Causes of Fluctuation 459 



are dependent upon the interplay of biotic potential and environmental 

 resistance. Ordinarily the full biotic potential of animals and plants 

 in nature is not reached— environmental factors usually keep a strong 

 check on the population. In some instances, however, even a slight 

 variation in environmental resistance may produce a marked effect on 

 abundance. Consider the accompanying hypothetical example for 

 the annual recruitment of a fish population. From the point of view 



Number of fish spawned each year: 1,000,000 

 1st year: 999,000 die; mortahty 99.9%; 1000 left to catch 

 2nd year: 998,000 die; mortality 99.8%; 2000 left to catch 

 = 100% increase in number surviving 



of the original number of fish spawned the variation in mortality ap- 

 pears to be trivial, but from the point of view of the survival of fish 

 to form the adult population a very great diff^erence is produced in 

 the two sample years. 



In addition to variations in physical features of the environment, 

 changes in the living elements, including both food supply and ene- 

 mies, will produce modifications of the environmental resistance. The 

 biotic potential of the prey is usually greater than that of the predator, 

 but the predator can destroy a much greater supply of its prey than it 

 actually assimilates. As a result predatory animals may sometimes 

 be the chief factor holding prey species in check. When an unusually 

 favorable combination of both physical and biological features of the 

 environment occurs, the species "breaks out" from its controls. If the 

 species possesses a high biotic potential, only a short period of un- 

 checked reproduction will cause the population to irrupt. One North 

 American species of field mouse can breed when it is 3 weeks old and 

 can produce 13 litters per year. A little arithmetic will indicate the 

 size to which the population could expand if most of the young sur- 

 vived for a year or two. Many invertebrates, including notably the 

 insects, can multiply even more explosively. After a population that 

 has escaped from its usual control has increased for a period, its ex- 

 pansion may eventually be stopped by another set of predators, by 

 disease, or finally by a lack of food. The maxima of snowshoe rabbits 

 are brought to an end reputedly by a disease that causes death from 

 shock, possibly induced by shortage of some mineral element in the 

 diet. 



Precisely which influence or combination of influences in the en- 

 vironment is responsible for the augmentation of a population and 

 which brings the increase of a population to a halt or causes it to re- 

 cede is extremely difficult to determine. In attempting to solve the 



