Final answer:
When the instantaneous per capita rate of growth (r) is equal to zero, there is no change in population size.
Step-by-step explanation:
The population size remains unchanged when the instantaneous per capita rate of growth (r) is equal to zero.
This condition is known as zero population growth, where the birth rate and death rate are equal. In this case, the number of individuals being born is equal to the number of individuals dying, resulting in no net change in population size.