COMMUNITY ORGANIZATION: PERIODISM 



53' 



stable major commvinity to adverse physical 

 influences. 



The habitat niche in which overwintering 

 is consummated is designated as the 

 hibernaculum. Animals begin their move- 

 ment into these winter residences, and 

 begin their physiological preparation for 

 overwintering, at differing times and in 

 diverse ways. 



At the latitude of southern Lake Mich- 

 igan, used here as a fairly well-known ex- 

 ample, bryozoans have maturing statoblasts 

 by middle August in certain species; fresh- 

 water sponges {Ephydatia and Spongilla) 

 are depositing a crust of gemmules on 

 submerged logs in middle October. By 

 the last week of October, garter snakes are 

 entering hibernation in loose mold and 

 burrows (cf. also Pope, 1937, pp. 113- 

 119). At this same time there is a large- 

 scale movement of many species of beetles, 

 bugs, spiders, and other arthropods into 

 relatively protected hibemacula. 



The primary impetus to such large move- 

 ments would appear to be the onset of ad- 

 verse weather, especially falUng tempera- 

 ture. The distribution of arthropods in 

 winter appears to be determined primarily 

 by the kind and amount of shelter, and by 

 moisture (Holmquist, 1926). In late 

 autumn this movement may be separated 

 into two converging streams of individuals 

 into more protected floors of dense forest 

 conmiunities (Shelford, 1913; Weese, 1924; 

 Blake, 1926; Holmquist, 1926; Park, 1930; 

 Cole, 1946). 



One of these migrations is from the less 

 protected grassland communities and such 

 allied and secondary habitats as pastures, 

 weed lots and forest-enclosed clearings. The 

 other migration is from the higher strata of 

 forests into lower strata. Thus the horizontal 

 and vertical migrations converge in the floor, 

 and upper portions of the subterranean 

 stratum of forest communities. Such an in- 

 flux swells the already heavily populated 

 lower strata. 



In general, the populations of soil-in- 

 habiting invertebrates move deeper into the 

 subterranean stratum during the autumnal 

 and hibernal periods (Fig. 184) and per- 

 form the opposite movement, to the upper 

 layer of soil, or into the floor stratum, the 

 following vernal season. 



Of interest here is the close correlation 

 of this seasonal vertical migration with the 



seasonal expression of the operating phys- 

 ical influences in a given community. 

 Dowdy (1944) finds that both the down- 

 ward hibernal movement and the upward 

 vernal movement of invertebrates of sub- 

 terranean and floor strata are close to the 

 temperature overturns, and in most cases 

 are coincident with them. 



140 

 14.3 

 15.0 



(l)=MEDIUM FINE SANDY 



LOAM 

 (2)SILTY CLAY LOAM 

 (3)'GPAVELLY CLAY 



FEB. MAR. APR. MAY 



38° 33° 62° 73° (I) 



45° 46° 56° 68° (21 



41° 43° 60° 75° (3) 



Fig. 184. Vertical migration of soil-inhabit- 

 ing invertebrates into the subterranean stratum 

 during the autumnal and hibernal periods. 

 (After Dowdy.) 



In tropical areas vertical seasonal move- 

 ments occur as regularly as in temperate 

 regions, but the general response of the soil 

 fauna is to moisture rather than tem- 

 perature. For example, Strickland (1947) 

 found that in Trinidad the downward move- 

 ment of soil arthropods was associated with 

 decreasing humidity as the dry season ad- 

 vanced. 



The distribution of arthropods in the 

 lower strata may be scattered, or certain 

 species may be gregarious in winter and 

 form sheltering aggregations (Allee, 1927a, 

 1931), in which case late arrivals continue 



