346 



POPULATIONS 



With these points in mind we now pre- 

 sent an illustration of food affecting popu- 

 lations in what appears to be primarily a 

 density-independent way, and several, later 

 in this chapter, in which such density-de- 

 pendent elements as competition, predator- 

 prey, and host-parasite relations can be 

 detected. 



Allen (1909) reported a suggestive rela- 

 tionship extending over a seven year period 

 between the amount of sunlight in February 

 and March and the abundance of mack- 

 erel caught off the West Cornish coast in 

 May of the same year. This correlation is 

 diagrammed in Figure 127, in which the 

 abscissa is time by years (1902 to 1908); 

 the left ordinate, the amount of sunUght; 

 and the right ordinate, the mackerel catch 

 in hundreds. The two curves show consid- 

 erable confluence, more than could be rea- 

 sonably expected on a chance basis. Bullen 

 showed (1909) by stomach content analysis 

 that the mackerel fed primarily on zoo- 

 plankton and especially on copepods, and 

 that there was a close association between 

 mackerel landings and the size of this zoo- 

 plankton population. He also showed that 

 there existed no consistent relation of a 

 positive sort between the phytoplankton as 

 such and the mackerel. The causal chain 

 linking the sunhght on the one hand with 

 the mackerel catch on the other is some- 

 what speculative, but the story runs some- 

 thing like this. The amount of sunUght in 

 February and March affects directly the 

 productivity of the phytoplankton. When 

 there are many hours of sunshine, the 

 plankton are more abundant, and vice 

 versa. Since it is well known that the zoo- 

 plankton feed on phytoplankton, it may 

 be assumed that years of lush phytoplank- 

 ton crops will favor large zooplankton 

 populations. The latter apparently reach 

 their maximum around May, and the 

 mackerel then move in from nearby to 

 feed, as reflected by the local fishery 

 statistics reporting size of catch. 



THE BIOTIC ENVIRONMENT AT THE 

 POPULATION LEVEL 



The discussion starting here includes, 

 first, an analysis of the nature and opera- 

 tional aspects of population density, and, 

 second, a consideration of problems in 

 which density plays a significant role. These 

 problems have been selected because of 

 their theoretical importance for population 



ecology; taken as a group, they encompass 

 both intraspecies and interspecies phenom- 

 ena. 



More specifically, population density is 

 first discussed from the viewpoint of gen- 

 eral orientation, followed by a treatment of 

 coactions within populations, of environ- 

 mental conditioning, and of microclimate. 

 Then certain problems are outlined, along 

 with which an effort is made to adduce 

 principles of general ecological merit. These 

 problems are considered: (1) population 

 dispersion; (2) the analysis of population 

 cycles; (3) the "mixed species" problem: 

 interspecies competition; (4) intraspecies 

 and interspecies predation; (5) organized 

 predation by man: the problem of the op- 

 timal yield; and (6) host-parasite interac- 

 tions. 



The intent behind our discussion of these 

 six problems is not to set forth in system- 

 atic fashion all ramifications of population 

 ecology, but rather to establish those 

 broader aspects that are considered, for 

 purposes of emphasis, reasonably discrete 

 imits by investigators in the field and that 

 are under study to a greater or less degree 

 at both the natural and the laboratory 

 levels.* 



POPULATION DENSITY 



General Aspects 



Certain preliminary comments about 

 population density are in order before pro- 



** A number of population problems other 

 than those mentioned are dealt with elsewhere 

 in this book, both in the present section and in 

 the sections on Communities and Evolution ( IV 

 and V). Thomas Park (1946), in a discussion 

 of the scope of population ecology, lists four- 

 teen subjects or problems of then-current in- 

 terest. These are: (1) studies describing the 

 growth form of populations; (2) the effect of 

 various physical-chemical factors on population 

 growth form; (3) analysis of population equi- 

 librium; (4) the problems of underpopulation, 

 optimal population, and overpopulation; (5) 

 the productivity of populations, and factors that 

 influence it; (6) the problem of the optimal 

 yield; (7) description and analysis of popu- 

 lation cycles; (8) analysis of dispersion within 

 and between populations; (9) analysis of range 

 and territory phenomena exhibited by natural 

 populations; (10) epidemiological aspects of 

 the interactions between host and parasite 

 populations; (11) intraspecies and interspecies 

 competition; (12) the organization of social 

 populations; (13) improvements and extensions 

 of population census techniques; and (14) the 

 integration of populations. 



