396 



POPULATIONS 



Under more usual conditions of competi- 

 tion, not all individuals benefit, and yet 

 the end result may be favorable for the 

 species under consideration; that is to say, 

 the final outcome even of such competition 

 is not necessarily disoperative. 



Certain aspects of survival values, and 

 other physiological efiFects associated with 

 aggregations of animals, are summarized by 

 the curves of Fig. 139. Curve A shows re- 

 sults of biological processes in which the 



Fig. 139. In some phases of population 

 physiology the optimum population is the 

 smallest possible (A); in others it is inter- 

 mediate in size (B). (Modified from Allee. ) 



liighest recorded value is given by the 

 smallest possible population. A pig born 

 as a fitter of one normally grows faster 

 than do members of larger fitters, and this 

 is a common relation tor many animals. 

 Other illustrations of curve A have been 

 provided in preceding chapters (cf. p. 

 347). 



In contrast, grouped animals often show 

 increased efficiency that is sometimes re- 

 flected in longer survival, or better growth, 

 if neither too few or too many animals are 

 present; they have an optimal population 

 density at some intermediate point of the 

 possible range of numbers. This situation 

 is summarized by Figure 139, B. In both 

 A and B the rate of action of the measured 

 process is indicated on the vertical axis, 

 and population density on the horizontal 

 axis, with the least crowded conditions 

 shown to the left. Under conditions sum- 

 marized by curve B, fife processes move 

 faster or more favorably as the population 

 increases until an optimum density is 

 reached. Beyond the optimum a further in- 

 crease in density produces a reversal of the 

 observed trend. The left-hand fimb of the 

 curve represents unconscious cooperation, 

 but after the peak is past, disoperation be- 

 comes steadily more and more the domi- 

 nant effect. 



Consideration of the two curves and of 



the phenomena on which they are based 

 indicates correctly that under most condi- 

 tions overcrowding produces harmful re- 

 sults. This fact has long been known and 

 can be verified easily. The existence ol 

 harmful results associated with under- 

 crowding is a more recent and less well- 

 known discovery. 



OVERCROWDING 



Disoperations from intense overcrowd- 

 ing are nearly or quite universal. Pending 

 direct experimentation, it is usually impos- 

 sible to determine the population density 

 at which such disoperations will be ini- 

 tiated. Some of the types of harmful efi^ects 

 include reduced fecundity (Pearl and 

 Parker, 1922; Robertson and Sang, 1944); 

 retarded cleavage of eggs (Allee and 

 Evans, 1937; Merwin, 1945); slowed asex- 

 ual reproduction of protozoans (Woodruff, 

 1911; Mast and Pace, 1946); decreased 

 growth rate of individuals (Winsor and 

 Winsor, 1935; Foerster, 1944). Animals 

 from an overcrowded population are often 

 stunted, and an inverse relation usually 

 exists, at higher population densities, be- 

 tween numbers present and size attained 

 (Davidson and Vaughn, 1941). 



The more readily observed ecological 

 factors promoting these disoperations in- 

 clude the reduction of available food, the 

 accumulation of excreta and of their de- 

 composition products, and the physical or 

 chemical alteration of the medium or sub- 

 strate. Diseases spread more rapidly 

 through a dense population than through a 

 sparse but otherwise comparable one 

 (Greenwood, 1932), and parasites pass 

 more freely from host to host under 

 crowded conditions (Hegner, Root, Augus- 

 tine, and Huff, 1938). 



Deleterious effects associated with dense 

 populations may result from direct contact 

 between individuals, from indirect contact 

 through the mutual environment, from 

 some other sort of environmental condi- 

 tioning, or from all acting together. Cer- 

 tain of these matters have been discussed 

 at some length in previous pages (346- 

 349) and need no elaboration here. In 

 their long experience with experimental 

 studies on the effect of population density 

 —studies directed primarily toward explor- 

 ing phenomena associated with under- 

 crowding— Allee and his associates have al- 

 ways been able readily to demonstrate ill 



