COMMUNITY ORGANIZATION: PERIODISM 



539 



ness was correlated with seasonal perio- 

 dicity of temperature, and (2) that degree 

 of cold hardiness in a series of species, from 

 a variety of habitats in terrestrial and 

 aquatic communities, was correlated with 

 the normal seasonal fluctuation of tempera- 

 ture in that community or habitat in which 

 a particular species was normally resident 

 (p. 99). 



These and similar data on toleration 

 (T. Park, 1945a; Park, Gregg, and Luther- 

 man, 1940) and mores (Allee, 1912, 1926b; 

 Shelford, 1914, 1914a) suggest that each 

 type of community has an inherent spec- 

 trum of toleration to normal extremes of 

 adverse physical conditions to which it is 

 subjected, and that this toleration is a 

 quantitative function that operates upon a 

 periodic seasonal rhythm. 



Much that has been said about the 

 general phenomenon of hibernation applies 

 equally well to the phenomenon of aestiva- 

 tion. The latter, too, has both organismal 

 and community aspects and involves 

 horizontal and vertical movements into the 

 floors of communities, desiccation of the 

 aestivating organism, the secretion of 

 epiphragms by mollusks, and the employ- 

 ment of cysts against drought. A lengthy 

 discussion* may be obviated by noting that 

 hibernation and aestivation are in reality 

 two similar facets of seasonal succession, 

 and that the essential differences lie in the 

 different communities involved. Aestivation 

 is typical of two great groups of com- 

 munities: the desert and semidesert type of 

 community, where the annual or pluri- 

 annual variation in precipitation is a domi- 

 nant influence; and tropical communities 

 with more or less sharply defined wet and 

 dry seasons (Hesse, Allee, and Schmidt, 

 1937; Bates, 1945). 



In our examination of the seasonal perio- 

 dicity of the community we have limited 

 the analysis to the resident organisms where 

 feasible, to avoid complexity of treatment. 

 There remains another seasonal response, 

 having broad community implications, that 

 involves species populations, or portions 

 thereof, that are temporary residents of 

 certain communities. It was pointed out 



* For a further study of aestivation, consult 

 Buxton (1923); Greaser (1931); Hesse, Allee, 

 and Schmidt (1937); Pearse (1939); Strandine 

 (1941a); Van Cleave (1931); Van Dyke 

 (1902). 



that an organism has but three choices 

 available when exposed to adversity: it may 

 die, adjust, or migrate. Hibernation and 

 aestivation are broad adjustments to ad- 

 verse weather or climate. Migration or 

 emigration are still different ways of avoid- 

 ing unfavorable conditions. 



As in dormancy, the subject of migra- 

 tion, in the strict sense, has an extensive 

 literature and a particular terminology. We 

 are concerned with the subject as it affects 

 the community concept, but for the student 

 interested in the several ramifications of 

 migration the following references are cited: 

 Chapman (1934); Coward (1912); Hamil- 

 ton (1939); Lincoln (1935); Williams 

 (1930, 1938); Wolf son (1945). 



In the general view, migration is thought 

 of as organismal movement from one place 

 of residence to another. With accumulation 

 of information, this concept has become 

 divided into at least three categories: 

 migration, a more or less continuous and 

 direct movement, under control of the 

 animal, coordinated with or controlled by 

 periodic environmental influences, from one 

 locality to another, in which there is a 

 periodic return to the original locality; 

 emigration or its converse, immigration, 

 a movement of a portion of a species popu- 

 lation, often over great distances, to another 

 locality, without a return to the original 

 area; and remigration, a movement of a 

 portion of a species population from one 

 locality to another, with a return movement 

 to the original locality by different in- 

 dividuals of the species (see also Dispersion, 

 p. 363). 



Animals exhibiting migration in the strict 

 sense of the word include the monarch 

 butterfly, herring, salmon, eels, certain 

 whales, fur seals, mule deer, and a great 

 many kinds of birds. True migration is such 

 a distinctive feature of bird behavior that 

 birds are classified frequently by their 

 migratory status. Such a classification 

 (Pough, 1946) includes the following 

 categories: permanent residents, species 

 that remain in one locality throughout their 

 lives. It should be remembered that even 

 in a resident species a few individuals may 

 exhibit migratorv behavior. Few species of 

 birds are whollv resident. Examples of this 

 first group are bobwhite, screech owls, and 

 English sparrows. Summer residents are 

 those birds that come northward to the 



