If it does it usually results from the absence of a strong competition among individuals. While the above patterns apply to individuals within a population, keep in mind that populations within a species show dispersal patterns. 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.