestion that, within the 

 principle of ecological replaceabifity, the 

 number of species tends to increase as 

 their relative importance in determining 

 the basic organization of the community 

 decreases. 



When what has been said is applied to 

 the human community, at a level of inte- 

 gration found in a large city, the imme- 

 diate appUcation is both interesting and 

 obvious. First, considering the human spe- 

 cies from the point of view of numerous 

 functional groups, each with its own oc- 

 cupation, it will be seen that the roles of 

 some of these "occupational species" are of 

 primary importance. That is, the essential 

 framework of the human community is a 

 product of their activities. On the other 

 hand, these relatively few groups of pri- 

 mary residential value serve as a direct 

 stimulus to more numerous "occupational 

 species;" these latter are of secondary res- 

 idential importance and by their manifold 

 activities serve to fill out this complex 

 frame. In addition to man, many other spe- 

 cies respond to the composite human stim- 

 ulus generated and may be considered as 

 tertiary residents whose physiological re- 



