Quick Answer: she's working on population structure. Demographics is the study of population variation. It is almost redundant.
It is not size, because she's finding how populations change with location and what influences that change (like water).
Population density is an entirely different topic. That tells how many people live in a unit of measurement. Usually is the number of people per square mile or square km.
Again growth is a different idea. This describes how fast something is increasing or decreasing.