332 



POPULATIONS 



just discussed, i.e., mortality increasing 

 with density in Smith's sense. Inverse den- 

 sity-dependent factors are those "eliminat- 

 ing influences which take a decreasing per- 

 centage of the individuals present as the 

 population increases." 



We are left, then, with the following 

 classification system of population factors: 



Density-independent mortality factors 

 Density-dependent mortaUty factors 



Direct 



Inverse 



Figure 124 schematizes a possible opera- 

 tion of these agents in a hypothetical popu- 

 lation."* 



"catastrophic," but for the most part inde- 

 pendent of density— hence, "density-inde- 

 pendent." A particular characteristic of cer- 

 tain biotic factors, e.g., competition, preda- 

 tors, parasites, and pathogens, is that they 

 exert an effect on numbers that is depend- 

 ent on density— hence, "density-dependent." 

 Fluctuations in population growth form ob- 

 viously are caused both by physical and 

 biotic factors. However, Nicholson (1933) 

 and Smith (1935) contend with cogency, 

 largely on theoretical grounds, that, by their 

 very nature, density-independent factors 

 operating alone cannot determine and 

 maintain an average population density 

 over long periods of time— a view that has 



q: 



I 



r^y 



INVERSE 

 DENSITY-DEPENDENT 



DIRECT 

 DENSITY-DEPENDENT 



^ DENSITY 



^ INDEPENDENT 



POPULATION DENSITY *► 



Fig. 124. Schematic representation of density-independent and density-dependent operations. 



(After AUee.) 



This classification has developed pri- 

 marily through theoretical considerations 

 substantiated here and there from observa- 

 tions derived largely from experience with 

 control of host-parasite insect populations. 

 Fundamentally, but not exclusively, the 

 classification distinguishes between the 

 operation of the physical environment from 

 the biotic environment as these determine 

 the "balance" or "steady state" of a popula- 

 tion in time, at least insofar as it is realistic 

 to speak of the existence of such a mean 

 density in nature. The eflFect of physical 

 factors on abundance may be great and 



' A general classification of ecological factors 

 that aifect populations has been proposed by 

 Voute (1943), and applied to forest insects 

 (1946). 



much to commend it. 



In this chapter we consider first the phys- 

 ical environment at the population level, 

 making frequent reference to the autecolog- 

 ical principles presented in Section II. This 

 is followed by consideration of food and the 

 biotic environment under which population 

 density 'per se, and selected problems of 

 population ecology, are discussed. Then a 

 general case is presented for which both 

 physical and biotic factors can be identified 

 in terms of a particular population along 

 with certain suggestions about their possible 

 interactions. The chapter ends with a brief 

 discussion of Population Integration in- 

 tended as a general summary. 



It would be desirable, of course, if each 

 factor considered, whether physical or bio- 



