334 Relations within the Species 



highest during the middle of the hfe span after the individual has 

 become mature and before it has become senile. Variation of mor- 

 tality with age or life stage differs greatly among different kinds of 

 animals and plants. In some species extremely high mortality is ex- 

 perienced in the egg, larval, or seed stages; in others high mortality 

 does not occur until late in life. 



The potential natality of every species of plant and animal is greater 

 than its potential mortality, and hence, under favorable conditions, 

 every species always has the capacity to increase. If under existing 

 conditions the realized natality also is greater than the realized mor- 

 tality, the population will actually increase. If the two rates are 

 equal, the population will be stationary; but, if the realized mortality 

 is greater, numbers will diminish. A birth-death ratio defined as 



100 .= =— is known as the vital index. 



deaths 



Biotic Potential and Environmental Resistance. The maximum 

 possible rate of increase (highest vital index) for a population of a 

 species occurs under ideal conditions in which the birth rate is the 

 highest possible for that species (potential natality) and the death 

 rate is the lowest (potential mortality). Maximum birth rate is de- 

 termined by the largest number of viable progeny (spores, eggs, 

 young, or seeds) that an animal or a plant can produce and the 

 frequency of reproduction. Minimum death rate is determined by 

 internal factors controlling survival when environmental factors are 

 all completely favorable. The values of maximum birth rate and 

 minimum death rate are thus fixed by life processes inherent within 

 the organism, and the maximum rate of population increase, or the 

 biotic potential, is an innate characteristic of each species. The 

 value of the biotic potential, or potential increase as it is sometimes 

 called, differs widely from species to species; contrasting examples of 

 differing rates were given in Chapter 1. As will be more fully dis- 

 cussed subsequently, evolutionary processes have established certain 

 relations between the biotic potential of each species and the exigen- 

 cies of its existence. 



Under natural conditions the full biotic potential of an animal or a 

 plant population is ordinarily not realized since conditions are rarely 

 completely favorable. Harmful climatic changes, attacks by preda- 

 tors and diseases, and other external circumstances curtail the growth 

 of the population. As the population grows, the increase in numbers 

 itself produces changed conditions. A moderate increase in density 

 may sometimes have an ameliorating effect, as we have seen, but 

 sooner or later the detrimental effects of overpopulation— scarcity of 



