19. BIOLOGICAL BACKGROUNDS FOR 

 POPULATION STUDIES 



In any scientific field there are focal points 

 of study. The geneticist stiesses the mode 

 of transmission and the biochemistry and 

 physiology of the gene. The cytologist 

 stresses the structure of the cytoplasm and 

 nucleus. The population ecologist is in the 

 final analysis concerned with three com- 

 posite factors: natality, mortaUty, and dis- 

 persion. These are the forces that shape the 

 course of population growth, the composi- 

 tion of the population, and its distribution 

 in space. In short, they are factors in the 

 statistical sense related to group survival. 

 We wish now to discuss these factors in 

 greater detail, for by so doing we develop 

 a partial "biological background" for the 

 population problem. 



NATALITY 



NataUty is the population-increase factor. 

 It can be defined in a general sense as the 

 "force" of total population reproduction. 

 There is some reason, despite their aca- 

 demic character, for recognizing two 

 aspects of this reproduction— potential and 

 realized. (Other discussions of this point ap- 

 pear in Chapman, 1931; Bodenheimer, 

 1938; Thomas Park, 1942.) 



Potential reproductive capacity is a theo- 

 retical concept in the sense that a species 

 potential is probably never reaUzed by a 

 natural population. We recognize absolute 

 potential and partial potential. Absolute po- 

 tential is the maximum reproduction pos- 

 sible for a species population. To attain 

 this maximum a species would exist under 

 ideally optimal ecological and genetic con- 

 ditions. Partial potential is the maximum 

 reproduction possible for the species popu- 

 lation under a given set of conditions. This 

 rate would not equal the absolute potential 

 unless the conditions were ideal. Species 

 with a high reproductive potential charac- 

 teristically have a great toll taken by death, 

 while those with a low potential have a 

 smaller death toll. We shall discuss shortly 

 and at greater length this interaction of re- 

 production with mortality. 



Several examples of high partial poten- 

 tial, or at least of great reproductivity, may 

 be of interest. GaltsoflF (1930) reported that 

 an individual oyster can produce 55 to 114 

 million eggs, while Pearse (1939) estimates 



that the blue crab of the Western Atlantic 

 carries 1,750,000 eggs at one time. The 

 capacities of queen ants and termites are 

 also well known. Emerson (1939a) reports 

 that an ant queen has been observed lay- 

 ing 341 eggs per day, while a capacity of 

 6000 to 7000 eggs per day is not unusual 

 for specialized queen termites. In a period 

 of about three weeks, the housefly {Musca 

 domestica) , under favorable conditions, can 

 lay six batches of eggs, each batch contain- 

 ing about 140 eggs. 



Hart and Tester (see Pearse, 1939) have 

 described the spawning activity of the Pa- 

 cific herring in the Strait of Georgia. There, 

 on four spawning grounds, a population of 

 1 to 9 milHon fishes annually produces 8 to 

 75 billion eggs. Of these about 0.1 per cent 

 reaches maturity, although 95 per cent may 

 hatch. Chapman (1931) suggests that the 

 shad lays ". . . from 30,000 to 100,000 

 eggs per season and the carp from two to 

 four milUon." Raillet (1895) concluded 

 that the parasitic tapeworm Taenia pro- 

 duces at least 8800 eggs in a single proglot- 

 tis and liberates as many as thirteen or 

 fourteen proglottids each twenty four hours. 



When graphed by generations, natality 

 potentials typically assume an exponential 

 or "compound interest" form. This is a sit- 

 uation in which increase at any moment is 

 proportional to the size already attained. It 

 is important that we understand the form 

 of such growth curves, since this concept 

 will be needed for later discussions. Boden- 

 heimer (1938) has shown, for example, 

 that an individual Paramecium under stated 

 conditions of culture multiplies by fission 

 with a consequent s-shaped or "logistic" 

 population growth-form (see page 301). 

 The early phases of growth coincide closely 

 with an unrestricted or exponential pattern. 

 However, after the fifth fihal generation the 

 exponential and observed curves begin to 

 diverge abruptly. The population reaches 

 its maximum possible size ("asymptote") of 

 about 300 paramecia per cubic centimeter 

 after twelve generations. Had the growth 

 for this interval been exponential, there 

 would be 4096 organisms instead of 300. 

 Between the twelfth and fifteenth genera- 

 tions the population remains at the 300 

 level, although presumably the reproduc- 

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