Another way to think of this experiment is in terms of birth and death rates. At the beginning of the experiment, birth rates greatly exceeded death rates, and the number of births relative to deaths (per unit time) defines the rate of population increase. As the population grew, the balance between births and deaths began to shift as deaths became proportionately more common. At the point where the maximum population was reached, the birth rate exactly equaled the death rate. When births equal deaths, a population is in equilibrium