POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



363 



mained quite stable over a range of 23° 

 to 29° C. This range, which presumably 

 coincides with the physiological optimum 

 for reproduction and development, is about 

 10 degrees higher than that of the sur- 

 rounding soil at the same depth. The tem- 

 perature appears to be regulated, first, by 

 insulation of the exposed, crater-hke part 

 of the nest, and, second, by certain specific 

 behavior activities exhibited by the work- 

 ers. The latter open or close the nest exits 

 that are either exposed or away from the 

 direct rays of the sun in such a way as to 

 admit heat or screen it out. 



Perhaps the best-known instance for all 

 poikilotherms is the regulation of tempera- 

 ture within beehives, a regulation resulting 

 from cooperative coactions on the part of 

 population members. A brief summary, 

 by Uvarov (1931, pp. 134-135), is here 

 quoted. 



"Reaumur (1740) discovered nearly two 

 hundred years ago that the temperature in- 

 side a beelxive is in winter much higher than 

 that of the outer air, and observed that the bees 

 can raise the temperature by wing movements. 

 Since his time, the problem of the temperature 

 conditions m beehives has been investigated 

 thoroughly by a number of authors. It has been 

 found that a fall in the outside temperature 

 induces the bees to form a dense cluster. With 

 a further fall in temperature (below about 

 13°), they become restless and begin to move. 

 As a result of these movements the temperature 

 in the cluster rises to about 25-30°, at which 

 point these movements begin to slow down. 

 The temperature inside the cluster then begins 

 to sink until it reaches about 13°. The fluctua- 

 tions in the external temperature are repro- 

 duced faithfully by the corresponding, but in- 

 verse, fluctuations inside the cluster, i.e., when 

 the air temperature falls very low, that of the 

 cluster becomes very high owing to the more 

 energetic movements of the bees. In this way 

 the micro-climate is kept practically independ- 

 ent of the climatic conditions." 



The quotation stresses the behavior of 

 bees as the outside temperature falls with 

 consequent increase of hive temperature. 

 The opposite of this, equally well known, 

 deserves mention: namely, the fact that in 

 warm weather the bees cool the hive. In 

 summer the hive is maintained at about 

 34° to 35° C, and if the temperature goes 

 beyond this range the bees cool the nest 

 by fanning movements of the wings and 

 possibly by the carriage of water. 



It seems obvious on a priori grounds 



alone that many natural populations of 

 wide taxonomic types must ameUorate 

 their surroundings to a certain extent (p. 

 211). This phenomenon cannot be dis- 

 cussed further because of space limitations. 

 For additional discussion the reader is re- 

 ferred to the Uterature, and he should 

 realize that the matter is not limited to in- 

 sect populations or even to terrestrial 

 groups. Allee (1931) cites examples in 

 which aggregations of marine invertebrates 

 modify their eflFective environment to an 

 appreciable degree, and sometimes this 

 modification confers added survival to the 

 aggregants. At the vertebrate level Gerstell 

 (1939) presents a convincing study of 

 temperature conservation by bobwhite quail 

 populations. The discussion of microcUmate 

 included in Uvarov's (1931) comprehen- 

 sive review should also be consulted (see 

 also p. 213). 



POPULATION DISPERSION 



The movements of organisms and their 

 populations constitute an important seg- 

 ment of modern ecology and afford a signi- 

 ficant point of distinction between the 

 ecological relations of plants and animals. 

 These movements are discussed from var- 

 ious points of view throughout this book. In 

 Section II the relation of length of day to 

 bird migration was reviewed (p. 121). 

 Migration receives further treatment and 

 definition in the section on Communities 

 (p. 539). There are also selected consider- 

 ations of animal distribution on a geo- 

 graphic scale (p. 580) and of the relation 

 of zoogeography to evolution and specia- 

 tion (p. 608). 



It has been stressed repeatedly in this 

 section that dispersion, along with natality 

 and mortality, can be one of the majoi 

 factors controlling population growth form 

 and behavior. In order to round out the 

 general treatment of dispersion we now 

 wish to discuss briefly an aspect, not con- 

 sidered elsewhere, that is pecuUarly popu- 

 lational in character. This is the distribu- 

 tion of organisms within an intraspecies, or 

 relatively simple interspecies, population 

 The focus here is on the pattern of distri 

 bution as it may be expressed quantita 

 tively over a circumscribed and defined 

 area (or volume). 



Within a particular area the population 

 components may be distributed essentially 



