286 



POPULATIONS 



until she is seven months old. After the fe- 

 male is one year old there is a sharp de- 

 cUne in fertility. Typically, the menopause 

 appears at about eighteen montljs. The 

 mother's age is a factor that affects litter size 

 as well as litter number. Young mothers 

 have small htters, and later Utters are large 

 until the female attains an age of seven 

 months. Thereafter the number decHnes to 

 a point where, for females near the meno- 

 pause, there are rarely more than three 

 young. 



Dublin and Lotka (1936) assembled 

 statistics for human populations that relate 

 the age of the mother to the rate of repro- 

 duction. The interpretation of these data 

 is self-evident, the facts being presented in 

 Table 21 (p. 294) in connection with an- 

 other topic. 



Bodenheimer (1938) stresses that, since 

 age distributions in populations are signifi- 

 cant in relation to reproduction, the Life 

 history of organisms should be studied to 

 emphasize this point. Accordingly, he pro- 

 poses three "ecological ages" into which all 

 animals fall: the period of development, ex- 

 tending from fertilization of the egg to the 

 first birth; the period of reproduction, cov- 

 ering the reproductive span; and the period 

 of postreproduction, or that time between 

 the end of reproduction and death. Need- 

 less to say, these periods vary greatly be- 

 tween species when expressed as percentage 

 components of the total life-span. In addi- 

 tion, the requisite information for most 

 forms is lacking. Bodenheimer presents a 

 table covering certain examples. We have 

 borrowed some of the material from this 

 table, added certain illustrations and 

 graphed the results in Figure 85. From this 

 figure three general points emerge: (1) 

 There seems to be no consistent relation be- 

 tween systematic position and percentage 

 of time spent in the three periods. Thus, the 

 two orthopterans, Schistocerca and Peri- 

 planeta, and the two beetles, Tenebroides 

 and Trogoderma, are quite different one 

 from the other, while Tenebroides hap- 

 pens to be rather similar to man. (2) In 

 general, the postreproductive period is the 

 shortest and the period of development, the 

 longest. (3) It is obvious that a species 

 population of, say the ephemerids, will have 

 a different kind of population dynamics 

 than, say, Tenebroides. This is an essential 

 point for the ecologist. It must be consid- 

 ered in any population analysis. 



DISPERSION 



The ecologist is frequently concerned 

 with population dispersion. This may take 

 the form of small movements or rearrange- 

 ments within the group or mass movements 

 of the group itself. The extent of the latter 

 depends upon the vagility of the species in- 

 volved. In most natural populations emi- 

 grations and immigrations are constantly 

 taking place. At times these are so slight 

 or extend over so long a time that the popu- 

 lation adjusts its growth trend and is not 

 altered in any statistical sense. At other 

 times the dispersion may be excessive and 

 result either in the depopulation or the 

 overpopulation of an area. When this oc- 

 curs, the compensations by the population 

 are more extreme. We discuss this question 

 of dispersion and migration in various con- 

 notations elsewhere in the book (see pp. 

 363 and 539). Here, our concern lies only 

 in emphasiang that such group movements, 

 along with natafity and mortality, form the 

 third set of factors that in final analysis 

 affect the populations' growth form and 

 accordingly must be included in this chap- 

 ter devoted to "biological backgrounds." 



It is self-evident that an immigration 

 temporarily increases population density, 

 while an emigration temporarily decreases 

 it. It is not so self-evident, perhaps, that a 

 population can react to these dispersions in 

 a number of different ways and thus bring 

 about various end results in terms of its size 

 and composition. The reactions, of course, 

 always take place through reproduction and 

 mortality and, occasionally, through addi- 

 tional dispersion. Some of them may be 

 stated in somewhat oversimplified and de- 

 scriptive fashion in the form of the follow- 

 ing rubrics: 



1. A population in equilibrium (i.e., nu- 

 merical stability) may return to that con- 

 dition rather quickly by increased mortality 

 of the increment added by an immigra- 

 tion. Or, the reproductive rate of the total 

 group may decrease, owing to the in- 

 creased density, until the equilibrium is re- 

 attained. Or, both things may happen. If 

 there is emigration, the population may 

 make up the decrement by lowered mor- 

 tality and/or increased reproduction. 



2. A growing population will have its 

 growth form altered by a dispersion if the 

 latter is of suflBcient intensity. Excessive 



