COMMUNITY ORGANIZATION: PERIODISM 



545 



have a long period of continuous darkness, 

 and an equally long period of illumination, 

 so that there are months when every diel 

 is equally dark or hght, as the case may 

 be. Between these two extremes are the 

 subtropical, temperate, and subpolar re- 

 gions, which are gradually and regularly 

 intermediate in tliis regard. 



Second, there is the variation of the rel- 

 ative lengths of day and night through 

 the year, at a given latitude or, to a lesser 

 degree, with altitude. This regular change 

 is closely associated with, and regularly af- 

 fected by, the first mentioned variation, and 

 is of equal importance to the community 

 (Harden, 1942). It is with this latter 

 rhythm that the associated photoperiodism 



of herbivores. Such a response would be a 

 general one, of unconscious cooperation at 

 the community level. 



In addition to, and associated with, the 

 length of day and night, are numerous pri- 

 mary and secondary influences of the phys- 

 ical part of the environment. Such in- 

 fluences usually follow a rhythmic diel pat- 

 tern (pp. 229, 232) in daylight intensity, 

 daylight quality, temperature, relative hu- 

 midity, rate of evaporation, and so forth. 

 The result is that the day is relatively 

 bright, warm, dry, with a high power of 

 evaporation; whereas the night is relatively 

 dark, cool, moist, with a low power of 

 evaporation (Fig. 187). 



The regular march of these influences 







^o-,^^.*'** 



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 (\n PM AM 



Fig. 187. Diagram of a diel in the late vernal period near the southern end of Lake Michigan. 



(p. 121) is concerned, which aflFects the 

 community in many ways (Garner and 

 Allard, 1920; Kellerman, 1926). The best 

 demonstrated effect is the series of photo- 

 periodic correlations between relative day 

 length and cycles in the leafing, fruiting, 

 and flowering of certain terrestrial plants 

 (Costing, 1948). Such rhythms affect plants 

 directly; for example, in the photosynthetic 

 key industry (p. 501). Herbivores are af- 

 fected indirectly, and carnivores still more 

 indirectly, until each trophic level is drawn 

 into the association. The photoperiodism of 

 a plant species may be viewed as a particu- 

 lar correlation, of those individuals of the 

 population involved, of endogenous and 

 exogenous factors, within the limits of in- 

 herited toleration. When the photoperiod- 

 isms of plant species populations of a com- 

 munity are viewed collectively, the over- 

 lapping mosaic that results affects the gen- 

 eral aspect of, for example, the canopy, 

 and may tend to equalize the food supply 



over the diel periods, throughout the sea- 

 sonally periodic year, imposes a periodic 

 physical environment upon the majority of 

 communities. Exceptions are generally 

 those communities that, because of their 

 locations, escape a seasonal environment or 

 have developed a complex social life, to be 

 discussed presently. 



What is not so generally recognized is 

 that the community is more restricted in 

 its reaction to adverse conditions than is 

 the organism or population. Organisms and 

 species have three choices: death, disper- 

 sion, or adjustment. A contemporary com- 

 munity, in the integrated sense used here, 

 has only two choices: death or adjustment. 

 A community may expand, contract, or 

 shift its area through time (Potzger and 

 Wilson, 1941), but it cannot migrate. Cer- 

 tain of its several component populations 

 may migrate, emigrate, or remigrate, and 

 they may articulate with another commu- 

 nity and attain survival. The independent 



