394 



POPULATIONS 



flies by the double handfuls from near 

 electric lights along Lake Erie, and hiber- 

 nating aggregations of ladybird beetles are 

 even larger and more widely distributed. A 

 flight of a chrysomehd beetle is on record 

 that formed a belt 15 feet thick and 100 

 yards wide that continued for two days. 

 Similar concentrations of various butterflies 

 and of migratory locusts are well known. 

 Massed populations of bats, of various 

 birds and mammals, including the emi- 

 grating hordes of lemmings, help round out 

 a hurried survey that will not be even ap- 

 proximately complete without mentioning 

 the dense shoals of marine fishes or the 

 water-coloring masses of smaller aquatic or- 

 ganisms, including various protozoans, 

 copepods, and the euphausid crustaceans 

 of the southern ocean. Dampier wrote of 

 these lobster-krills in 1700: "We saw great 

 sholes of small lobsters which colored the 

 sea red in spots for a mile in compass" 

 (vide Matthews, 1932). 



With this rich background of naturalistic 

 lore, it is rather surprising to find that 

 ecologists were slow in bringing forward 

 statistical proof of the common existence 

 of contagious distributions of animals. One 

 of the more complete of these, although by 

 no means the first, that by Cole (1946), 

 was reviewed in the foregoing chapter (p. 

 364). Aggregations of animals are a special 

 phase of population density, and the pre- 

 ceding discussion of principles and prob- 

 lems related to density form a needed 

 background for the present treatment. 



FORMATION OF AGGREGATIONS 



The least social animal aggregations are 

 represented by more or less accidental col- 

 lections. For example, animal-drift lines 

 occur along the margins of many bodies of 

 water. These accumulations are particularly 

 noticeable on the sandy beaches of the 

 Great Lakes, where they may extend for 

 miles. In the animal drift near the margin 

 of Lake Michigan one may find sizable col- 

 lections of aquatic snails, small bivalves, 

 crayfish, or even fishes thrown up by the 

 waves. Similar concentrations of insects are 

 also deposited with many of the individuals 

 still alive. These may have been carried 

 out over the lake by an offshore breeze, 

 then caught in the water and tossed back- 

 on the land by currents and waves. So far 

 as the different individuals are concerned, 

 their aggregation is entirely accidental. 



They may remain aggregated through ina- 

 bility to move or as a result of inertia. In 

 themselves, these aggregations are near the 

 zero mark for sociality; even the quaUty of 

 inertia shown under such conditions has 

 only slight social significance, if any. 



The animal collections along the drift line 

 are a source of food for diverse animals, 

 making what has been called, in the non- 

 technical language of nature study, the 

 "lunch counter" of the beach. The life his- 

 tories of some insects fit the rough periodic- 

 ity of recurrent storms that replenish the 

 animal drift (Herms, 1907). Predaceous 

 and scavenger insects, and some birds and 

 mammals, including skunks, feed on the 

 animal drift ( Shelf ord, 1913). 



Animals congregate under diverse con- 

 ditions as a result of their own movements. 

 Perhaps the simplest of a long, intricate 

 series of types of aggregating behavior re- 

 sults from so-called kineses, that is, from 

 the unoriented reflex action of the whole 

 animal. Thus animals that are stimulated 

 by high light intensity slow down when 

 they enter shaded areas. If many are mov- 

 ing about in a given locality, such photo- 

 kinesis will cause them to aggregate in the 

 shade somewhat as automobiles, moving 

 freely along a through highway, aggregate 

 automatically when slowed by entering a 

 village or by encountering other agencies 

 that locally reduce their rate of movement. 



Aggregations result from oriented reac- 

 tions to light intensities, or to various gradi- 

 ents of environmental forces, when an 

 oriented reaction brings numbers of indi- 

 viduals into a restricted region. A positive 

 response to odors, leading animals to a 

 restricted source of food, affords a common 

 example. The collected forms must have a 

 certain amount of toleration for the pres- 

 ence of, often for contact with, other indi- 

 viduals if the aggregations are to occur and 

 particularly if they are to continue. 



In contrast with the more or less auto- 

 matic aggregation in response to odors, 

 light or shade, moisture, favorable niches, 

 and other environmental factors, there are 

 the much more definitely social situations 

 (p. 419) in which animals collect as a re- 

 sult of positive reaction to the presence of 

 others like themselves. The aggregation of 

 male midges "dancing" in the quiet atmos- 

 phere, or the formation of schools of fishes 

 or flocks of birds illustrates this wide- 

 spread phenomenon. 



