The number of individuals per unit area is referred to as ...

• A. Population density
• B. Gene pool
• C. Immigration
• D. Carrying capacity

Answer: (A)

Population density is the measure of how crowded a population is in a given area. It counts how many individuals occupy each unit of space, usually per square unit (or per unit volume in water). You determine it by dividing the total number of individuals by the area they occupy. This concept helps explain how organisms share space and resources, and how crowdedness changes with population size or area.

For example, if 500 plants are found in a 10-by-10 meter plot, the density is 5 plants per square meter. The other terms describe different ideas: the gene pool is about genetic diversity, immigration is the movement of individuals into a population, and carrying capacity is the maximum population size an environment can sustain over time.