Division of Labor 



327 



spread but also, after it has been reduced below a critical size, it may 

 be unable to hold its own. This latter possibility introduces the prin- 

 ciple of the minimum population which states that in order for a popu- 

 lation to survive indefinitely in an environment its numbers must be 

 maintained above a critical minimum. 



A classic illustration of this principle is furnished by the fate of the 

 heath hen, a bird that was formerly abundant in Massachusetts and 

 may have been distributed from Maine to Delaware (Gross, 1928). 

 By 1880 the heath hen was restricted to Martha's Vineyard Island, 

 and a realization of its low abundance led to the establishment of a 

 large reservation for the birds on the island. The heath hens in- 

 creased to about 2000 in 1916, but a fire, a gale, and a hard winter 

 with a great flight of goshawks decimated the population, leaving 

 fewer than 50 breeding pairs. Numbers continued to decline irregu- 

 larly until only 20 birds were counted in 1927, and the last bird was 

 seen in 1932. In spite of elaborate protection this species could not 

 be saved from extinction once the population had been reduced 

 below its critical size. 



Division of Labor. Beneficial effects of increasing numbers may 

 also be brought about by a division of labor made possible within 



Fig. 9.7. The discontinuous range of Iris setosa (open circles) and its varieties: 

 canadensis ( small black dots ) and interior ( large black dots ) . Hatched lines in- 

 dicate the maximum extent of the Pleistocene glaciations. (Cain, 1944, modified 

 from Anderson, Copyright, Harper and Bros., New York.) 



