ECOIOGY 773 



the age ratios of commercial fish catches are of great use in predicting 

 future catches and in preventing overfishing of a region. 



The term biotic potential, or reproductive potential, refers to the 

 inherent power of a population to increase in numbers, when the age 

 ratio is stable and all environmental conditions are optimal. The biotic 

 potential is defined mathematically as the slope of the population growth 

 curve during the logarithmic phase of growth (Fig. 37.6). When environ- 

 mental conditions are less than optimal, the rate of population growth 

 is less. The difference between the potential ability of a population to 

 increase and the actual change in the size of the population is a meas- 

 ure of environmental resistance. Even when a population is growing 

 rapidly in numbers, each individual organism of the reproductive age 

 carries on reproduction at the same rate as at any other time; the in- 

 crease in numbers is due to increased survival. At a conservative esti- 

 mate, one man and one woman, with the cooperation of their children 

 and grandchildren, could produce 200,000 progeny within a century, and 

 a pair of fruit flies could increase to 3368 X 10^" individuals in a year. 

 Since optimal conditions are not maintained, such biologic catastrophes 

 do not occur, but the situations in India and China indicate the tragedy 

 implicit in the tendency toward overpopulation. 



The sum of the physical and biologic factors which prevent a species 

 from reproducing at its maximum rate is termed the environmental 

 resistance. Environmental resistance is often low when a species is first 

 introduced into a new territory, and the species increases in number 

 at a fantastic rate. The introduction of the rabbit into Australia, and 

 the English sparrow or Japanese beetle into the United States, are ex- 

 amples of these. As a species increases in numbers the environmental 

 resistance to it also increases in the form of organisms which prey upon 

 it or parasitize it, and the competition between the members of the 

 species for food and living space. 



When a few individuals enter a previously unoccupied area, the 

 increase in numbers is slow at first (called the positive acceleration 

 phase), then becomes rapid and exponential (the logarithmic phase), 

 slows down as environmental resistance increases (the negative accelera- 

 tion phase) and finally reaches an equilibrium or saturation level (Fig. 

 37.6). 



346. Population Cycles 



Once a population becomes established in a certain region, and 

 has reached its equilibrium level, the numbers will vary up and down 

 from year to year, depending on variations in environmental resistance 

 or on factors intrinsic to the population. Some of these population 

 variations are completely irregular, but others are regular and cyclic. 

 One of the best known of these is the regular 9 to 10 year cycle of abun- 

 dance and scarcity of the snowshoe hare and the lynx in Canada which 

 is based on the records of the number of pelts received by the Hudson 

 Bay Company. The peak of the hare population occurs about a year 

 before thi peak of the lynx population (Fig. 37.8). Since the lynx feeds 



