COMMUNITY ORGANIZATION: PERIODISM 



543 



in the profunda! strata of certain deep lakes, 

 while their topmost strata exhibit marked 

 seasonal rhythms. Cave communities ex- 

 hibit more or less aseasonal structure de- 

 pending upon (1) the portion of the com- 

 munity examined, and (2) the degree to 

 which the cave foodweb is dependent upon 

 an epigean seasonal replenishment of foods. 



On the other hand, the majority of com- 

 munities show some seasonal phenomena 

 at some parts of their organization. Two 

 principles emerge from the survey of sea- 

 sonal aspects of communities. The kind and 

 degree of seasonahty of a given community 

 is correlated directly with the kind and de- 

 gree of periodicity of the operating physical 

 influences. The kind and degree of season- 

 ality of any given part of a community is 

 correlated directly with the kind and de- 

 gree of periodicity of the operating physi- 

 cal influences at that part of the structure 

 considered. 



There remain several community phe- 

 nomena that are often confused with the 

 strictly seasonal aspects of activity: the 

 aseasonal remigrations, and emigrations, as 

 defined previously. 



Remigrating animals (migratory grass- 

 hoppers) and emigrating animals (lem- 

 ming, gray squirrel, snowshoe hare, beaver, 

 bushy-tailed wood rat, Norway rat, among 

 others) have been the subject of much 

 study (Elton, 1942; Hamilton, 1939; Seton, 

 1909; Uvarov, 1928), and the relatively 

 sudden appearance of the moving swarms 

 or hordes are spectacular phenomena. The 

 economic damage that often results, when 

 the animals feed upon the agricultural 

 products of man, may be large and create 

 temporary shortages. 



From the point of view of the major 

 community, as a relatively stable and inde- 

 pendent unit, such movements are not of 

 great or lasting value. The chief effects are 

 (1) the immediate and abnormal effect 

 upon the food webs of communities along 

 the line of travel. This is a two-way inte- 

 gration, in that the incoming animals feed 

 upon a lower trophic level, usually as her- 

 bivores, and at the same time are available 

 as food to a higher trophic level. A time lag 

 is required for the community to read- 

 just the food web, after the remigrants or 

 emigrants have moved on. In the case of 

 remigration, this effect is repeated later, 

 when the return is made by the next gen- 



eration, or by a succeeding generation, of 

 the species involved. This is not necessarily 

 always true, since the remigrants may re- 

 turn by a different route. 



Such effects may be important at the 

 time of occurrence, but no case is known 

 where such movements have impaired or 

 destroyed a community beyond redemption. 

 Eventually the imbalance is corrected. This 

 corrective process is especially interesting 

 when the community is seen as a long-time 

 unit with seasonal periodicity. The rela- 

 tively abrupt drop in community activity 

 during dry or cold seasons, the ensuing 

 period of dormancy, and the resumption of 

 plant growth in the following season, are a 

 sequence of events most likely to correct 

 exceptional ravages on the food supplies of 

 the whole community. The community, 

 then, would appear to have a seasonal pro- 

 tective factor against the effects of remigra- 

 tions and emigrations. 



Finally, there are the aseasonal, but pe- 

 riodic, increases in such forms as certain of 

 the cicadas (Beamer, 1928; Davis, 1925; 

 Marlatt, 1907; Mills, 1929; Osborn, 1902, 

 Strandine, 1940). These insects, after pass- 

 ing a relatively long period of development 

 in the subterranean strata of terrestrial 

 communities, chiefly forest communities in 

 which oaks are prominent, emerge to make 

 a dramatic and noisy debut in the upper 

 strata. The numerous species have widely 

 differing periods of development, and are 

 to be considered as permanent residents of 

 certain types of communities, although their 

 adult span is seen for a relatively brief 

 period. The species of cicada that has at- 

 tracted the most attention in the United 

 States is the periodical cicada or seventeen- 

 year "locust" {Masicicada septendecim) . 

 Some twenty broods of this species have 

 been traced out, and one or more broods 

 appear each year, at some part of the 

 range. 



The appearance of the adults in the 

 higher strata of the occupied communities 

 sets up the same two-way integration re- 

 ferred to previously for remigrants and 

 emigrants. The act of oviposition kills a 

 conspicuous amount of foliage in oak can- 

 opies, and the insects are an abundant and 

 readily available source of food for the bird 

 populations. The amount of food thus avail- 

 able is large. Brood XIII, resident in the 

 Chicago area, appeared in 1922 and more 



