344 



POPULATIONS 



water was low, this blanketing effect of the 

 quiet air probably resulted in a sharp rise 

 in the carbon dioxide concentration of the 

 water, a rise great enough to induce asphyx- 

 iation. It is not likely that the acidity of 

 the water was caused by something other 

 than the carbon dioxide, because the pH at 

 the other station was not 5.6, but 6.1. As 

 the fresh wind blew over the stream, the 

 pH rose from 5.6 to 6.0, and the popula- 

 tion recovered. The following summer, 

 when a school of only 10,000 salmon was 

 in this region of the stream, repeated water 

 analyses showed the pH to be 6.3 at 61° F. 

 for both stations, and there was no such 

 catastrophic mortality. In short, this partic- 

 ular and unique series of events did not 

 repeat itself. 



Davidson cites the work of others to 

 make the points (1) that carbon dioxide 

 pressure increases rapidly with a lowering 

 of pH in water with a low alkaline con- 

 tent, and that theoretically this pressure at 

 the place and time of death should exceed 

 that in other regions of the stream where 

 the pH was 6.1 by approximately 50 per 

 cent; and (2) that experimental studies 

 show that an increase in carbon dioxide 

 pressure in water causes an increase in 

 blood acidity in the fish with consequent 

 destruction of red blood cells. 



If this admittedly speculative analysis is 

 correct (it seems reasonable so far as the 

 observations and evidence go), Davidson's 

 study affords a striking, even if rare, illus- 

 tration of the toxic effect of sudden accu- 

 mulations of carbon dioxide on a natural 

 population. 



Miscellaneous Chemical Factors 



A number of chemical factors, discussed 

 in Section II, that have relation to our 

 present interest cannot be considered fur- 

 ther at the population level because of 

 space limitations. However, reference 

 should be made to this section, particularly 

 to passages dealing with silicon, copper, 

 phosphorus, nitrogen, osmotic balance, oxi- 

 dation-reduction potentials, hardness of 

 water, and trace chemicals in soils. 



The reader should not infer from the 

 foregoing discussion that the physical en- 

 vironment exerts an effect on populations 

 which is exclusively independent of den- 

 sity. In fact, we believe that this general 

 position has been overstressed. Smith 



(1935) has written cogently about this 

 point, and the following quotation expresses 

 the matter in balanced perspective and 

 serves as something of an admonition as 

 well: 



"Climate does not always act as a density- 

 independent factor, but often operates quan- 

 titatively in much the same way that is char- 

 acteristic of biotic factors, that is, it destroys a 

 percentage which increases with density. So far 

 as the writer is aware, no careful studies have 

 been made to elucidate just how climate has 

 this effect. It would require a type of study 

 which would be extremely dijBBcult to carry out, 

 since it would necessitate the determination of 

 the causes of death of a large number of insects 

 at two or more densities, under perfectly nat- 

 ural conditions in the field. But climate so 

 obviously limits geographic distribution and 

 determines the average number of so many 

 species that, even in the absence of proof, we 

 must admit that under certain conditions it is 

 capable of acting as a density-dependent factor. 



"It seems most probable that this takes place 

 through the existence of protective niches in 

 the environment which are more or less limited 

 in number. Individuals in excess of this number 

 and which cannot therefore attain these niches 

 are destroyed by unfavorable climate, while the 

 others survive and prevent extermination of the 

 species. Climate affecting the numbers of a 

 species in this way would operate as a density- 

 dependent factor, since its relative effect would 

 increase and decrease with increasing and de- 

 creasing density. Climate can also affect the 

 equilibrium position indirectly by modffying 

 the efficiency of the density-dependent factors" 

 (p. 894). 



FOOD 



The importance of food as a factor of 

 population significance is obvious in the 

 sense that there is some sort of relation be- 

 tween population growth form and food 

 availability. But it is difficult to generalize 

 about this factor and particularlv difficult 

 to place it neatly into a classification of 

 density-independent or density-dependent 

 categories or, for that matter, into other 

 classificatory schemes. For example, .sup- 

 pose an essential limitation of a particular 

 food substance for a defined population 

 results in an insufficient amount of the sub- 

 stance for all. On logical grounds it 

 could be assumed that this would have 

 an effect on the population irrespective of 

 its density, or that the limitation might 

 establish a competition within the group for 

 the substance, in which case density would 

 certainly enter as a factor, or there might 



