546 



THE COMMUNITY 



community must remain, and either survive 

 through adjustment, or perish thiough lack 

 of it. The community is more complex, 

 stable, independent, and less mobile tlian 

 its parts. Ihis inability to migrate is a 

 major difference between the major com- 

 munity and its interdependent populations. 



Consequently, just as the community ad- 

 justs to the seasonal rhythm, it also adjusts 

 to the diel rhythm (O. Park, 1940); and 

 where certain portions of its structure are 

 exposed to the diel rhythm, it tends to ad- 

 just locally to those portions so exposed. 



These adjustments cover a wide range 

 of activities, and are as diverse as possible 

 within the heritable limitations and tolera- 

 tions of the reacting populations. All such 

 responses take the form of periodic diel 

 adjustments to food, shelter, and reproduc- 

 tion. Such responses are also characterized 

 by alternating periods of relative activity 

 and relative inactivity, the ampUtude and 

 frequency of such rhythms depending upon 

 the interplay of external, or exogenous, in- 

 fluences, and of organismal, internal, or en- 

 dogenous influences (O. Park, 1940, 1941). 



These responses are classified with ref- 

 erence to the period of activity, rather 

 than the period of inactivity, into three 

 major categories (Park, Lockett, and 

 Myers, 1931). Activity occurring during 

 the day is termed diurnal; at night, it is 

 said to be nocturnal. Activity occurring 

 both during the day and the night, under 

 normal conditions, is said to be arrhythmic 

 with respect to the diel cycle. In this re- 

 spect we may speak of diurnal, nocturnal, 

 or arrhythmic activities, organisms, or popu- 

 lations. An organism may carry out cer- 

 tain activities that are diurnal, and some 

 that are nocturnal. The majority have a 

 well-defined diel period of relative activity 

 and of relative inactivity. 



The periods of overlap, between day 

 and night, when the diurnal animals are 

 becoming quiescent, and the nocturnal ani- 

 mals are becoming active, or vice versa, 

 represent a fundamental shift in the general 

 community activity. These periods of dusk 

 and dawn may have their own faunas, or 

 peculiar activities. Carpenter (1935) di- 

 vided the diel period into four periods of 

 activity: those occurring in the dawn 

 {auroral period), those in the day (diurnal 

 period in a limited sense), those in the eve- 

 ning (vesperal period), and those at night 



(nocturnal in a limited sense). Carpenter 

 (1938) used crepuscular to embrace both 

 dawn and dusk activities, but the more 

 familiar usage, of restricting crepuscular 

 response to the evening, will be adhered 

 to here, and crepuscular and vesperal will 

 be used interchangeably. Similarly, for pur- 

 poses of convenience, unless otherwise 

 stated, nocturnal activities will embrace the 

 crepuscular, and diurnal activities will in- 

 clude the auroral, responses. 



Since the majority of animals have a diel 

 periodicity in general behavior, the com- 

 munities of which they form a part have 

 similarly well-defined diurnal and nocturnal 

 faunas. Although there is much autecologi- 

 cal information on this subject, few com- 

 munities have been studied from this point 

 of view. In fact, most of the general prin- 

 ciples attributed to synecology were based 

 upon diurnal activities or populations, and 

 the analogous study of nocturnal activities 

 and populations has lagged far behind 

 (Park, Lockett, and Myers, 1931). Many 

 years ago Verrill (1897) remarked on the 

 importance of nocturnal studies, and later 

 AUee (1927) emphasized that nocturnal 

 ecology was a practically untouched field 

 of investigation. 



At the organismal level there are numer- 

 ous data on nocturnal individuals, species, 

 and higher taxonomic categories. These are 

 too numerous to consider here and have 

 been discussed (O. Park, 1940). Our con- 

 cern is with their integration in the several 

 communities of which they form an impor- 

 tant part. 



The adjustments of nocturnal animals are 

 even less well understood. In the most gen- 

 eral terms, nocturnal adjustments are best 

 known in two categories, the photogenic or- 

 gans of photurid beetles as mating adapta- 

 tions (Buck, 1937, 1937a), and the visual 

 adaptations of invertebrates (Bennitt, 1932; 

 Horstmann, 1935; Rau, 1935; Welsh, 

 1935, 1938) and of vertebrates (Walls, 

 1942). Of especial value is the study of 

 parallel adjustments to day and to night 

 in diurnal and nocturnal species in fairly 

 close taxonomic relationship. This has not 

 been done often. An example is the work 

 of Walls (1931) on the lenses of squirrels. 

 Thus flying squirrels (Glaucomys) are noc- 

 turnal, and have colorless lenses, whereas 

 the tree squirrels (Sciurus), where inves- 

 tigated, have yellowish lenses and are diur- 



