In a population of an area, the difference in the natality rate and mortality rate is zero in a particular year, but the population density has decreased. Which of the following may be the reason for that decrease?

Number of emigrants (people who moved out of the geographical area permanently) is more than immigrants (people who moved in) and so the population size decreased although natality and mortality rate is zero.

The density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes, two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.