Natality and Mortality 333 



POPULATION DEVELOPMENT 



The organisms inhabiting an area at a given time constitute a 

 population. If the organisms all belong to one species, they form a 

 single-species population. In most natural situations several to many 

 kinds of plants and animals coexist in the same habitat. The in- 

 habitants may then be regarded as composing a corresponding num- 

 ber of single-species populations that are intermingled, or alternatively, 

 as forming one mixed or 7nulti-species population. The interspecific 

 relations of mixed populations will be treated in the next chapter. 

 Here we shall discuss the quantitative relations that arise during the 

 growth and fluctuation of single-species populations under the in- 

 fluence of the harmful and beneficial interactions considered in the 

 foregoing sections. 



Principles governing population dynamics are found to apply 

 equally to special situations, such as laboratory cultures, in which 

 only one species is present, and to natural areas in which many popu- 

 lations exist together. The presence of other species acts as part of 

 the environment in influencing the changes in the population of the 

 species under consideration. 



Natality and Mortality 



The abundance of a species in an area tends to increase because of 

 reproduction, and to decrease because of death. The rate of repro- 

 duction depends upon the birth rate or natality, and the rate of death 

 is referred to as 7nortality. The dispersion of members of the species 

 also affects abundance— positively in the case of immigration, and 

 negatively in the case of emigration. We shall consider first the sim- 

 plest situation in which no dispersion is taking place, leaving a discus- 

 sion of the influence of migration into or out of the area to a later 

 section. The maximum possible rate of reproduction for a given 

 species under optimal conditions is termed the potential natality. In 

 natural situations the potential natality is rarely, if ever, attained be- 

 cause the birth rate is inevitably reduced by one adverse circumstance 

 or another. The actual birth rate under the existing conditions is 

 referred to as the realized natality. In parallel fashion the lowest pos- 

 sible death rate for a given species in the best of circumstances is the 

 potential mortality, and the actual death rate is the realized mortality. 



Natality and mortality vary not only from species to species but 

 also according to the age of the individuals. Natality is usually 



