The population grows in an exponential or geometric ratio. If in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (per capita death rates) as d, the increase or decrease in N (growth rate) during a unit time period t (dN/dt) will be dN/dt = {b - d) × N Let (b - d) = r, then dN/dt = rN. The r in this equation is called intrinsic rate of natural increase.