328 



POPULATIONS 



lations have reached (probably) a peril- 

 ously low level: e.g., the house mouse of 

 St. Kilda, the sea otter, the ivory-billed 

 woodpecker, the white-winged dove, the 

 trumpeter swan, and the CaUfornia con- 

 dor." The difficulty with these examples 

 from the point of view of the population 

 student, however, is that they are reported 

 after the event, so that quantitative data 

 describing the decline and/or extinction are 

 rarely available. This means that we must 

 turn primarily to a few experimental illus- 

 trations to show how a group decUnes from 

 period to period. 



Before mentioning these experimental 

 studies, however, it is appropriate to dis- 



have been restricted to Martha's Vineyard 

 alone. The course of the population from 

 1890 to 1926 is shown in Figure 122. 

 There it can be seen that approximately 200 

 birds remained in 1890 with a slight de- 

 cline extending until 1907, This decline 

 may have been caused by the introduction 

 of prairie chickens that probably interbred 

 with the heath hens and by an increase in 

 the desirabihty of the bird as a collectors' 

 item. 



In 1907 Massachusetts inaugurated sound 

 conservation procedures with a resulting in- 

 crease in population size, to a total number 

 of 2000 by 1916. That year a combination 

 of factors (a fire, a gale, a cold winter, and 



1890 



'16 '18 '20 



'25 



1907 



TIME IN YEARS 

 Fig. 122. Population trend of the heath hen on Martha's Vineyard (Massachusetts) from 



1890 to 1926. (From Gross.) 



cuss the decUne and extinction of the heath 

 hen. This case involves a natural popula- 

 tion for which census data are available 

 and certain of the factors understood. The 

 matter is summarized by Gross (1928) and 

 put in more general perspective by Alice 

 (1938). 



Populations of heath hens were originally 

 abundant in Massachusetts and probably 

 much of New England. Through hunting, 

 these birds were gradually driven eastward 

 until about 1850 the species existed only on 

 Martha's Vineyard and contiguous islands, 

 and among the pine barrens of New Jer- 

 sey. From 1880 the heath hen seems to 



" An interesting, theoretically probable, 

 erratic fluctuation concerns hibernating frogs 

 and toads. An exceptionally severe winter, if 

 also without adequate snow cover, may kill a 

 large proportion of the hibernating individuals. 

 Severe cold alone will not do it. Frogs intro- 

 duced in Iceland were said to have been ex- 

 terminated by this combination of factors. 



an unusual predation pressure by gos- 

 hawks) reduced the population to fifty 

 breeding pairs or less. Again, the numbers 

 rose slightly by 1920, but thereafter a 

 steady and inexorable contraction set in so 

 that by 1928 only one male could be found. 

 This bird was later banded and released 

 and was last seen alive on February 9, 1932. 

 It is reasonable to conclude that the spe- 

 cies is now totally extinct. Gross suggests 

 that the decline of the heath hen was accel- 

 erated by an inadaptability of the species, 

 excessive inbreeding, and, during the later 

 period, an excess of males. 



Gause, Nastukova, and Alpatov (1934) 

 studied dechne and extinction in popula- 

 tions of two species of paramecia, P. Cauda- 

 turn and P. aurelia, when cultured in homo- 

 typically and heterotypically conditioned 

 media, i.e., media altered by and contain- 

 ing waste products liberated by the organ- 

 isms hving in them. Their report is useful 



