Final answer:
Population density refers to the average number of individuals of a species per unit area or volume and is a key concept in population ecology.
Step-by-step explanation:
The number of individuals of a single species per unit area is known as population density. Population density can be thought of as the average number of individuals within a specific area or volume. For example, if we have a population of 100 insects in a region of 100 square meters, the density would be 1 insect per square meter. The density would change if the same number of insects lived in a smaller or larger area.
Population ecology is the field that studies population size and density, considering how and why these numbers change over time. A species' population density is a critical characteristic as it affects reproductive success and resource availability. For instance, the federally endangered Karner blue butterfly's population density is heavily influenced by the abundance of its habitat's wild lupine.