THE BIOTIC ENVIRONMENT OF ORGANISMS 563 



organisms, it is at once obvious that no organism ever does approach its 

 potential reproductive rate. L. O. Howard, of the U.S. Bureau of Ento- 

 mology, determined the biotic potential of the common house fly living 

 under optimum conditions in a laboratory in Washington, D.C. He used 

 experimentally determined values for n, R, and Z: n = 7 generations per 

 year; R = 0.5; and Z = 120 eggs per female. With these values, a pair 

 of flies should be potentially able to produce 1 X 120 7 X 0.5 6 = 5,598,- 

 860,000,000 progeny in the course of a year. 



Actually, of course, nothing like this could happen, not because the 

 values or calculations are false, but because optimum conditions could 

 not be maintained. Even if temperature, humidity, light, and other 

 physical factors should stay at optimum, limits to food and space would 

 soon produce such an intense competition that only a portion of each 

 generation (beyond the first few) could possibly survive. Moreover, there 

 are many organisms that feed upon flies, and not only would their toll 

 on the population greatly cut down the number of flies that would live to 

 produce offspring but the population of fly-eating organisms would tend 

 to increase rapidly as the fly population increased. 



Population Size. Even though the biotic potential of any organism is 

 never attained, the concept and the actual or approximate values for a 

 given species have a much greater use than as mere exercises in arithmetic. 

 Since the biotic potential (as an inherent maximal reproductive rate) is 

 a constant for each species, we can use it to gain an idea of the toll that 

 the total environment levies on that species. This is because the existing 

 population of any species of organism — the number of bass in a given 

 lake, for instance — is a quotient of its biotic potential and the resistance 

 it encounters from all adverse environmental factors. It may be stated as 



Biotic potential . . 



Environmental resistance " & P P 



Since the biotic potential may be determined (at least approximately) 

 by experiment and observation and the existing population may be 

 counted or estimated, we can thus obtain a quantitative measure of the 

 total resistance of all environmental factors. 



When the English sparrow was first introduced into this country, it 

 was freed from its native European competitors and enemies, and for a 

 time it multiplied at a tremendous rate. In 1889, when agriculturists 

 and others were alarmed by its rapid increase and spread across the 

 country, it was estimated that a single pair of English sparrows could 

 in 10 years give rise to 275,716,983,698 descendants. Gradually this 

 species met a stronger environmental resistance. This was partly due 

 to the development of sparrow-eating habits by a number of our hawks, 

 owls, and shrikes; partly to decreasing food supply brought about by 



