CERTAIN DEMOGRAPHIC BACKGROUNDS FOR POPULATION STUDIES 287 



emigration could result in extinction, al- 

 though this probably happens but rarely in 

 nature." A more likely result is that the 

 growth is merely greatly retarded and may, 

 in temperate latitudes, be even postponed 

 for a season. Excessive immigration might 

 either hasten the population's progress to- 

 wards its equiUbrium or exceed the equilib- 

 rium position. The latter would result in 

 compensations of the type mentioned in 

 the preceding paragraph. 



3. Excluding radical changes in the ex- 

 ploitable resources of an environment or 

 some equally radical biotic maladjustment, 

 a natural population rarely reaches a den- 

 sity so low as to be in danger of extinction. 

 Should either of these events occur along 

 with considerable emigration, the popula- 

 tion could die out in that particular ecologi- 

 cal area. An immigration would help to re- 



• Does not include geological extinction. 



store the equilibrium, provided the checks 

 initiating the dechne had been removed. If 

 they had not, the immigrants would either 

 die or become emigrants. 



We shall make some attempt later to 

 provide some actual illustrations. The 

 literature, although it is rich in descriptive 

 information about dispersion and migration, 

 unfortunately boasts little that can be called 

 truly quantitative and analytical. The illus- 

 trative propositions just listed, however, 

 have some value in connection with this 

 chapter: They point out certain of the ob- 

 vious methods by which a population reacts 

 to dispersion patterns through the medium 

 of mortahty and natahty; they focus atten- 

 tion on dispersion as a third factor that 

 aflFects population growth form, and they 

 set the stage for discussions in this and 

 later sections. 



20. CERTAIN DEMOGRAPHIC BACKGROUNDS FOR 

 POPULATION STUDIES 



fiarlier we pointed out that a population 

 has a series of "group attributes" which can 

 be dealt with in numerical terms only (see 

 pp. 263, 264). In short, any analysis of 

 population phenomena requires that the 

 data be subjected to statistical tabulation 

 and treatment. This does not imply that 

 such procedures are necessarily esoteric. In 

 fact, many of the important statistics are ob- 

 tained by simple arithmetic, graphs, and 

 the elementary operations of classical sta- 

 tistics. Nor does it imply, on the other 

 hand, that population investigations do not 

 profit both in their design and their analysis 

 from the intelligent application of certain 

 of the newer statistical methods such as 

 analysis of variance and co-variance, factor 

 analysis, the method of small samples, the 

 methods of probits, path-coeflRcient analysis, 

 and so on. Our only point is one of em- 

 phasis: that population ecology, as in other 

 phases of ecology and much of biology 

 is, of necessity, quantitative ecology and 

 must be dealt with accordingly. 



Statistical methods and biometry ob- 

 viously fall outside the scope of the present 

 work. We do assume that the reader has a 

 modicum of statistical knowledge, so that 

 commonplace statistics, where used, are 

 not defined. 



Certain expressions, however, developed 

 by the population student, especially in 

 connection with human population studies, 

 merit brief attention here. These have some 

 application in the pages that follow and 

 should also form a larger part of the equip- 

 ment of the modem ecologist than they do 

 now. Our intent is not to develop a coherent 

 treatment of demography, but, rather, to 

 suggest by examples that these techniques 

 have real methodological value. In this 

 chapter we discuss briefly the following: 

 birth rates; death rates; the true rate of 

 natural increase; the life-table; and the 

 logistic curve of growth. This is far from 

 being an exhaustive list, but it will in- 

 troduce the reader to certain essentials of 

 demography. 



THE BIRTH RATE 



The demographer recognizes two forms 

 of the birth rate; the crude rate and the 

 specific rate. The former is expressed 

 merely in terms of population size; the lat- 

 ter is expressed relative to some specific 

 criterion such as age, sex, race, economic 

 status, and so on. The specific rate gen- 

 erally has more meaning in the sense that 

 it has greater analytical value; the crude 

 rate is easy to compute and has the virtue 



