logical and physiological capacities of the species, the 

 amount of parental care that the offspring receives, 

 and weather conditions. Death rates correlate directly 

 with the number of young produced. Reproduction 

 cannot be considered successful unless the young 

 reach sexual maturity. 



Life tables tabulate, in condensed form, the vital 

 statistics of survival and mortality by time intervals. 

 They provide essential data for calculating longevity 

 and age composition of populations. Survivorship 

 curves show three characteristic survival patterns, 

 but most populations exhibit a relatively high death 

 rate early in life and a lower, more constant death 

 rate thereafter. 



Sex ratios are often correlated with mating be- 

 havior. The age of full reproductive maturity varies 



widely between species. Young birds, surplus adults 

 of either sex, and birds unsuccessful in establishing 

 breeding relations sometimes constitute a relatively 

 large non-breeding population in addition to the more 

 conspicuous breeding one. Perhaps this is true also 

 for other animals, but evidence is scanty. 



Ratios of young animals to adults often indicate 

 whether a population is expanding, contracting, or is 

 stabilized. In stabilized populations the number of 

 offspring reaching reproductive maturity can never 

 be greater or less than the number of adults them- 

 selves. The number of young that must be produced 

 to permit such a population turnover gives a measure 

 of the rigor of the environment, and how well adapted 

 a species is to its niche. 



218 Ecological processes and dynamics 



