22. POPULATION FACTORS AND SELECTED 

 POPULATION PROBLEMS 



A classification and illustration oi the more 

 important factors affecting populations is 

 here presented; and the integration of popu- 

 lation phenomena is attempted by the dis- 

 cussion of the interdependence and interac- 

 tion of such factors in laboratory and natu- 

 ral populations, of both terrestrial and 

 aquatic species. 



Classification of the factors affecting 

 populations presents certain difficulties. The 

 problem, on the one hand, is to avoid a 

 classification so general that it is meaning- 

 less, and, on the other hand, to avoid one 

 so specific that it is inflexible. It is helpful 

 to examine the classification developed by 

 certain students interested in host-parasite 

 populations and the control of insect pests, 

 since this exhibits some agreement on basic 

 principles. These students have been pri- 

 marily concerned with the factors that efim- 

 inate certain members from a particular 

 population; that is, with sources of mortal- 

 ity. 



Howard and Fiske (1911), in their paper 

 on the gipsy moth, discuss the natural 

 causes of mortafity in insect populations. 

 They divide this mortality into two large 

 categories: "catastropliic," and "facultative." 

 Catastrophic refers to factors that destroy 

 a constant percentage irrespective of the 

 abundance of the form. Facultative refers 

 to factors that destroy a percentage increas- 

 ing as the density increases. This distinction 

 was recognized by Thompson (1928), an 

 important worker on this subject, who re- 

 named catastrophic, "general or independ- 

 ent," and facultative, "individualized or de- 

 pendent." The first group of factors com- 

 prises largely the physicochemical aspects 

 of the environment and those "intrinsic de- 

 fects in adaptation characteristic of the 

 species; the second group comprises 

 the predaceous and parasitic organisms" 

 (Thompson, 1939, p. 331). 



Thompson (1939), in further discussion 

 of these distinctions, points out that the 

 mortality caused by general factors was 

 considered by Howard and Fiske as "inde- 

 pendent of the numerical value of the 

 population in which they act" and that 

 this view depends on the assumption that 

 "the distribution of individuals with respect 

 to these factors remains unaltered, in spite 



331 



of increase or decrease in numbers." He 

 has tliis comment regarding the individ- 

 ualized factors: 



"The 'individualized or dependent' factors 

 are . . . limited in this action by the fact that 

 they are not all-pervading ioHuences, but con- 

 crete beings of a certain kind, restricted to 

 spots where the environment is not too distant 

 from the optimum and subsisting for a limited 

 period of time. One of the most important 

 factors in determining the status of parasites 

 and predators is the relative abundance of die 

 food supply, i.e., of their hosts. The more per- 

 fectly uniform and continuous the distribution 

 of the hosts, the more likely are the parasites to 

 flourish. Furthermore, since the essential char- 

 acteristic of the parasite or predator is that 

 it increases at the expense of its host, the 

 regions in which the host is abundant are those 

 in which it is likely to be most effective as a 

 controlling factor" (p. 331). 



In 1935 H. S. Smith reconsidered this 

 classification on theoretical grounds from 

 the point of view of host-parasite interrela- 

 tions, adopted it in essence, and again 

 changed the terminology. He proposed the 

 terms "density independent" and "density 

 dependent" mortafity factors. The former 

 factors destroy a constant percentage of or- 

 ganisms in sparse, intermediate, or dense 

 populations and thus are basically analo- 

 gous to the "catastrophic" of Howard and 

 Fiske and to the "general" of Thompson. 

 The density-dependent factors destroy a 

 percentage that increases with density and 

 thus are basically analogous to the "facul- 

 tative" and "individualized" factors of the 

 other authors. Smith also recognized an- 

 other group of factors, which destroys a 

 percentage that decreases as density in- 

 creases, but he made fittle of it. Since 

 Smith's terms are essentially self-defining, 

 we shall adopt them in preference to those 

 of the earher writers. 



A further step in the growth of this 

 classification system appeared in a paper by 

 Alice (1941), who was interested in for- 

 malizing in a more specific way the fact 

 that mortality can decrease, as well as in- 

 crease, with density. Alice, accordingly, 

 proposed that density-dependent factors be 

 split into "direct" and "inverse" categories. 

 Direct density-dependent factors are those 



