Technical term in biology and ecology for a group of individuals of the same species living in a particular area.
The population on this island is genetically isolated.
Investigate the dynamics of the population.
個体数 means 'number of individuals' and is a count, whereas 個体群 refers to the population as a collective group.
population ecology
Compound of 個体 (individual organism) and 群 (group). The term is a modern scientific coinage for biological populations.