THE GROWTH FORM OF POPULATIONS 



327 



housefly) and its pupal parasite, Mormon- 

 iella vitripennis. These, again, will be men- 

 tioned in Chapter 22. Suffice it to say here 

 that DeBach and Smith also concluded that 

 "the populations of both host and parasite 

 followed closely the theoretical predictions" 

 —that is, predictions of oscillations. 



It is well to stress that the examples just 

 discussed and others Hke them concern os- 

 cillations reported from experiments con- 

 ducted under rigorous, simpHfied, and care- 

 fully controlled conditions; in fact, the au- 

 thors were interested in trying to demon- 



to oscillations in natural populations as we 

 have been able to find. 



Decline and Extinction 



Our next consideration under growth 

 form concerns population decline and ex- 

 tinction. As stated on page 305, by decline 

 is meant consistent and progressive reduc- 

 tion of the population below equilibrium or 

 below the lower range of usual fluctuation 

 and/or oscillation: by extinction is meant, 

 obviously, the final dying-out of the group, 

 whether of the species as a whole or of an 



6 8 



TIME 



16 



18 



10 12 14 



DAYS 



Fig. 121. Fluctuations in population density of Paramecium aurelia (broken line) and yeast, 

 Saccharomyces exiguus (solid line). The first species censused per 15 cc. of media; the sec- 

 ond species, per 0.1 c.mm. of media. (From Gause. ) 



strata their existence. It is not possible, so 

 far as we are aware, to present cases for 

 natural populations that approach these in 

 symmetry of trend Hues. This is to be antic- 

 ipated on the ground that animal groups 

 in nature characteristically inhabit a rather 

 labile and changing environment when con- 

 trasted with those of the laboratory. To put 

 it differently, oscillations, in contradistinc- 

 tion to fluctuations, are the exception rather 

 than the rule when such departures from 

 equilibrium are considered. However, the 

 reader should glance back through the sec- 

 tion on fluctuations and reexamine espe- 

 cially Figures 117, 118, 119, and 120, 

 dealing with the varying hare, the lynx, the 

 field mouse, and the colored fox, respec- 

 tively. These are, plainly, regularized 

 fluctuations, and are as close an approach 



individual population. It seems to us that 

 these two growth forms, though separable 

 by definition, actually belong with each 

 other. That is, once a population gets well 

 under its normal hmits of abundance and 

 barring important ecological or genetic 

 changes, it frequently, though not neces- 

 sarily, becomes extinct. Therefore, for pur- 

 poses of convenience, these two aspects are 

 considered together. 



There are, of course, many instances of 

 known decline and extinction of species, 

 and certain of these are discussed in the 

 section on Evolution (p. 679). Notable ex- 

 amples mentioned in passing include the 

 Arizona elk, the great auk, the Labrador 

 duck, the passenger pigeon, the Carolina 

 parrakeet, the Eskimo curlew, and the heath 

 hen. There are also instances in which popu- 



