_800 



FIG. 17-6 Changes in population 

 and reproductive and physiological 

 vigor of muskrats in Iowa, 

 correlated with the 9-10 year 

 grouse-rabbit cycle (peak years 

 1941-1942, 1950-1951) elsewhere 

 over North America (Errington 

 1957). (A) autumn populations; 



(B) grouse-rabbit cycle; 



(C) siie of litters; (D) young 

 breeding in year of their birth; 

 (E) tolerance of crowding; (F) 

 resistance to disease. 



(A) t 



(B) 



(C) 



(D) 



(F) 



'36 '38 '40 '42 



•44 '46 '48 '50 '52 "54 '56 



YEAR 



become evident with variations of fecundity and suc- 

 cess in raising young, in susceptibility to disease, and 

 in individual behavior (Errington 1945, 1954, 1957). 

 When chinch bugs were cultured experimentally in 

 the laboratory, starting each year with new individ- 

 uals collected out-of-doors, there were marked dif- 

 ferences during the nine-year period of study both 

 in number of generations raised per year and in 

 number of young per generation. The rating of re- 

 productive vigor rose from 1.4 in 1917 to 31.2 in 

 1919, dropped to 1.4 in 1921, and rose again to 270 in 

 1925. These differences from year to year were ap- 

 parently not related to the density of population, and 

 could not be otherwise explained (Shelford and 

 Flint 1943). Changes in reproductivity and behavior 



of muskrats in Iowa did not correlate closely with 

 variations in population size of the species, but did 

 coincide in a general way with the grouse-rabbit cycle 

 in other parts of North America. 



Cycles 



the environment 



We have considered so far only factors intrinsic 

 to populations; extrinsic factors (Hutchinson and 

 Deevey 1949) may play a contributing role. Cer- 

 tainly the amount of synchrony within a species evi- 

 dent over extensive areas and between different 

 species or events that are otherwise clearly unrelated 

 (Dewey 1960) is greater than can be explained by 



242 Ecological processes and dynamics 



