A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths, respectively. The rates, hence, are expressed is change in numbers (increase or decrease) with respect to members of the population. Here is an example, If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, then the birth rate calculate as 8/20 = 0.4 offspring per lotus per year.