POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



333 



tic, could be factually examined in terms of 

 its density-independent and density-depend- 

 ent components. Unfortunately, the present 

 state of knowledge does not permit this, so 

 that the best we can do is to deal syste- 

 matically with factors that seem of more 

 general significance and to suggest, or spec- 

 ulate about, their mode of action whenever 

 possible. In our treatment we extend this 

 classification beyond the insect populations 

 for which it was originally proposed to in- 

 clude other terrestrial and aquatic groups. 

 Also, our attention is not hmited to mortal- 

 ity; natality and, in some instances, disper- 

 sion are reviewed. 



THE PHYSICAL ENVIRONMENT AT THE 

 POPULATION LEVEL 



Uvarov (1931) outlined the principal 

 climatic factors that afiFect insect popula- 

 tion growth form primarily through density- 

 independent action. This outline serves as 

 an appropriate point of departure for our 

 discussion and is reproduced in the next 

 paragraph. Since we also wish to consider 

 aquatic populations, it is necessary to list 

 the more important factors that can afiFect 

 them, irrespective of their density, and 

 these, too, appear. It must be stressed here 

 that the purpose of this chapter is not a 

 systematic analysis and synthesis of the 

 physical and biotic components of the en- 

 vironment at the autecological level. That 

 was done in Section II and will be con- 

 tinued in the discussion of Communities. 

 Our concern rather is to provide selected 

 cases in which density-independent oper- 

 ations can be detected, or at least sug- 

 gested, at the group level. Our treatment of 

 this topic is limited to the presentation of 

 enough illustrations to bring forth the 

 fundamental ecological principles involved. 

 A number of related topics have already 

 been considered in Section II dealing with 

 the Physical Environment, and we shall 

 refer to certain of these where appropriate. 



The following, in somewhat modified 

 form, is Uvarov's classification of the cli- 

 matic factors that afiFect abundance of ter- 

 restrial species: 



1. Temperature as a controlling factor: 



(a) Direct injury by high and medium 

 temperatures 



(b) Effect of high temperature on re- 

 production 



(c) Winter mortality and general effects 

 of low temperatures 



(d) Favorable effects of temperature 



2. Precipitation: 



(a) Excess precipitation 



(b) Deficient precipitation (drought) 



(c) Winter precipitation 



( d ) Effects of rainfall in tropical climates 



(e) Rainstorms and hailstorms 



3. Wind 



4. Atmospheric pressure 



5. Thunderstorms 



6. Temperature and rainfall 



7. Temperature and humidity 



8. Temperature and light 



9. Food quality and quantity as related to 

 climate 



The following factor-categories seem 

 most significant for aquatic populations 

 from the point of view of the physical en- 

 vironment" : 



L Physical properties of the water 



2. Chemical properties of the water 



3. Water movements, both regular and 

 irregular 



4. Light penetration 



5. Substratum effects 



6. Certain aspects of quality and quantity of 

 food. 



Our task now is to provide illustrations 

 of certain of these items by choosing ex- 

 amples from the hterature dealing with ter- 

 restrial and aquatic, natural and experi- 

 mental populations. As was mentioned, we 

 do not discuss every possible type of opera- 

 tion, nor do we imply, even though for rea- 

 sons of analysis we treat the various factors 

 individually or at best in combinations of 

 twos, that a single factor is the only one 

 eflFectively operating. Obviously, in many 

 cases a number of environmental factors are 

 afiFecting the population, and these may be 

 both density-independent and density-de- 

 pendent in character. Later, we attempt to 

 show something of this in discussing inte- 

 gration. Our general philosophy about the 

 wholeness of the environment has been 

 stated on page 1. 



TEMPERATURE 



Experimental Populations 



Terao and Tanaka (1928) showed that 

 the water flea Moina macrocopa, when 

 abundantly fed, produces difiFerent-sized 



" These general categories receive detailed, 

 systematic treatment in Welch (1935) and 

 Sverdrup, Johnson, and Fleming (1942). 



