542 



THE COMMUNITY 



spring migration, on the other hand, ap- 

 parently coincides with the resumption of 

 plant growth. This food factor is dependent 

 upon the disappearance of snow, which, in 

 turn, is dependent upon a rise in tem- 

 perature, but the deer appear to react to 

 the growth of fodder, rather than to melting 

 snow and increasing temperatm'e. If suit- 

 able fodder is available throughout the year, 

 some or all of the deer in a particular area 

 may not migrate. The exact migration 

 routes taken appear to depend upon habit. 



Similarly, groups of diverse stimuH, more 

 or less complex in their origin and effects, 

 play their parts in the migratory phenomena 

 of the numerous kinds of animals cited. We 

 are a long way from complete compre- 

 hension of any migratory problem, whether 

 we are deaUng with anadromous and 

 catadromous fishes (p. 170), whales, moun- 

 tain sheep, certain bats and butterflies, or 

 with the relatively better-known migratory 

 habits of birds. 



As more and more migratory species are 

 drawn into a discussion of migration as it 

 affects the community, it becomes increas- 

 ingly clear that the essential differences be- 

 tween broad, truly migratory patterns and 

 broad shifts in community populations are 

 less than at first appear. Length of migra- 

 tion route is no criterion of difference. 

 Among migratory birds, distance traveled 

 varies from thousands of miles in many 

 species, to a few miles in the case of partial 

 migrants, and similar variation occurs in 

 other groups of migratory animals. En- 

 vironmental and internal stimuli are in- 

 volved in both geographic migration and 

 in smaller intracommunity patterns. Many 

 seasonal movements, either between dif- 

 ferent strata of the same community or 

 between communities of the same type, or 

 between different types of communities, are 

 diflBcult to classify as migratory in the 

 strict sense of the word, or partially migra- 

 tory, or not migratory. As we learn more 

 about population movements within and 

 between communities, the dissimilarities 

 become no more remarkable than the agree- 

 ments. 



To illustrate this diflBculty, let us con- 

 sider an example of seasonal movements 

 in a terrestrial, in a fresh-water, and in a 

 marine community. A population of the 

 ladybird beetle, Ceratomegilla ftiscilabris, 

 moves from the meadow grasses and forest 



leaves into the forest floor during the 

 autumn, stays in the deeper portions of the 

 floor mold during the winter, and moves 

 into the meadow and upper forest strata the 

 following spring, where it breeds and feeds 

 upon aphids and other organisms (O. Park, 

 1930). Seven species of pulmonate snails 

 move into the deeper water of Douglas 

 Lake, Michigan, at the approach of cold 

 weather, remain in the deep water through 

 the winter, and return to shallow water of 

 the lake in the spring (Cheatum, 1934). 

 Certain species of the snail, Littorina, move 

 into relatively deep, salty water for the 

 winter season, and move back into brackish 

 water during the summer (Batchelder, 

 1915, 1926). Numerous species of crabs, 

 prawns, lobsters, and squids move into 

 deeper ocean water for the winter, and 

 move back to the marine httoral water in 

 the spring (Pearse, 1939). 



How shall we classify these movements? 

 They are all seasonal. They are apparent 

 adjustments to apparently changing condi- 

 tions. They are periodic journeys, of dif- 

 ferent distances and of different rates of 

 tiavel. Since they are regular population 

 shifts between two or more communities, 

 with a return to the original area, they 

 cannot be called emigrations. Are they, 

 then, migrations or remigrations? Who 

 can say for all of them? The complete 

 answer includes a knowledge of the fife 

 span of the several species. Second, if they 

 live long enough, do the same individuals 

 make the return journey (which is another 

 way of saying that an individual makes the 

 trip twice instead of once)? But if twice, 

 is the species truly migratory or partially 

 migratory; that is, does the whole popula- 

 tion regularly make the round trip, or is a 

 part of the species sedentary? What is 

 the relative importance of exogenous and 

 endogenous factors? The questions cannot 

 be answered at this time. The best that 

 can be done now is to note that these 

 seasonal movements are integrated aspects 

 of the periodism of the communities in- 

 volved. 



Before leaving the strictly seasonal 

 periodic aspects of communities, it should 

 be remembered that few show seasonal 

 rhythms in all portions of their structure. 

 The abyssal stratum of the marine com- 

 munity may be regarded as strictly 

 aseasonal, and that condition is approached 



