Populations ... population by changes in reproduction or survival. Wauters & Lens (1995) studied how food availability and density combine to limit red squirrel reproduction rates.

Populations of organisms do not remain constant; the number of individuals within a population changes, sometimes dramatically ... food increased hare density three-fold, whereas fertilizer ...

Population density (often abbreviated PD) is a compound measure that tells us roughly how many people live in an area of known size. It is commonly used to compare how 'built-up' two areas are.