POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



347 



ceeding to an illustration of selected den- 

 sity-dependent operations. Population den- 

 sity was defined in Chapter 18 (p. 266) 

 under the heading "Space-Relative Popula- 

 tion" as the number of organisms per unit 

 of space they occupy. This index affords a 

 statistical measure of their concentration 

 and, as was pointed out earlier, can be very 

 meaningful when intelligently appUed.* 



Practically all students of populations 

 have given attention to the study of den- 

 sity. This is so, not simply because the 

 eflFect of density on groups lends itself to 

 analysis, but also because such analysis fre- 

 quently yields data that are, in themselves, 

 of biological interest. Density effects, re- 

 sulting either directly or indirectly from the 

 coactions between the group components, 

 are influential in aflFecting population 

 growth form. Pearl (1930, p. 145) referred 

 to this when he said, "In general there can 

 be no question that this whole matter of in- 

 fluence of density of population, in all 

 senses, upon biological phenomena, de- 

 serves a great deal more investigation than 

 it has had. The indications all are that it is 

 one of the most significant elements in the 

 biological, as distinguished from the physi- 

 cal, environment of organisms." 



A logical analysis of population density 

 can be developed under three arbitrary 

 categories: first, the kinds of processes and 

 events that have been shown to be in- 

 fluenced by density; second, the type of 

 end result induced by density, irrespective 

 of the process involved; and, third, the 

 nature and constitution of density per se. 

 These will be briefly considered merely for 

 purposes of orientation and then developed 

 in more detail by examination of particular 

 cases. 



In discussing the processes influenced by 

 population density, it is only necessary to 

 present a partially complete list to make 

 the point that the crowding of organisms 

 within a restricted environment elicits many 

 diverse responses on the part of the com- 

 ponent members. The following events 

 (among others) have been shown to be af- 

 fected by population density: natality, mor- 

 tality, and dispersion, the three primary 

 factors that control population growth 

 form (p. 272); responses such as the post 

 embryonic development of insects, the 



• A criticism of the term "density" and some 

 of its implications will be found in Hogben 

 (1931) and Robertson and Sang (1944). 



growth of individual organisms, rate of oxy- 

 gen consumption, protection from environ- 

 mental poisons, resistance of marine forms 

 to hypotonic sea water and other unusual 

 physical conditions, communal activity of 

 bacteria and protozoa, the determination of 

 sex in certain organisms, and morphological 

 expressions such as the development of 

 wings by aphids and the initiation of phases 

 in locusts. It is obvious that these and other 

 diverse events, while always of intrinsic 

 physiological interest, are not necessarily of 

 especial populational significance unless 

 they eventually affect in the statistical sense 

 birth, death, and/or dispersion (see Chap- 

 ters 19 and 20). 



In terms of its eflFect on the growth form 

 of populations, density usually has one of 

 two general influences: Either population 

 growth is inhibited, or else it is stimulated, 

 at least temporarily until new density rela- 

 tions are established. Growth inhibition has 

 been the subject of much study and is 

 clearly related to population control and 

 "balance"; growth stimulation has received 

 considerable attention primarily by Alice 

 and his students (Alice, 1931, 1934a, 1938). 



It has long been recognized that the in- 

 creased crowding of organisms in a popula- 

 tion reduces the population growth rate and 

 even brings about population decline. This 

 view has been demonstrated for laboratory 

 and natural groups and has been advanced 

 for human societies. As early as 1843 Farr" 

 proposed an equation in which he at- 

 tempted to establish that human mortality 

 is a function of crowding. Experimental and 

 field studies have corroborated findings of 

 this sort. On the other hand, Allee has 

 marshalled evidence from diverse sources to 

 substantiate the point that "undercrowding" 

 as well as overcrowding can be a hazard. 

 Examples illustrating both types will be 

 presented in this chapter (see also Chap. 

 23). 



The nature of population density, while 

 obviously a problem of importance, is fre- 

 quently one about which precise informa- 

 tion is scanty. Density has been defined in 

 formal terms as the number of ors;anisms 

 per Tmit of space they occupy. Although 



" Farr's equation states that if tlip death rate 

 has the notation R, and population density the 

 notation D, then, 



R = cDm 

 with c and m being constants. 



