10 



Productivity and Populations 



POTENTIAL AND ACTUAL PRODUCTIVITY^ 



Productivity is defined by Leopold ( 1933 ) as "the rate at which 

 mature breeding stock produces other mature stock," Before an 

 appraisal is made of this, the actual productivity, let us consider tlie 

 potential productivity— that constant property of the species which 

 is the theoretical rate at which the species could increase if no mor- 

 tality occurred. From this base line, we may examine the factors that 

 affect this theoretical possibility and derive the actual result. Being 

 promiscuous, breeding the first year after birth and laying a large 

 clutch of eggs (averaging over eleven eggs per clutch), the ruflFed 

 grouse has a high-breeding potential. Like the plant aphid that we 

 learned about in elementary biology, it would overpopulate the 

 earth in relatively few years if nothing checked its increase. Assum- 

 ing a balanced sex ratio, which is normal, a pair of perfectly health)' 

 grouse producing twelve eggs would result in fourteen birds the 

 first season; continuing with equally successful progeny, this orig- 

 inal pair of birds becomes 110 the second year, 782 the third, 5,486 

 the fourth, and so on. 



That such productivity is never even remotely approached is due 

 to the perpetual interplay of numerous delimiting conditions, en- 

 vironmental influences, and decimating factors. To begin with, uni- 

 versal breeding is rarely attained; the occurrence of a few non- 

 nesting females, nonbreeding males, or infertile eggs makes an 

 initial reduction in the potential. Then losses in the egg, immature 

 and adult stages result from a never-ending gamut of hazards. 



Subtracting these losses from the potential productivity, we have 

 remaining the actual. When the fall and winter losses bring the 

 population back to the original number of breeders, we have a 



^ The first portion of tliis chapter is taken largely from Productivity of the Ruffed 

 Grouse in New York, Edminster ( 1938). 



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