RELATIONS WITHIN A SPECIES 



341 



MORTALITY RATE 



The frequency of death within a population really 

 is not a single rate, because different segments of any 

 species vary as to mortality rate. For example, adult 

 males and females, difTerent age groups of immature 

 organisms, and embryonic stages have unlike poten- 

 tials for death. Moreover, the deaths within any 

 biologically meaningful grouping might have seasonal 

 and other fluctuations. Therefore, "the mortality 

 rate" of a population is an average of many in- 

 dividual, often variable and independent, mortality 

 rates. 



The causes of death can be stated simply as being 

 heredity and/or environment, or a detailed listing 

 can be made of genetic and ecological factors. The 

 hereditary list would contain many traits that are 

 either lethal or semilethal in one or more stages of a 

 life history. The environmental causes, collectively 

 being environmental resistance, are either physical or 

 biotic. Physical causes would include such things as 

 freak weather or fire. Biotic causes would include in- 

 sufficient food, predators, parasites, disease, competi- 

 tion, overpopulation, and underpopulation. 



POPULATION VARIATIONS 



Species groups fluctuate in size. These variations, 

 whether they cause permanent increase, permanent 

 decrease, or cyclic changes in numbers, are further 

 modified by movements into and out of a population, 

 reproductive rate, and mortality rate. Moreover, 

 movements, births, and deaths are so difficult for the 

 naturalist to record that in many species little is 

 known about the cause of population variations. 

 Even if fluctuations in numbers of a group were re- 

 stricted to the birth and death rates, the obstacles to 

 research still would be great. However, discussion of 

 what is known about these two rates will reveal some 

 important features of populations. 



The reproductive potential, a precise kind of birth 

 rate, is the product of the number of breeding fe- 

 males, the average number of ofTspring, and the aver- 

 age number of litters. However, this ignores the fact 

 that while the many older females are breeding, new 

 females become sexually mature and contribute to an 

 even greater reproductive potential. In terms of 

 possible population increase, the rate would become 

 greater geometrically through time. However, this 



potential for geometric increase is not realized in 

 nature. The potential is checked by mortality rate, 

 the ratio between the potential rate of increase and 

 the actual population density. This check operates 

 even when a new population is introduced into a 

 favorable habitat. When a group originates, there is 

 at first a rapid increase in its numbers; soon, however, 

 there is a tendency towards equilibrium in which the 

 biotic potential and mortality rate are the same. On 

 the other hand, recall that in spite of any tendency 

 toward equilibrium under a constant environment, 

 ecological factors in nature vary so much that equi- 

 librium is unlikely. Because environments have 

 regular and noncyclic fluctuations, populations typi- 

 cally are either increasing or decreasing. 



CAUSES OF POPULATION CYCLES 



Reasons for permanent increase or permanent de- 

 crease in populations are often relatively simple to 

 explain. Permanent increase frequently is related to 

 some improvement in the environment. More spe- 

 cifically, one or more of the factors contributing to 

 species mortality rate are either eliminated or re- 

 duced. Permanent decrease, up to and including ex- 

 tinction, would then be allied to increase in factors 

 favoring a higher mortality rate. 



The big mystery in population fluctuations is the 

 cause of cyclic changes in numbers of individuals. 

 Many plants and animals, but especially mammals, 

 are known to display such variations. A large num- 

 ber of these cycles are joined to regular fluctuations 

 in the physical environment; however, the cycles of 

 certain rodents in particular are not closely related to 

 these factors. Many attempts have been made to 

 connect rodent population cycles with many factors. 

 Early studies indicated that many climatic features — 

 and even sunspots — might be relevant to population 

 cycles, but the association of none of these factors is 

 really close. Another explanation is that a rodent 

 population decreases as a result of increase in 

 predators and/or greater contacts among individual 

 rodents, thus spreading disease. As a supplement to 

 this hypothesis, after the rodent population is re- 

 duced, there is a natural tendency for increase be- 

 cause of the lag in increase of predators, parasites, 

 and disease-causing organisms. However, this hy- 

 pothesis does not explain why individuals during 

 cyclic increase are more vigorous, why there is less 

 susceptibility to disease, why litter sizes are larger 



