ECOtOGY 



771 



POSITIVE 



ACCELERATION 



PHASE 



Logarithm 

 of number 



of 

 Individuals 



in 

 population 



Time 



Figure 37.6. A typical growth curve of a population, one in which the logarithm 

 of the total number of individuals is plotted against the time. The absolute units of 

 time and the total number in the population would vary from one species to another, 

 but the shape of the growth curve would be similar for all populations. 



there is an upper limit to the number of men that can be supported 

 on the earth is perfectly sound. 



The birth rate, or natality, of a population is simply the number 

 of new individuals produced per unit time. The maximum birth rate 

 is the largest number of individuals that could be produced per unit 

 time under ideal conditions, wlien there are no limiting factors. This 

 is a constant for a species and is determined by physiologic factors such 

 as the nimiber of eggs produced per female per unit time, the propor- 

 tion of females in the species, and so on. The actual birth rate is usually 

 considerably less than this, for not all of the eggs laid are able to 

 hatch, not all the larvae or young survive, and so on. The size and 

 composition of the population and a variety of environmental condi- 

 tions affect the actual birth rate. It is difficult to determine the maxi- 

 mum natality, for it is difficult to be sure that all limiting factors have 

 been removed. However, under experimental conditions, or by careful 

 field studies, one can get an estimate of this value which is useful in 

 predicting the rate of increase of the population and in providing a 

 yardstick for comparison with the actual birth rate. 



The mortality rate of a population refers to the number of indi- 

 viduals dying per unit time. There is a theoretical minimum mortality, 

 somewhat analogous to the maximum birth rate, which is the number 

 of deaths that would occur under ideal conditions-deaths due simply 

 to the physiologic changes of old age. This minimum mortality rate is 

 also a constant for a given population. The actual mortality rate will, 

 of course, depend upon physical factors and upon the size and compo- 

 sition of the population. By plotting the number of survivors in a 

 population against time, one gets a survival curve (Fig. 67./). U the 

 units of the time axis are the percentage of total life span, one can 



