a. In a population, the birth rate and death rate refer to per capita births and deaths, respectively.
Examples of birth rate and death rate are :
Birth rate : If in a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added, talking the current population to 28, calculate the birth rate 8/20 - 0.4 offspring per lotus per year.
Death rate : If 4 individuals in a laboratory population of 40 fruit flies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruit fly per week.
b. A population at any given time is composed of individuals ages. If the age distribution is plotted for these population, the resulting structure is called an age pyramid.