COMMUNITY ORGANIZATION: PERIODISM 



557 



As to terrestrial environments, dial verti- 

 cal movements of animals are known for 

 relatively few communities, although such 

 phenomena undoubtedly form a regular 

 feature of such stratified communities as 

 forests, and the movement into and out of 

 desert floors is a periodic response to the 

 rigors of the physical portion of the envi- 

 ronment. 



In general, in the communities studied, 

 there are definite vertical migrations in ter- 

 restrial communities. These movements ap- 

 parently are under the control of, or at 

 least are coordinated \vith, the rhythmic 

 march of the operating diel factors, among 

 which may be noted especially (D light 

 intensity, (2) air temperature, (3) the cor- 

 related soil temperature, (4) relative hu- 

 midity, and (5) rate of evaporation. Grav- 

 ity, important in aquatic communities, is 

 relatively inoperative, since the moving ani- 

 mals can climb or crawl up the stable, 

 stratified plant portions of the community 

 in prairies and forests. 



More or less satisfactory demonstrations 

 of vertical movements are known for a 

 sufficient variety of terrestrial communities 

 to attest to its probable general occurrence. 

 For example. Chapman and his colleagues 

 (1926) were able to show a diurnal move- 

 ment into the subterranean stratum, and a 

 nocturnal movement to the sand floor, of 

 certain insects of a Minnesota sand dune 

 community, and Sanders and Shelford 

 (1922) found movements between the her- 

 baceous and shrub strata of a pine dune 

 community near Gary, Indiana. Similar 

 diel vertical and horizontal movements have 

 been described for a variety of flood plain 

 communities near Perm, Russia (Gudosh- 

 chikova, 1927); grassland, grassland-forest 

 ecotone, and deciduous forest near Nor- 

 man, Oklahoma (Davidson and Shackle- 

 ford. 1929; J. R. Carpenter, 1936); and a 

 beech-su^ar maple forest near Michigan 

 City, Indiana (Park and Strohecker, 1936). 



Manv small invertebrates pass their 

 period of diurnal or nocturnal inactivity in 

 the forest floor stratum. This is evident 

 from certain of the studies fust cited, and 

 also from quantitative sampling of leaf 

 mold. In the Panamanian rain forest, Wil- 

 liams (1941) found a relatively low popu- 

 lation density within auadrats studied at 

 night as compared to those studied during 

 the day, and certain of the animals he col- 



lected in the floor by day were not dis- 

 covered there at night. In one instance one 

 of these animals, a pselaphid beetle, Arth- 

 miiis sahomba, was collected by lights at 

 night (O. Park, 1942). 



There is evidence of two vertical diel 

 movements in forest communities. These 

 are (1) a diurnal movement into higher 

 strata during the day, with a return to lower 

 strata at night; and (2) a dusk nocturnal 

 movement into the herbaceous, shrub, and 

 tree strata during the night, with a return to 

 the forest floor during the early morning 

 hours. Such vertical movements may be ac- 

 companied by horizontal spreading over 

 herbaceous, shrub and tree strata. 



The well-defined vertical movement of 

 the marine and fresh-water zooplankton 

 has its counterpart in the less studied verti- 

 cal movement of nocturnal animals in for- 

 ests. Herbivores and predators are involved 

 in both movements. The vertical and hori- 

 zontal movements of diurnal animals in ter- 

 restrial communities apparently have no 

 counterpart in the zooplankton as a whole; 

 instead, the analogous community function 

 may lie in vertical or horizontal movements 

 of diurnal populations in littoral zones, or 

 in vertical movements of larger aquatic ani- 

 mals. 



From a more general view of terrestrial 

 communities, there is a definite correlation 

 between the vertical movements of smaller 

 invertebrates and the active period of the 

 larger and better-known vertebrates. Few 

 data are available on either the relative 

 proportion of nocturnal to diurnal species, 

 or on the relative size of the populations in- 

 volved, per unit of community area. 



As seen from Table 50, in one case there 

 is a striking convergence between the per- 

 centages of diurnal and nocturnal mammals 

 from two relatively well-known areas (O. 

 Park. 1940), the north temperate decidu- 

 ous forest in the vicinity of Chicaeo, Illi- 

 nois, and the Panama Canal Zone equato- 

 rial forest. 



Contemplation of the activities of diurnal 

 and nocturnal animals poses the question: 

 Are such activities induced by the periodic 

 diel rhythm in phvsical factors, or are they 

 more or less endogenous, and only in ap- 

 parent correlation with the cycle of day and 

 night? The answer lies in an analvsis of 

 activity patterns under controlled labora- 

 tory conditions. Hardly more than a start 



