This branch of science deals with the affects that immigration and emigration have on a given population. Other than new organisms leaving or entering the population birth and death rates are also studied. Age Structure and Sex Ratio . Many populations have overlapping generations where individuals of more than one generation coexist. This situation produces an age structure in most populations.
Every age group has a characteristic birth and death rate.
A standard rule of thumb sets a high mortality for the lower and upper age groups and a low mortality rate for the intermediate age group. Body size has a major effect on generation time. Mice,which are small,can produce a litter of individuals ever 21 days. Their offspring usually are ready to reproduce in 3 to 4 weeks after birth. On the other hand, the Elephant takes 2 years to produce an offspring and several more years before the offspring can successfully reproduce.